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234 Cards in this Set
- Front
- Back
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Define KERMA
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Kerma (kinetic energy release in mass) is the mean energy transferred from indirectly ionizng radiation to charged particles in a medium
K - dEtr/dm Unit =J/kg |
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Define LET
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is the amount of energy deposited per unit length along the track of an ionizing radiation particle
- track average - energy average Units: keV/um |
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What is a curie?
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3.7 x 10 exp 10 disintegrations per second
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Define Air Kerma Strength
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Air Kerma strength is the product of air kerm in free space and the square of the distance of calibration to the center of the source (using a perpendicular bisector)
:Sk = Kt x l2 - usually at 1m - units: uGy/m2h |
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what is the size of an atom? nucleus?
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10 exp-10 m
10 exp-15 m |
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Describe an atom.
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a central positively charged nucleus containing protons and neutrons (nucleons) surounded by a "cloud" of orbiting electrons which occupy distinct energy levels
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What is binding energy?
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Nuclear binding energy: is the energy is required to keep the nucleons together and is provided by the mass defect.
Electron binding energy: the energy required to remove an electron from its shell. |
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What is a proton?
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a subatomic particle with an electric charge of +1 (1.6x10exp-19 C).
Mass = 1.00727 amu consists of 3 quarks: 2 up and 1 down |
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What is a neutron?
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a subatomic particle with neutral charge. Mass 1.00866 amu.
Consists of 3 quarks, 2 down and 1 up |
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What is an electron?
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a subatomic particle which caries a negative charge of -1 (-1.60x10exp-19 C)
Weight is 1/1836 proton, 9.11x10exp-31kg First generation members of the lepton particle family |
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What is an isotope?
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Same number of protons, different number of neutrons (i.e. same Z, different A)
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What is a isotone?
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Same number of neutrons, different number of protons.
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What is a isomer?
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Same number of neutrons and protons, but the nucleus is in a different nergy state
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Define an amu.
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It is 1/12th of the weight of a carbon-12 nucleus.
1 amu = 1.66x10-27kg |
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What is the law of conservation of energy?
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The law of conservation of energy states that energy can neither be created nor destroyed, but only convereted from one for to another (in a closed system)
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What are the types of photon matter interactions?
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Coherent/classical/raleigh scattering
Photoelectric effect Compton scattering Pair production |
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What is coherent scattering?
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incoming photon causes oscillation of electron in orbit, electron reradiates the same energy in a different direction (small angles).
Low energies, high Z No relevance in RT |
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List 5 interactions of photons with matter.
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Classical/Rayleigh scattering
Photoelectric effect Compton Scattering Pair production Photodisintegration |
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At what energy ranges are the various interactions most relevant?
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Photoelectric effect - 10-40keV
Compton- 40KeV - 24 MeV Pair produciton > 10MeV Photodisintegration >> 10MeV |
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What variables influence the probability of the various interactions with matter?
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Photoelectric
- Z3 -1/E3 Compton - E - electron density Pair Production - Z2 - Z/electron - Z/gram - exponential increase above threshold energy |
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What is the formula for efficiency of XR production?
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E = ZV x 9x10(exp-10)
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What is the relative rate of energy loss of an electron (in water)
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2MeV/cm
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Define Fluence
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Number of photons dN per unit cross section da.
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Define intensity.
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Intensity, is the energy fluence rate, energy flux density and is the energy fluence per unit time, that is the energies of all photons that enter sphere of cross sectional area da. i.e. dE/da
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What is exponential attenuation?
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Applies to the decrase in intensity with an absorber and applies only monoergetic beam.
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What is the linear attentuation coefficient
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Linear attenuation coefficient defines the change in intensity with a beam through an abosrber and depends on the nergy of the photons and nature of the material
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What is the amss attenuation coefficient
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Linear attneuation coefficient with density factored out.
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What is energy transfer?
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Energy transfer coefficient defines the fraction of photon energy trnasferred into kinetic energy of a charged particle
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What is the energy absorption coefficient.
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Energy absorption refers to the fraction of energy which is absorbed local as a proportion of the energy of secondary charged particles that are lost to bremsstrahlung in the material.
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What is the formula for efficiency of XR production?
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Efficiency = 9x10exp-9 x ZV
where Z is atomic number V is tube voltage |
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Define stopping power
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Stopping power is the rate of kinetic energy loss per unit path length of the particles dE/dx
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Define mass stopping power
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Rate of kinetic energy loss per unit path length divided by density (S/ro).
The total S/ro is the sum of S/ro (coll) + S/ro (radiative) |
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What is mass scattering power?
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Mass scattering power (t/ro) is defined by the quotient
d(theta)squared/(ro)l Increases with square of Z, and inversely with square of kinetic energy of electrons |
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Draw a Linac.
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Draw it.
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Draw a XR tube.
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Draw it.
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What is a magnetron?
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Produces microwaves.
Consits of a cnetral cathode and outer anode bored out of a solid piece of copper A static magnetic field is applied perpendicular to the plane of cross section of the cavities. The cathode is heating a produces elctrons via thermionic transmission. A pulsed DC electric field is applied between cathode and anode Electrons spin and oscillate under this fields in complex spirals which radiate energy in the form of microwaves suitable up to 6MV |
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What is a klystron?
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Suitable for >6MV machines.
Is attached to a low frequency microvwave generator Hot wire filament cathode produces electrons which enter the klystron at the bunch cavity where they interact with the low amplitude microwaves Under the influence of the microwaves some electrons are accelerated, some decelerated result in bunches of electrons. they drift through to the catcher cavity. The electrons induce a negative charge on the end of cavity,which causes deceleration of the incoming electrons and this loss of energy is transferred into the form of microwaves, at a much greater energy then the enterig microwaves |
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What is cerrobend made of?
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50% Bismuth
26.7% Lead 13.3% Tin 10% Cadmium |
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What are independent jaws and MLC's made of?
