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230 Cards in this Set

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Rationale for electrons in radiation therapy.
1 Relatively uniform dose between surface and 80% isodose
2 Sharp fall off of dose with depth beyond 80% line - finite range in tissue
Electron Production
Target and flattening filters are moved out of the path of the electron beam in the LINAC.
Replaced by dual scattering foil system.
Electron beam initially pencil beam - very tight and nearly monoenergetic.
First foil spreads beam.
Second foil flattens it.
Primary Electron Interactions
More complex than photons.
Four categories
1 Inelastic collisions
A With atomic electrons
Creates ionization and excitation
B With nuclei
Bremsstrahlung production
2 Elastic collisions
A With atomic electrons
Multiple Coulomb Scattering
B With nuclei
Z effect on electron interactions
Low Z - primarily atomic electrons
A MCS
B Excitations and ionization
High Z - primarily with nuclei
A More chance of Bremsstrahlung

The incident electrons lose energy continually until the surrounding tissue absorbs all of their energy - unlike photons.
For the given energy, give
A Surface Dose
B 80% Depth (cm)
C Practical range (cm)
6 MeV: 78% / 1.8cm / 2.8cm
9 MeV: 81% / 2.8cm / 4.1cm
12 MeV: 86% / 3.9cm / 5.8cm
15 MeV: 89% / 5.0cm / 7.4cm
18 MeV: 91% / 6.2cm / 9.2cm
As energy increase,
A
B
C
D
E
As energy increases,
A Surface dose increases (70-90%)
B Therapeutic depth (R90 or R10) increases
C Dose falloff (R90 - R10) increases
D Practical range increases
E Bremsstrahlung dose increases
As field size DECREASES
A
B
C
D
As field size DECREASES
A Therapeutic depth decreases (isodose constriction) - most important effect
B Surface dose increases (very little)
C Depth of Dmax decreases (very little)
D Practical range stays constant
Effects more significant at higher energy.
What is isodose constriction?
High isodose curves tuck in, ie, 80% coverage at depth is less than the actual field size.
This effect is worse for high energies and small field sizes.
Calibration
Electron beams are calibrated to produce 1.00 cGy/MU in water at 100cm SSD at the dmax of each beam.
An output factor is measured separately for each cone.
For electrons,

MU=
For electrons,

MU = Dose / (Output Factor) (PDD)
Inverse Square for Electrons
Behave as if they originate from a pont source that DOES NOT corresponde to the actual source position.

Behavior as if they originate from a point closer to the patient.

This point is called the virtual source position.

Means that using simple inverse square will not work.

As long as SSD <= 103, use normal inverse square.

If SSD > 103, seek physics help.
Effects of Oblique Incidence on Dose Distribution

Central Axis Effects (A-E)

Penumbra (A-B)
Effects of Oblique Incidence on Dose Distribution

Central Axis Effects
A Surface dose increases
B Depth of max dose (R100) decreases
C Therapeutic depth decreases
D Rp increases
E Effects worsen as angle increases

Penumbra
A Decreases for surface closer to source
B Increases for surface further from source
Effect of Irreg Surface on Electron Distribution

Protrusion (Nose): (A-B)

Depression (Ear canal): A-B
Effect of Irreg Surface on Electron Distribution

Protusion (nose)
A Decreases dose in shadow of protrusion
B Increased dose around the periphery

Depression (Ear canal)
A Increased dose in shadow of depression
B Decreased dose around periphery
Effect of Internal Heterogeneities on Dose Distribution
Hard bone: A-D
Air cavities: A-B
Lung
Effects of Internal Heterogeneities on Dose Distribution
Hard bone
A Backscatter - small increase in dose to upstream tissue (~4%)
B Small increase to dose in bone (~7%)
C Therapeutic isodose (80-90%) moves toward surface - most significant effect
D Hot and cold spots have small effect (~20%)
Air cavities
A Dose falloff region penetrates deeper
B Hot/cold spots can become significant (~20%)
Lung - penetration in lung can be 3-4 times that of unit density tissue
Moral of the story

When using electron beams,

Three steps
Moral of the story

the patient needs to look as much like a flat water phantom as possible.

1 Rotate gantry to make beam incidence flat
2 Use bolus around protrusions and in depressions
3 Avoid tissue heterogeneities
Bolus: Use (1-3)
Bolus

Use

1 Increase surface dose for low energy electron beams or in electron arc therapy

2 Homogenize dose distribution at irregular patient surface (eg, nose)

3 Variable depth of target volume (eg, chest wall, parotid)
Bolus: Basic Rules (1-6)
Bolus

1 Make patient as much like a bucket of water as possible

2 Use tissue equivalent material

3 Place directly on skin surface

4 No sharp edges in field

5 Extend edges outside of radiation field

6 Verify
A Dose calculation using bolus & CT
B In-vivo dosimetry
Field abutment: Criteria (1-3)
Field abutment: Criteria

1 Broad and matched penumbra

2 Overlap at 50% dose ratios (usually coincides with light field)

3 Place virtual source in same position for both fields
Electrons:

Rate of energy loss
Greater for low Z than high Z

Between 1-40 MeV, rate of energy loss is approx 2MeV/cm

Below 1 MeV, the rate of electron energy loss increases as the particle's energy decreases.
Electron scattering
Electrons scatter due to Columbic interactions with (mostly) nuclei.

The spatial and directional distribution of electrons is approx Gaussian.

The mass angular scattering power is ~ Z2 and 1/E2.
Energy of an electron beam
Beam energy E0=C4*R50 C4=2.33MeV/cm
Rp=practical range an electron can travel.
Graph does not reach zero due to x-ray contamination after x-ray production secondary to collisions with foil, sidewalls
Chars of %dd curve

Rules of thumb
Rp (cm)
R80 (cm)
R90 (cm)
Rp (cm) = 1/2 E (MeV)

R80 (cm) = 1/3 E (MeV)

R90 (cm) = 1/4 E (MeV)
Electron energy at depth
Ez = E0 (1 - z/Rp)

where z=depth
Electron dose components
80% directly for electrons
20% from other sources
- Applicator
- X-ray contamination (photons)
- Secondary (eg, delta rays)
Dose Falloff
Slower dose falloff with higher energies.

Using higher E defeats the purpose: increased dose with gradual falloff --> why not use photons?

It isn't that more electrons are absorbed superficially; instead, more electrons are required to get close to a point. As a result more energy is absorbed superficially.

"Higher surface dose for higher energy electron beams."
Isodose lines
Higher isodose lines pull in.

Lower isodose lines bulge out.

Scattering angle increases as farther from depth

Scattering power increases as energy decreases 1/e2.
Flatness
Dose at off-axis d/Dose at central axis * 100%

Flatness is defined for central 80% width of beam:

F = [maxdose-mindose/(max+min)] * 100%
Electron beam flatness and symmetry requirements
Flatness within 5%

Symmetry within 1%
Rule of thumb:

Field size dependence
When field size is less than Rp, changes in OF and %dd are possible.
Treating with electrons at extended distance
Reduction in output at dmax

Broadening of penumbra

Slight change in %dd values
Reduction of output at dmax with electron tx at extended distance
Inverse square law - fewer electrons in central region with increasing distance

Loss of side scatter equilibrium --> electrons are "flaring out" in air and cannot get back to dmax

Characterization
- Air Gap Factor
- Effective Point Source "Effective SSD" (Khan)
- Just measure it!
Treating with electrons at extended distance: field size and energy impact
Small energy & small field size --> huge dose reduction at extended distances.

As energy increases for some cutout size, the effect is less.
Treating with electrons at extended distance: broadening of penumbra
Increased scatter in the air increases the angular spread (ie, blurring the direction of electron travel)
At extended distance, the electrons are further from their original vector --> more difficult to block a region if extended distance due to electrons variable directions at extended distance.
Why more penumbra bulge with lower energies with electrons?
The penumbra with (80-20%) at shallow depths increases with increasing distance from the collimators, esp at lower electron energies (<10 MeV).
More bulge with lower energies due to increased likelihood of scatter with low-energy electrons.
Remember when treating with electrons at extended distance.
1
2
3
1 Unique dosimetric chars --> verify with physicist that ext SSD is considered in calcs
2 Diffs increase with decreased energy, decreasing field size, and increasing distance.
3 Skin collimation can restore penumbra when treated at extended distances.
Remember when treating with electrons at extended distance.
4
5
6
4 Increase in penumbra may be beneficial for abutting radiation fields.
5 If cutouts not standard, must measure.
6 Apply for extended distances within 120cm.
Gray
Absorbed dose.
Becquerel
Activity.
Rem
Equivalent dose.
Sievert
Equivalent dose.
Curie
Activity.
Roentgen
Exposure.
Charge on particles:
1 Alpha particle
2 Neutron
3 Electron
4 Positron
5 Photon
Charge on particles
1 Alpha particle +2
2 Neutron 0
3 Electron -1
4 Positron +1
5 Photon 0
Isotope
Elements which have the same Z but different A
Isotone
Nuclides having the same number of neutrons in the nucleus.
Isomer
Nuclides having the same atomic and mass numbers but differing in energy states for a finite period of time, eg 99mTc and 99Tc.
Isobar
Nuclides having the same total number of protons plus neutrons but with different distribution, eg, argon-40 with 18 protons and 22 neutrons and potassium-40 with 19 protons and 21 neutrons; the product of beta-disintegration is an isobar of its parent.
Proton captures neutron to create a deuteron. Energy emitted?