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Lead and Tungsten
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What are the relative transmission of cerrobend, independent jaws and MLC;s respectively
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cerrobend - 3.5%
jaws - <1%, MLC's - <2% (interleaf <3%) |
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Define wedge angle.
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wedge angle is the slope of the isodose line at the central axis at a specified depth, usually 10cm.
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What are the types of wedge systems?
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Physical wedge (hard wedge, flying wedge)
Dynamic wedge |
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What is Kramer's equation?
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Describes intensity of a photon beam
IE = KZ (Em - E) |
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How can beam quality be defined?
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Photon spectrum
HVL Effective energy Dosimetrically i.e. TPR20,10 |
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What is the energy spectra of a photon beam?
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The most precise way to define a xray eam but characteristising its spectral distribution.
scintillation spectometry can measure energy spectrum. |
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What is effective energy?
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effective energy is the beam which is attenuated to the same degree as a monoenergetic beam to the beam in question. (i.e. same linear attenuation coefficient)
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What is Half value layer?
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Half value layer is the thickness of an absorber (which should be specified) suffieicnty to attenuate the intesnity of the beam to half of its original value
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How is linac beam quality given?
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Peak kilovoltage
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How does a beam change with voltage? filament current? tube current?
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voltage - exposure increases supra linearly
filament current - increases greater than linearly (small changes in filmaent current = large changes in exposure) Tube current = exposure increases linearly |
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What is the inherent filtration in most XR machines?
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1mm Al
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What are K lines of tungsten
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69 and 57.
(K shell -69 keV, L shell 12keV) |
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How does a thoraeus filter work?
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The characteristic x rays of tungsten are 58-69keV
A tin filter heavily absorbs radation in the 30-70 keV range as its K shell is 29keV, which results in its characteristic radiation Copper has K edge of 9keV so it heavily absorbs the characteristic xrays of Tin Aluminium absorbs small XR from copper Largest Z must be closest to tungsten i.e. Sn |
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What is the range of filters used in superficial
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1-8mm Al.
(orthovoltage may use copper filters) |
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What are two methods of delivering an electron beam?
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Scanning beam and via scattering foil.
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What is the energy spectrum of a clinical photon beam?
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Essentially the beam is monoenergetic before it leaves the treatment head, but as it passes through the exit window, scattering foils, ionization chambers, collimators and air there is a broadening of hte beams spectum as well as bremsstrahlung radiation which causes the tail.
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Define Rp
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Rp is the depth at which the tangent to the steepest part of the electron depth dose curve intersects with the extrapolation line of the bremsstrahlung tail.
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What are and define the methods of specifying electron energy.
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1. Most probable energy
- Ep,0 = C1 + C2Rp + C3Rp2 2. Mean Energy E0 = C4 x R50 3. TRS 398 defines beam quality index as R50 |
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Describe variation of energy at depth and why is it important in clinical dosimetry?
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Most probable energy and mean energy (approx). decrease linearly with depth. This is important in dosimtry because for absorbed dose measurements it is is necessary to know th eman electron energy at the lcoation of the chamber
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What is photon contamination?
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Photon contamination occurs as a result of elelctron - bremsstrahlung interactions with the collimators, air and the patient
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What are the typical contaimination ranges of electron beams? When is it of clinical concern?
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6-12MeV: 0.5%
12-15MeV: 2% 15-20MeV: 5% In total body electron treatment i.e. mycosis fungoides |
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What is the TRS398 beam quality index for electron beam dosimetry? What is it best measured with?
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R50.
Parallel plate ionisation chamber. |
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What detectors can be used for measuring output.
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Output changes widely between machine with changes in field size.
Output variations can be measured with ionisation chambers, but also TLD;s, film and diodes - which are all suitable for calculation dose ratio;s, however only a ionisation chamber is useful for absolute dosimetry. |
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How does surface dose change with electron energy?
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Increases.
Because with low energy electrons the electrons are scattered through larger angles and the buildup occurs over a much more rapid rate. The ratio of the surface dose to the maximum dose is therefore less. |
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What is lateral constriction?
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Lateral constriction is seen with high energy electron beams at the high isodose curves, particularly with small fields at
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How is the flatness of an electron beam determined?
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ICRU defines beam flatness in terms of a uniformity index.
It is defined in a reference plane and at a reference depth as the ratio of the area where the dose exceeds 90% of its value on the central axis to the geometric beam cross sectional area at the phantom surface. It should exceed a given fraction i.e. 0.80 for a 10x10cm field at Dmax In addition the dose at any arbitrary point in the reference plane should not e xceed 103% of the central axis value. (note the AAPM and IEC have different definitions) |
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Hows is beam symmetry define for electron beams?
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The cross beam profile in the reference plane should not differ more than 2% at any pair of points located symmetrically on opposite sides of the central axis.lm
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What is penumbra?
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Penumbra is the region of rapid dose gradient as a function of distance from the central axis.
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How does ICRU define penumbra?
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distance between the 80% and 20% isodose curves at depth of R85/2 (distal 85%)
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What factors determine penumbra?
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Beam energy
Distance between application and patient depth in tissue whether surface shielding isused |
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What effects are seen with a decrease in field size?
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Field size must be large enough for lateral equilibrium to occur otherwise dose distribution unpredictable.
Surface dose increases |
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What is CET?
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Coefficient of equivalent thickness. Used to calculate tissue heterogeneities in electron beams.
Works well for bone. Lung is more unpredictable |
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What is the effect of small heterogeneities on electron beam dosimetty?
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Complicated. Hot and cold spots depending on density of heterogeneity. (not can get a hot spot behind high atomic numbers)
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What effects are seen on dose distriubtion with changes in field size in electron dosimetry?
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with small field sizes below the size required for electronic equilibrium (Rp/2 = MeV is safe for edge) :
1. surface dose increases 2. Dmax increaes and becomes more shallow 3. PDD is very sensitive due to loss of lateral scatter to the central axis |
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What effects does an increasing SSD have on electron dosimetry?