proton 1.00727
neutron 1.00866
deuteron 2.01355
1 amu = 931.50 MeV
E=diff in total mass before and after reaction:

1.00727 (pro) + 1.00866 (neu) = 2.01593
2.01593 - 2.01355 (Deu) = 0.00238
0.00283 amu)(931.5 Mev/amu)=2.22 MeV
Electron binding energy in the outer shell is dependent on
1
2
3
4
Electron binding energy in the outer shell is dependent on
1 Atomic number
2 Total number of shells
3 Number of electrons in shells
4 Charge of nucleus
The maximum number of electrons in the outer shell of an atom is
8.

2n**2 , where n is the quantum number, is the total number of electrons in all shells.
Equation describing number of radioactive atoms left as a function of time
N(t)=N0*exp(-t/tau)
where tau is the average lifetime of a radioactive atom.
Effective half-life considering both physical and biologic clearance.
1/Teff=1/Tp+1/Tb

= Tp*Tb/(Tp+Tb)
Alpha decay formula.
Z --> Z-1.
Beta minus decay formula.
Z --> Z+1.
Beta plus decay formula.
Z --> Z-1.
Isomeric decay formula.
Z --> Z.
Medical radioisotope production methods.
1
2
3
4
5
Medical radioisotope production methods.
1 Separation from spent reactor fuel rods.
2 Bombaring with neutrons in a reactor.
3 Bombarding with protons in a cyclotron.
4 Elution of a metastable daughter from a parent.
5 bombarding samples placed inthe neutron flux of a reactor, eg, 60Co and 192Ir

Not bombarding with neutrons in a cyclotron because cyclotrons can accelerate only charged particles.
How are the following isotopes produced?
1 60Co
2 15Oxygen
3 226Ra
4 99mTc
5 137Cs
How are the following isotopes produced?
1 60Co - 50Co bombarded with neutrons = 60Co
2 15Oxygen - Cyclotron-produced radioisotopes are typically short-lived positron emitters.
3 226Ra - Part of radium decay series.
4 99mTc - 99Mo decays to 99mTc in a Tc generator. Transient equilibrium is usually established before elution of the Tc.
5 137Cs - fission product of 235U; a gamma emitter used for brachy.
When will radioactive equilibrium be achieved if a parent nuclei is sealed in a vial?
Radioactive equilibrium is defined as equality rates of daughter production and decay. They always reach equilibrium whether transient or secular.
How many hours does it take 99mTc (t1/2=6h) to reach equilibrium with its parent 99Mo (t1/2=67h)?
Transient equilibrium is established after 4 half-lives.
One microCurie in Bq?
37000 Bq.
Based on the original definition, 1 Ci is equal to the activity of 1 g of which isotope?
I-125

Radium activity is usually expressed in mg.
The exposure rate at 1 meter from a point source of 10 mCi of 137Cs?

for 137Cs is 3.3 R.cm2/mCi.h
Exposure rate = Exp Rate cons * Activity x 1/d2
= 3.3 R.cm2/mCi.h x 10 mCi x (1/100**2)cm**-2
= 3.3 mR/hr
X-ray characteristics
EM waves that travel at speed of light (3.0 X 10EE8 m/s)

eV - kinetic energy gained by an electron accelerated by one volt of electrical potential.

Transverse waves with electric and magnetic fields perpendicular to the direction of motion.
Ultrasound characteristics
Longitudinal waves with compressions and rarefactions in the direction of travel.

Mechanical waves
- Do not travel at the speed of light
- Do not have energy stored as EM fields.
Wavelength (lambda)
Distance between peaks in the intesity of EM waves

Varies according to a sine wave
Frequency (nu)
Number of wavelengths that pass a point in space in one second.

A wavelength crossing a point per second is a hertz (Hz).
wavelength x frequency =
c.

So if given either wavelength or frequency, can determine the other.
Relationship between freq and wavelength.
Inversely related.