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1. Decrease surface dose secondary to loss of low energy electrons from treatment head
2. Dmax becomes deeper 3. Decrease PDD |
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What effects are seen with increasing beam quality in electron dosimetry?
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1. Increase surface dose
2. Dose buildup is less rapid 3. Increase dose at depth i.e. loss of rapid fall off (particularly over 15MeV) |
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What effects are seen with patient irregularities on electron beams?
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1. Dmax closer to surface
2. Larger dose at Dmax 3. Decrease penetration (defined by D80, Rp is relatively stable up to 60 degrees) |
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Why are applicators requried in electron beams?
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Because the photon beam collimators are too far away to be effective and after scattering there would be an unacceptable penumbra.
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What is acceptable transmission with blocks in electron treatments.
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5%.
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What thickness of lead is required to appropriately block electron beam?
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Approximately Mev/2 in mm Pb + 1mm for safety
Cerrobend - 20% extra |
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What parameters are important in the change of dose with field shaping
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Electron energy
Thickness of lead/cerrobend Field size |
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How imporant is backscatter factor in internal shielding? How can this be overcome?
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Up to 30-70%.
Using a low atomic layer such as bolus or aluminium will reduce this. EBF is related to average electron energy at the interface (i.e. energy at depth) |
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What are the dose limits for public and for occupational exposure?
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Occupational
- 20mSv per year averaged over 5 years, no more than 50mSv in a single year - Hands and skin 500mSv/year - Lens 150mSv Public -1mSv per year averaged over 5 years - 50mSv for skin -15mSv for ens - no hands and feet |
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Why are the skin, hands and feet and lens stipulated with difference dose levels?
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The normal dose limits as applied are unlikely to result in determinstic effects, except for these three regions, in which effective doses are stipulated.
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What are the steps of radiation protection control (in context of dose limiits)
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1. Justification
- more harm than good 2. Optimization - best use of resources to reduce dose 3. Limitations (dose/risk) - dose limits |
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What are the three types of exposures (in terms of radiaion protection)?
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1. Occupation
(all work related, excludes natural background radiation unless part of job i.e. radionuclides in mining) 2. Medical (patient, volunteer, carer) 3. Public |
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How imporant is backscatter factor in internal shielding? How can this be overcome?
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Up to 30-70%.
Using a low atomic layer such as bolus or aluminium will reduce this. EBF is related to average electron energy at the interface (i.e. energy at depth) |
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What are the dose limits for public and for occupational exposure?
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Occupational
- 20mSv per year averaged over 5 years, no more than 50mSv in a single year - Hands and skin 500mSv/year - Lens 150mSv Public -1mSv per year averaged over 5 years - 50mSv for skin -15mSv for ens - no hands and feet |
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Why are the skin, hands and feet and lens stipulated with difference dose levels?
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The normal dose limits as applied are unlikely to result in determinstic effects, except for these three regions, in which effective doses are stipulated.
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What are the steps of radiation protection control (in context of dose limiits)
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1. Justification
- more harm than good 2. Optimization - best use of resources to reduce dose 3. Limitations (dose/risk) - dose limits |
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What are the three types of exposures (in terms of radiaion protection)?
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1. Occupation
(all work related, excludes natural background radiation unless part of job i.e. radionuclides in mining) 2. Medical (patient, volunteer, carer) 3. Public |
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Are the dose limits proposed by ARPANSA/ICRP appropriate for dose limitation?
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The dose limits do not represent a safe level (there is no such level) for stochastic effects, but are unlikely to cause deterministic effects. Doses under limitation should be ALARA.
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Are medical, occupation and public exposures all subject to the principles of justification, optimisation and limitation?
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Except for the principle of limitation for medical exposure(although there are limits for volunteer exposures from which there are unlikely to benefit from the intervention).
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Define absorbed dose.
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Absorbed dose is the energy, dE, imparted to a mass of tissue, dm.
D = dE/dm SI Units = Gray = J/kg |
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Define intebgral dose.
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Integral dose is the total dose received by the body and is the sum of the mass of tissue by the absorbed dose to tissue
Units: Gy.kg |
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Define Kerma.
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Kerma is kinetic energy release in medium and is the sum of initial kinetic energies of all secondary particles liberated by uncharged particles in a mass, dm.
dEtr/dm Units: Gy (J/kg) |
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Draw the relationship between KERMA and dose.
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Define equivalent dose.
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Equivalent dose is the dose received by tissue taking into consideration the differences in biological effectiveness of different types of radiation.
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What are radiation weighting factors.
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Photons/electrons 1
Neutrons 5-20 Protons 5 Alpha particles 20 |
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Define Effective Dose.
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Effective dose is the dose received which considers the radiosensitivity of different tissues, as well as the different effectiveness of different radiations types.
SI Unit is Sievert |
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What is the role of the dose limit recommedations?
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The system of radiation protection described in these
Recommendations is designed to keep the probability that stochastic effects will occur from exceeding a level that is regarded as unacceptable. The dose limtis will prevent deterministic effects from occurring, except for skin, hands/feet and lens. |
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Define radioactivity.
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Radioactiveity is the phenomonenon whereby an unstable nucleus emits energy in the form of ionising radiation (particulate or energy). It is a stochastic event.
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List 6 radionuclide decay processes.
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Beta decay
Alpha decay Internal Conversion Electron capture Gamma decay Spontaneous fission |
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Give example of alpha decay.
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226, 88 Radium to 222,86 Radon.
Only occurs in elements above 82 - ? strong nuclear force unable to overcome strong coulomb forces |
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What are the daughter products of (negative) beta decay? When does it occur.
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A,Z-1 nuclide + antineutrino + energy (as kinetic energy).
Example is 137,55 Cs - 137,56 Ba with electron and antineutrino. Excess neutrons. |
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What are the daughter products of positron decay. When does it occur.
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A, Z-1 Nuclide + neutrino + positron.
Example is 22,11 Na to 22,10 Neon Excess protons |
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What is internal conversion?
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Internal conversion results from the transmission of excess nuclear energy to a electron in the electron shell (there is no emission of a gamma ray first, however)
I-125 is an example |
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What is electron capture.
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Electron capture occurs when a nucleus "captures" (generally a K shell) electron into the nucleus, converting a proton in a neutron.
Example is 22,11Na to 22,10Ne Occurs if too many protons. |
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What is an Auger electron.
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An Auger electron is an electron which may absorb characteristic XR resulting from electron transitioning to be ejected as an electron.
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What is gamma decay?
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Gamma radiation is emitted from the nucleus. Internal conversion is a competing mechanism of energy loss. Isomeric transition
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What is photodisintegration and what is its relevance to radiotherapy?
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A gamma ray may be absorbed by a nucleus resulting in emssion of a neutron (most common emission, but also can result in proton, deuteron, tritium emission). ? cause of neutron production in high energy beams.
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Define equivalent dose.
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Equivalent dose is the dose received by tissue taking into consideration the differences in biological effectiveness of different types of radiation.
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What are radiation weighting factors.
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Photons/electrons 1
Neutrons 5-20 Protons 5 Alpha particles 20 |
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Define Effective Dose.
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Effective dose is the dose received which considers the radiosensitivity of different tissues, as well as the different effectiveness of different radiations types.
SI Unit is Sievert |
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What is the role of the dose limit recommedations?
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The system of radiation protection described in these
Recommendations is designed to keep the probability that stochastic effects will occur from exceeding a level that is regarded as unacceptable. The dose limtis will prevent deterministic effects from occurring, except for skin, hands/feet and lens. |
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What is the phsyical half life of a nuclide?
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Physical half life is the time it takes for activity of a nuclide to halve or for the number of nuclides to decay to half of the original number.
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What is the biological half life of a nouclide?
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The time it takes for a subustance to lose half its concentration due to metabolism, degradation or excretion.
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What is the effective half life of a nuclide.
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Effective half life of a nuclide is the time required for the acitivty of the nuclide in a biological system to halve due to both the physical and biological half life.
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Define the mean life of a radioactive nuclide.
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Mean life is the average lifetime for decay of radioactive atoms.
MA = 1.44 x T1/2 |
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What are the two types of radiation equilibrium?
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1. Transient - if half life of parent is not much longer than the daughter.
2. Secular - if half life of parent is much longer than the daughter product. |
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Define activity.
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Activity is the rate of decay (or disintegrations) i.e. the number of events/unit time.
A = (lambda)N or A = A0 x e (exp - lamda x t) SI unit = Becquerel (=1dps) 1 Curie = 3.70x10(exp10) dps |
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Define apparent activity.
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apparent activity is the activity of a bare (unfiltered)( point source of the same nuclide that produces the same exposure rate as the source to be specified
It is calculated by dividing the calculated dose rate at 1m by the exposure rate constant of the unfiltered source at 1m. |
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Define specific activity.
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Activity/unit mass.
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Define reference Air Kerma Rate.
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Reference Air Kerma Rate is defined by the ICRU as the air kerma rate in air at a distance of 1m, corrected for attenuation and scattering
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What is the air kerma rate constant?
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The air kerma rate constant is unique for each radionculide and descirbes the relationship between kerma at a distance to the source per hour.
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How does CT work?
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1. Uses ionisation radiation in the KV range via a rotating tube head and a fixed or rotating detector system.
2. In modern helical scanners the patient moves in a longitudinal plane through the axis of the gantry 3. At a single slice in the patient, as the radiation is attenuated through the patient and measured by the detector, a linear attuenation coefficient can be calculated for each small part of that slice. 4. The linear attenuation coeffiicents are converted into CT numbers of Hounsfeld units (a grey scale applied to the density of the tissues) by the relationship: HU = 1000 x (utissue-uwater)/(uwater) Where air has a value of -1000, water a value of 0, and bone a value of +1000 5. electron density can be inferred from the CT numbers (linear attenuation coefficients) for dose calculations in radiation therapy, although the relationship is not linear due to difference in atomic number and compton/PEE in the CT scanning range. |
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What is the use of CT in RT?
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1. Patient target volume, critical structure position and contour information for planning system
2. Inhomogeneities NB point 1 is much more important than 2 |
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What are the advantages and disadvantages of CT?
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ADVANTAGES
- quick, rapid, relatively cheap (in comparison to MRI/PET) - readily available - contour information - tissue inhomogeneity information - electron density for dosimetric calculations - IV contrast allows further anatomical information - multiplanar recontructions - allows for 3D conformal RT planning DISADVANTAGES - ionising radiation - blurred images from motion artefact i.e. breathing - metal artefacts - IV contrast reactions - soft tissue resolution not as good as MRI - aperture of CT may not allow for ideal positioning of patient - table limit i.e. 150kg |
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Describe the principles of plain film radiography used in RT?
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Plain films may either be taken in the KV range (diagnostic imaging) or the MV range (i.e. EPIs).
Plain films have largely been replaced with digital imaging systems in both diagnostic and therapeutic imaging. Differential attenuation of radiation is exploited to show the characteristic differences between air, soft tissue and bone. High atomic number materials attenuate radiation in the kV range differential, in the therapeutic range the differential is based on electron density and tissue density. Digital imaging uses amorphous silicon flat panel detectors with a fluorescent layer for image production |
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Describe principles of fluoroscopy.
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Uses ionisation radiation and differential attenuation within a patient when it interacts with fluorescent detector. When linked to a XR image intestifier and CCD video screen the images can be viewed live with continuous streaming.
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Describe the principles of MRI.