Short-wavelength x-rays have high frequencies; long-wavelength x-rays have low frequencies.
Clinically useful x-rays
- Typical wavelengths
- Typical frequencies
- Typical energies
Clinically useful x-rays
- Wavelength: 0.08-1.24 angstroms (1 ang = 1xEE-8cm)
- Freq 2.4x10EE18-3.7xEE19 Hz
- Energies: 10-150 keV
Photon or quantum
A single x-ray.

The smallest packet of EM energy
Planck's law
Energy of an x-ray is directly proportional to its frequency: E = h v.

High freq x-rays have more energy than lower freq x-rays.
Formula for energy of x-rays
E (keV) = 12.4/wavelength

E (keV) = (4.13EE-18)v (hz)
keV
The energy gained by accelerating a single electron by a electrical potential difference of 1000 volts.
Characteristics of the most penetrating x-rays
Characteristics of the most penetrating x-rays
1 Higher energy
2 Shorter wavelengths
3 Higher frequencies
EM waves from shortest to longest wavelength
EM waves from shortest to longest wavelength
1 gamma rays
2 x-rays
3 ultraviolet waves
4 visible light
5 infrared waves
6 microwaves
7 radiowaves
EM waves from lowest to highest frequency
EM waves from lowest to highest frequency
1 radio waves
2 microwaves
3 infrared waves
4 visible light
5 ultraviolet waves
6 x-rays
7 gamma rays
All of the following are EM waves except
A Heat
B Radiation used in MRI
C Sunlight
D Sound
E FM radio signals
D Sound is a longitudinal pressure wave that travels at about 330 m/s; it is not an EM wave.
If the wavelength of an x-ray is reduced to half, its energy is
A Increased by 4
B Increased by 2
C Unchanged
D Decreased to 0.5
E Decreated by 0.25
B. E=12.4/wavelength --> 12.4/wavelength(0.5) = (2)(12.4)/wavelength
Energy is increased by 2.
If the wavelength of an x-ray is reduced by half, its speed is
A Increased by 4
B Increased by 2
C Unchanged
D Decreased to 0.5
E Decreased by 0.25
D. All EM waves travel at the speed of light; only the wavelength and freq change.
If the wavelength of an x-ray is reduced by half, its frequency is
A Increased by 4
B Increased by 2
C Unchanged
D Decreased to 0.5
E Decreased by 0.25
B.
c=(wavelength)(freq)
c=(wavelength/2)(freqx2)
The x-rays that penetrate best through patient tissue have the smallest
A Frequency
B Energy
C Speed
D Wavelength
E Quanta
D. Wavelength.

The most penetrating x-rays have high energy, short wavelengths, and high frequency.
The wavelength of a 60-keV x-ray is about
A 4.5 microns
B. 0.2 angstroms
C. 0.001 mm
D 1.25EE-8cm
E 6.3MB
B.
E = 12.4 / wavelength
60 keV = 12.4 / wavelength
wavelength = 12.4/60 ang
wavelength = 0.2 ang

1 ang = 10EE-8cm
As the energy of the x-rays is increased, the x-rays have
A Faster speed
B Lower freq
C Shorter wavelength
D Longer wavelength
E Same speed, wavelength, and freq
C.
E=hv so as energy increases, frequency must increase.
c=(wavelength)v so if v increases, wavelength must decrease.
For equal image quality, radiographs have lower radiation doses if the x-rays have
A Higher frequency
B Longer wavelengths
C Faster speeds
D More quanta
E Longer period
A.
Fewer high-energy x-rays can be used to produce a given quality image than lower energy x-rays. This reduces the patient's radiation dose. High-energy x-rays mean both short wavelengths and correspondingly high frequencies.
All of the following are transverse waves except
A Gamma rays
B Ultraviolet rays
C Microwaves
D X-rays
E Diathermy waves
E.
Diathermy waves use high-power ultrasound waves which are longitudinal waves with force (mechanical pressure) traveling in the same direction as the motion of the waves.
To make certain that the x-rays have dissipated after clnical radiography, one should wait at least ___ before entering the room.
A 50 nanoseconds
B 50 microseconds
C 50 femtoseconds
D 50 milliseconds
E 50 deciseconds
Teaching points:
1 No need to wait; x-rays dissipate nearly instantly
2 Travel at speed of light --> cover large distances quickly
Exposure rate
Exp rate constant * activity * (1/d)**2
The exposure rate at 1 meter from a point source of 10 mCi of 137Cs with exposure rate constant for 137Cs 3.3R.cm**2/mCi.hr?
C.
Exp rate = exp rate cnst * activity * (1/d)**2
= (3.3 Rcm**2/mCihr)(10 mCi) (1/100cm)**2
= 3.3EE-3R/hr=3.3mR/hr
The exposure rate constant for a radionuclide is 12.9 Rcm2/mCihr. How many HVLs of shielding are required to reduce the exposure rate from a 19.5 mCi source at 2m to les than 2 mR/hr?
B.
Exp rate=exp rate cnst * acty * (1/d)**2
= (12.9Rcm**2/mCihr)(19.5mCi)(1/200cm)**2
= 0.00628R/hr = 6.29mR/hr
1 HVL = 3.15mR/hr
2 HVL = 1.58mR/hr
Smaller vs larger filament in an x-ray tube.
Smaller filament
- Small dimensions
- Enables smaller geometric penumbra with less blurring