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1. Uses the principles of nuclear magnetic resonance
2. A magnetic field is applied around which certain nuclei precess (the frequency of which is known as the Larmor frequency) 2. A second field is applied from RF coil 3. the second field is turned off, and the nuclei align to the original field, and during this transition induce a signal in the receiving coil (NMR) 3. Relaxation occurs in the transverse direction (T2) as well as in the direction of the magnetic field (T1) 4. Most MR imaging uses a 180 degree RF pulse immediately after the 90 degree pulse. - the signal is received 2x the difference in pulses and is known as the echo time (TE) - the time interval between 90 degree pulses is known as the repitition time (TR) 5. By manipulated the TR and TE, different contrast patterns can be seen, which results in the differences seen in different sequences. - i.e. short TR, short TE - T1; long TR, long TE |
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What is nuclear magnetic resonance?
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It is the underlying principle of MRI. There is a resonance transition between nuclear spin states when a RF pulse is applied to a nucleus aligned within a magnetic field. Only applies to nuclei with a non zero spin.
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What are the principles of PET imaging?
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Positron Emission Tomography uses positron-electron annihilation reaction to locate isotope accumulation.
Positron emitters are attached to a pharmaceutical which is biologically active at a particular site (i.e glucose, misonidazole). At the site of uptake positrons collide with electrons in a annhiliation reaction resulting in the destruction of the particles and creating of two 511keV photons which travel 180 degrees apart These two photons are measured by detectors and the point of the creation of that event is mapped (resolution 4mm) Due to the poor spatial resolution it is best overlayed with a CT scan at the same time to correlate underlying anatomy (i.e. functional and anatomical information fused) |
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What are the other types of MRI scanning.
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1. Function MRI (change in oxygenated Hb concentration)
2. MR spectroscopy (measures amount of different metabolites, as they all have there own spectroscoy signature) 3. MRA 4. Diffusion MRI (diffusion of water molecules in tissue) |
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What are the principles of Ultrasound?
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1. Uses sound waves in the range of 1-20MHz
2. The pizoelectric effect is the phenomenon whereby a crystal lattice will emit ultrasonic waves under the influence of an electric current/energy. This releases energy into the patient. 3. Sound waves are reflected and transmitted within the interfaces in the body. - the degree of reflection or transmission is due to the differences in acoustic impedance between the two tissues of the interface (which is the product of the density and velocity of sound waves in that media) - the greater the difference in acoustic impendance the greater the reflection i.e. bone and soft tissue 4. Attenuation of the signal occurs exponentially as the wave passes through the media 5. The reflected energy is recieved by the transducer which via the pizoelectric effect translates the ultrasonic energy into electrical energy. The pizoelectric effect cuases the crystals to oscillate mechanically, generating acoustic waves. |
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What are the principles of nuclear medicine scanning?
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1. A gamma emitter is attached to a pharmaceutical which accumulates at the organ of interest (ie 99m-Tc with MDP a bisphosphonate to sites of bone turnover)
2. A gamma camera is used to locate the position of these events over a period of time. This can be in the form of a simple planar evalulation as is used in traditional nuclear medicine imaging, or via a rotating gamma camera with SPECT which can provide better spatial resolution 3. The gamma rays cause ejection of electrons in the crystal (usually NaI) in the camera which results in energisation of the surrounding atoms which emit blue light. This light is recieved by a photomultiplier tube which causes an electron ejection which is accelerated and causes further ejection and so on, the original signal is amplified some 1 million times) |
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What abre the types of ultrasoudn?
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A - amplitude mode -
B - brightness mode - cross sectional imaging M - motion mode - echo |
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What is meant by cone beam CT?
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Cone beam CT means that the detector panel is flat rather than circular. Ct planar projection images are obtained from multiple directions through 180 degrees or more and a rectonstructed to provide volumetric information.
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What advantages does MV cone beam have of kV
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1. Uses generally the same device (i.e. no need for extra tube and detector device)
2. Less metallic artefact in MV 3. No need to extrapolate attenuation coefficients from kV to MV for dosimetric calculations Note that the contrast is much better in KV imaging |
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Describe the construction of a cobalt unit.
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SOURCE
- Co-60 is produced by bombarding C0 59 with neutrons in a reactor - Decays to Ni-60 via beta emssion with release of two photon beams 1.17 and 1.33 MeV - Comes as a disc or capsule which is welded into a stainless steel capsule, and then again to avoid contamination - the stainless less absorbs the electron, a small amount of bremsstrahlung radiation is made, which does not significantly contribute to the dose - there primary beam interacts with the housing, the capsule and does impact on beam dosimetry, the lower energy beams consituting some 10% of the dose to the patient - the fact that the source is not a point source is important as it increases geometric penumbra HOUSING - housing is steel shell filled with lead - Cobalt source is kept in off position and there are a number of devices to bring to on position including drawer, wheel mechanisms, large primary collimators, mercury flowing between source and patient to absorb beam. |
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What are the components of physical penumbra.
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Transmission penumbra
Geometric penumbra Scatter penumbra |
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How does penumbra vary with SSD?
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Decreases
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What options are available for characterisation of beam quality?
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1. Complete Spectrum
2. HVL 3. Efffective energy 4. Mean energy 5. peak voltage |
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What options are available to characterise MV beams?
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1. Peak voltage
2. PDD i.e. TPR20,10 |
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What dosimeter is best used for measuring beam quality?
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cylindrical chamber.
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How does the TRS 398 define beam quality? What dosimeter is required to perform this?
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TPR20:10. Cylindrical chamber.
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How does the TRS 398 define electron beam quality? What stipulation is made on the measuring dosimeter?
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R50 (same as TG51). Parallel plate for <10MeV, either parallel or cylinderical for above. (field size 10x10 up to 16MeV, 20x20 for higher)
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What advantage is the their for the TPR2010 compared to TG51 for photon beam dosimetry?
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TG51 at dmax which is easier, but also suffers from electron contamination.
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What is a horn?
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It is an off axis region which has a higher dose than the central axis as a result of beam flattening.
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What is the surface dose, dmax, d5cm, d10cm and d15cm for:
1. 6MV 2. 10MV 3. 15MV |
1. ?, 1.5cm, 87, 65, 50.
2. ?, 2.5cm, 92, 75, 60 3. ?, 3cm, 97, 80, 65 |
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Define penumbra. How is it measured?
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Penumbra is the region of rapid dose gradient as a function of lateral distance from the central axis.
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What is PDD?
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Percentage depth dose is the absorbed dose, expressed as a percentage of a reference dose (usually Dmax) at the central axis.
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What is primary dose.
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Dose as a result of the initial or primary photon.
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What is scattered dose>
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Dose as a result of scattered photons. Consists of collimator and phantom scatter.
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What is a beam profile?
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a graph which demonstrates the variation across of beam field at a particular depth.
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Define Beam Flatness for a photon beam.
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F = 100 x (Dmax-Dmin)/(Dmax + Dmin)
<3% is requirement at depth of 10cm for maximum field size at SSD = 100cm. |
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What is a consequence of achieving flatness of a beam at the reference depht?
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Horns superficially. Underflatting distantly.
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Define beam symmetry for a photon beam.
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2 methods.
1. Simple method is to find to points equidistant from the cetnral axis should be within 2% of eachother. 2. More complicated. Area under the z max beam profile on each side of the central axis extending to the 50% dose level are determined. S = 100x (AreaL - AreaR)/(AreaL + AreaR) |
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What is transmission penumbra?
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Transmission penumbra is a component of penumbra attributable to the radiation passing through the edges of the collimator blocks.
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What is geometric penumbra?
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Geometric penumbra is a result of the source being of a finite size.
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What is the approximate surface dose for 6 and 18MV beams?
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15 and 10%
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The surface dose in MV beams is a result of what?
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1. Photons scattered from the collimators, air, flattening filter
2. Photons backscattered in the patient 3. High energy electrons from interaction in air, shielding structures near patient |
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What is skin sparing?
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Skin sparing is the effect seen in MV range photons whereby the dose is characterised by a low surface dose and a maximum dose at a certain distance under the surface which is a function of the beam energy and field size.
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What effect does increasing SDD have on PDD in photon beams?
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PDD increases with SDD because the ISQ law becomes less imporant with increasing SSD.
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What effect does increasing beam quality have on PDD?
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Increases. Increasing KERMA.
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Draw a MV beam passing through soft tissue and then bone and soft tissue. Draw a 6MV and 10MV. Draw a superficial voltage diagram.
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See answer.
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Draw a PDD chart for a beam passing through soft tissue, lung and soft tissue.
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See answer in book.
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What are the 4 methods of inhomogeneity corrections for MV beams?
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TAR method
Baltho power TAR method equivalent TAR Isodose shift |
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What are the 3 methods of tissue irregularity corrections?
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TAR method
Isodose shift method Effective SSD method |
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What are the apprxoimate corrections for dose beyond healthy lung?
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3% - 4MV
2% - 10MV 1% - 20MV |
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Wha is the result of a small soft tissue inhomogeneity within bone (i.e. blood vessels etc.)?
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Increased dose.
mass attenuation coefficent lower in Compton range, S/(ro) greater for sot tissue energies. |
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What is the ideal dosimeter for measuring in the buildup range?
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Extrapolation chamber. However these are not generally readily available so parallel plates and TLD's are used.
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What are the relative transmssion factors for Cerrobend, jaws and MLC's?
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3.5%, 1% and 2%. NB the interleaf transmission for MLC's generally <3%
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What are the effects of irregular field size on dose rate?
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- change in transmission penumbra
- decrease dose rate with decreasing field size as a reult of decreased phantom and collimator scatter. |
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What are the effects of field shaping with MLC's, blocks and jaws.
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Jaws give sharp cut offs for field matching. Least transmission.
MLC's give the largest penumbra, and with the jagged boundary they give the least conformal treatment which is generally only signifcant with small fields and when field matching is required. |
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Discuss differences with bolus for orthovoltage and megavoltage.
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Orthovoltage tissue compensators can be position on the skin. In megavoltage the bolus must be placed at a distance from the skin and must be adjusted for beam divergence, linear attenuation coefficients and decreased scatter at various depths. It must be of reduced thickness compared to the deficit it is replacing or it will overcompensate
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Why are tissue compensators not placed on the skin? How far away are they placed?
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Loses skin sparing advantage. 20cm.
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What is the tissue lateral effect?
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As patient thickness increases or as beam energy decreases the dose at the periphery will increase relative to the midpoint.
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How does integral dose change with photon energy?
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As photon energy increases integral dose generally decreases.
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What are the advantages of a fixed SSD over isocentric.
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Small dosimetric advantages with SSD versus a much simpler technique with isocentric setup.
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What are potential disadvantages with using multiple field techniques? Advantages?
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Disadvantages
1. Reporducibility 2. Set up accuracy 3. Certain angles may be prohibitive Advantages 1. Spare normal tissue damage |
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What is rotational therapy?
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Beam moves around the patient, or the patient is rotated around the beam.
Best for small, deep seated tumours, if the the tumour does not extend more than half way fromthe centre of the controur tumour. Not use ful for large tumours, the external surface is markedly different from a cylinder nad the tumour is too far from the centre. Offers little advantage over multifield isocentric techniques. |
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Define TAR.
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Tissue Air Ratio is the ratio of the dose at a point Dq in a phantom to the same point in free space Dfs
TAR(d) = Dq/Dfs |
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How is TAR used to calculate dose?
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Originally used for rotation treatment. Now is used for stationary isocentric techniques and irregular fields.
Independent of SSD (ratio of doses at two identical points - i.e. no distance factor). TAR varies much like PDD. Increases to Dmax and then decreases exponentially. Increases with increasing field size. |
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What is the back scatter factor? Peak scatter factor?
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Same thing. TAR at Dmax.
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Define scatter air ratio.
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Scatter air ratio is the dose at a point Q from scatter in a phantom to the dose from scatter at the same point in free space.
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How is scatter air ratio used clincially?
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To calculate dose as a result of primary and scattered radiation, which is particularly useful in irregular fields.
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Briefly describe how Clarkson's method for dose calculation in irregular fields is used.
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Dose in irregular fields in calculated by divided area into 10 degree segements. Using a SAR table the dose from scatter at a point from each of the 36 segments can be calculated.
The total TAR is TAR(0) + SAR The PDD can be calculated from the TAR via a conversion formula. |
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Define TPR.
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Tissue phantom ratio is the ratio of a dose at a given point in the phantom to the dose at the same point at a fixed reference depth, usually 5cm.
TPR(d) = Dd/Dr |
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Define TMR.
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TMR is a special case of TPR where the reference depth is Dmax.
i.e. TMR is the ratio of a dose at a given point in a phantom to the dose at the same point at Dmax. |
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Define Output factor.
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Also known as Relative Dose Factor, or Collimator Scatter Factor, Sc.
It is the ratio of dose output in air for a given field to that of a reference field (10x10cm) NB The RDF is actually measured in a phantom. |
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Define OAR.
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Is the ratio of dose at an off axis point to the dose on the central beam axis at the same depth in the phantom.
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What is the clinical use of PDD and TMR?
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TMR is the measurement of choice for isocentric techniques. PDD is the measurement of choice for SSD calculations.
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What is a Bragg-Gray cavity?
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When a gas filled cavity is placed in a medium and the cavity is sufficiently small that its introduction does not alter the number of electrons that would exist in the medium without the cavity.
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Draw a free air chamber.
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Describe the use of a free air ionisation chamber.
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1. Radiation from point source passes through diaphragm.
2. Radiation continues through chamber to collecting region which lies between collecting plates, which are protected by guard rings. 3. Ionisation events in the air as a result of photon events result in ions which under the influence of an applied voltage are attracted to the opposing plates. 4. The charge is measured via a electrometer. 5. Stipualtions: - the electron range in air must be less than: a. the distance between the plates b. from the diaphragm to the collecting region |
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What are the disadvantages of a free air ionisation chamber?
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Unsuitable for high energy beams, as distance between plates becomes prohibitely large, which also causes:
- increase in recombination - large size |
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What corrections are required for a free air ionisation chamber?
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- temperature
- pressure - humidity - recombination - scattered photons |
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What is the general range of ionisation chambers? How can this be improved? To what range?
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400kV.
Pressured air can increase the useful range. 3MeV |
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Draw a farmer chamber.
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Describe an farmer thimbel chamber.
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1. Central collecting cathode held at a high voltage bias
2. Air cavity ~0.6mL volume, surrounded by graphite (approx air equivalent, but compressed so that the thickness is at or slightly thicker than the range of electrons in the wall. The wall is coated with a conducting material. 3. The collecting electrode section outside of the cavity is surrounded by a electrical insulator PTCFE 4. Guard electrode: a. prevents leakage from high voltage elecrode b. defines ion collecting volume |
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What is the stem effect?
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The stem effect is seen when the stem of the thimble chamber is irradiated it results in measurable ionisation, which means that the ionisation measurement is dependent on the amount of stem irradiated.
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What is the significance of the stem effect.
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4% at 4MV
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Describe a condensor chamber.
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A condensor chamber is a thimble ionisation chamber.
1. Air thimble with outer wall of Zeff of air and thickness required to achieve > range of electrons. 2. Inner layer of outer wall electrically insulation. 3. Inner electrode is connected to a inner layer of conducting material surrounded by a insulator, which forms a condensor, capable of storing charge. 4. Chamber is charged to ~400V. 5. When chamber is exposed, electrons formed in the wall flow to the inner electrode and positive ions to the outer electrode, which results in a decrease of charge, which is proportional to the dose received. |
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What is an extrapolation chamber useful for?
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Surface dose.
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Draw a extrapolation chamber.
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How does a diode work?
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1. Silicone is doped with impurities to form n and p type silicone
a. p is doped with group III element (electron receptor) b. n is doped with group V element (electron donor) 2. A depletion region forms at the boundary from the motion of holes and electrons to the two sides, this reaches equilibrium and sets up an intrinsic electric field 3. When irradiated electron- hole pairs are form and flow out of the depletion zone (and from within the diffusion distance of the depletion zone) which results in a current which is measurable and proportional to the dose absorbed |
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What are the two types of semiconductor dosimeters?
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1. Silicone diode
2. MOSFET (Metal oxide semiconductor field effect transistor) |
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What are the advantages and disadvantages of diodes?
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Advantages
1. Good spatial resolution 2. Far more sensitive than ionisation chambers (due to less dose required to cause ion pair and relative density of silicon to air) Disadvantages 1. Energy dependent (and as such cannot be used for absolute dosimetry) 2. Temperature Dependence 3. Angular dependence 4. Radiation induces damage (requires calibration) |
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What are the clinical uses of diodes?
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1. Relative photon beam dosimetry
2. Electron beam dosimetry 3. In vivo measurements (require calibration) particularly useful in high dose gradients |
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What does MOSFET stand for?
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Metal Oxide Semiconductor Field Effect Transistor
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Describe the operation of a MOSFET.
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Metal Oxide Semiconductor Field Effect Transistor.
Small silicon transistor. Based on measurement of a threshold voltage. Must be attached to a bias voltage. Radiation penetrating the oxide cause a charge that is permanent trapped until reading. (there is some drift) |
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What are the advantages and disadvantages of MOSFETS?
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Advantages
1. Spatial resolution 2. Response covers full range of electrons and photons 3. Small anistropy Disadvantages 1. Temperature dependence 2. Radiation damage/limited lifespan 3. Drift 4. Must be attached to voltage bias |
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What are the uses of MOSFET?
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In vivo and phantom measurements
i.e. TBI, IMRT, Brachy, SRS |
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What is thermoluminescence?
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TL is the phenomenon whereby energy is trapped within a crystal lattice and can be released in the forms of photon (light) energy after heating.
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Describe theory of TL.
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1. crystal lattice has "allowed" and "forbidden" energy levels
2. Impurities create energy traps 3. Radiation causes electron and postive to hold to move from valence band to conduction band, where they either recombine and return to valence level or drop into metastable energy "trap". 4. Heating results in release of energy as visible light which is received and amplfied by a photomultiplier tube into electrical energy. 5. Known glow curves for individual crystals are known, and they may have various peaks corresponding to different traps. The measurement of the this glow curve results in calculation of absorbed dose. |
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What is annealing?
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Annealing is the process of removing residual trapped energy in a TL crystal. for LiF crystal heat for 1 hr at 400C and then 24hrs at 80C.
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How precise are TLD's?
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~3%
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What is the most common type of TLD?
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LiF (Zeff = 8.2)
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Clinical uses of TLDs?
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1. Staff monitoring
2. In Vivo dosimetry (very small, good orientation) |
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Describe how radiographic film can be used as dosimeter.
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1. Silver based emulsion
2. Radiation causes a chemical change in the silver (latent image) 3. Film is develop which causes a chemical change to metallic silver (if exposed by radiation) 4. Film is developed and the non exposed silver is washed away 5. The degree of blackening is proportional to the dose received in that region. 6. A densitometer can be used to assess the degree of blackening (optical density) which, after correcting for the base fog (degree of blackening of unexposed film) can be measured against a known dose response curve (sensitometric cruve) |
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What are the advantages and disadvantages of radiographic film as a dosimeter?
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Advantages
- cheap, quick - no directional dependence Disadvantage - can't be used for absolute dosimetry - light sensitive - There are small low energy photons from scatter which may be affect the dose as Silver has a relatively high Z - processing costs and workplace safety issues, requires darkroom |
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Describe radiochromic film.
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A blue leucodye is bonded or sandwhich between a ester base.
On exposure to radiation the leucodye polymerises causes a permanent change chemically and physically (changes to blue) The change in blue can be read by a spectrophotometer or a densitometric camcer |
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What are the advantages and disadvantes of radiochromic film?
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Advantages
- good spatial resolution - Z eff close to soft tissue (6-6.5) - low spectral sensitivity - good dynamic range - no processing Disadvantages - sensitive to UV light |
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What are uses for radiographic and radiochromic film?
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Radiographic
- light/field coincidences - collimoatr axis checks Radiochromic - brachy - IMRT - SRS fields |
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Describe a Fricke dosimeter.
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Radiation dose to a chemical medium can cause changes into that medium, which if it can be quantified can give a assessment of the dose received.
Fricke dosimeter contains ferrous sulfate, which when irradiated causes Fe2+ to be oxidised to Fe3+. The concentration of Fe3+ can be measured by a spectrophotometer. Using a G value the concentration can be related to the dose received. |
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How does a GM counter work?
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Cylindrical cathode with a fine wire stretched through centre.
Special mix of gases used to 100mmHg. A very large voltage is applied across the tube (much higher than an ionisation chamber) Radiation traversing the tube that causes an ionisation event results in an avalanche of secondary electrons due to the high voltage. Current flows betweent the electrodes, which is usually attached to audio output and is heard as an audible click. GM tube is very sensitive and can detect a single photon, but is not a dose measuring device. Suffers from signficant dead times which would seriously underestimate dose. |
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How does a scintillator work for dosimetry?
|
Basically:
- crystal traps energy - energy released as light and coupled to PMT which converts light to electrical signal Used in brachy. |
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What are uses for radiographic and radiochromic film?
|
Radiographic
- light/field coincidences - collimoatr axis checks Radiochromic - brachy - IMRT - SRS fields |
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Describe a Fricke dosimeter.
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Radiation dose to a chemical medium can cause changes into that medium, which if it can be quantified can give a assessment of the dose received.
Fricke dosimeter contains ferrous sulfate, which when irradiated causes Fe2+ to be oxidised to Fe3+. The concentration of Fe3+ can be measured by a spectrophotometer. Using a G value the concentration can be related to the dose received. |
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How does a GM counter work?
|
Cylindrical cathode with a fine wire stretched through centre.
Special mix of gases used to 100mmHg. A very large voltage is applied across the tube (much higher than an ionisation chamber) Radiation traversing the tube that causes an ionisation event results in an avalanche of secondary electrons due to the high voltage. Current flows betweent the electrodes, which is usually attached to audio output and is heard as an audible click. GM tube is very sensitive and can detect a single photon, but is not a dose measuring device. Suffers from signficant dead times which would seriously underestimate dose. |
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How does a scintillator work for dosimetry?
|
Basically:
- crystal traps energy - energy released as light and coupled to PMT which converts light to electrical signal Used in brachy. |
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|
What are the three types of radiation that must be accounted for in considering structural shielding of a bunker?
|
Primary radiation from primary beam
Secondary radiation from scatter Leakage radiation from treatment head |
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|
What 4 factors are considered in the development of a barrier for radiation?
|
Workload (weekly dose at 1m from the source)
Use factor (fraction of total time for which radiation is directed at that barrier) Occupancy factor (relative time for which that area of interest is occupied by an individual) Distance (distance from source, ISQ) |
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Which is more important primary or secondary barriers?
|
Primary barrier is much thicker than secondary barrier and if the primary barrier is included then this will adequately meet the requiredments of the secondary barrier.
If only secondary radiation is considered then the thickness for leakage and scattered radiation are considered separately, if the thicknesses differ by at least three HVL's the thicker willb e adequate, if the difference is less than three than one extra HVL should be added to obtain the required secondary barrier |