Larger filament
- Higher heat capacity
- Ability to support higher currents
- Last longer
Which type of x-ray circuit provides the best image quality?
A. Single-phase
B. Three-phase
C. Constant voltage.
D. A or B.
E. A, B, and C provide the same.
Which type of x-ray circuit provides the best image quality?
C.
The three-phasecircuit minimizes ripples in voltage.
The constant voltage circuit completely elimantes ripples
- Provides constant x-ray spectrum
- Yields best image quality
- Higher average beam energy
- Lowest dose to thepatient
Match the type of radiation with its description.
Betas Ionizing elementary particles
Heat radiation Non-ionizing elementary particles
Visible light Ionizing phtons.
X-rays Non-ionizing photons.
Ultrasound Other.
Match the type of radiation with its description.
Betas Ionizing elementary particles
Heat radiation Non-ionizing elementary particles
Visible light Ionizing phtons.
X-rays Ionizing photons.
Ultrasound Other.
Which of the following types of EM radiation can be blocked by a wire mesh?
A Visible light
B Ultraviolet light
C Infrared light
D Microwaves
E X-rays
Microwaves
Which of the following types of EM radiation has the highest penetration in tissue?
A Visible light
B Ultraviolet light
C Infrared light
D Microwaves
E X-rays
X-rays
For a 100 keV photon the dominant mechanism of attenuation in musle tissue would be
A Pair production
B Coherent scatter
C Photoelectric
D Compton interaction
E None of the above
Compton scatter is the dominant photon interaction with muscle tissue in the energy range of 25 keV to 25 MeV.
If the linear attenuation coefficient is 0.05cm**-1, the HVL is
A 0.0347 cm
B 0.05 cm
C 0.693 cm
D 1.386 cm
E 13.86 cm
E.

HVL=0.693/linear attenuation coefficient
The photoelectric mass attenuation coefficient is proportional to
A Z.E
B Z**2.E**2
C Z**3.E**3
D Z**3.E**-3
E Z**2.E**-2
D.

The probability increases as Z**3 and decreases approximately at 1/(E**3).
Regarding photoelectric interactions, all of the following are true except:
A K, L, and M characteristic x-rays may be emitted if the photon energy is great than the K shell binding energy, and L amd M shells are at least partially filled.
B The photoectron's energy is the energy of the incident photon less the binding energy of the emitted electrons.
C The probability is greatest when the photon energy is a little less than the electron binding energy.
D In tissue, most of the released energy is locally abosrbed.
C.
The probability is greatest when the photon energy is just greater than the electron binding energy.
The ratio of Compton interactions in one gram of hydrogen to one gram of carbon is approx
A 0.5:1
B 1:1
C 2:1
D Dependent on the photon energy.
E The ratio of the density of hydrogen to water.
C.
The number of Compton interactions depends on the number of electrons present. Most elements have the same number of electrons per gram except hydrogen. It has one electron per nucleon whereas all other have have one electron for every two nucleons. So hydrogen has twice as many electrons per gram was other atoms.
A 3MeV photon interacts by the pair production process. What is the combined initial kinetic energy of the positron and electron pair?
A 1.02 MeV
B 1.98 MeV
C 2.49 MeV
D 3.00 MeV
E 4.02 MeV
B.
In pair production, the creation of the electron-positron pair requires 1.02 MeV. The difference between this and the original photon energy is shared between the two particles.
The pair production mass coefficient is proportional to
A Z
B Z**2
C Z**3
D Z**-1
E But independent of Z.
A.
The mass attenuation coefficient for photons
1 Is the linear attenuation coefficient divded by the density.
2 Has units of cm**-1.
3 Varies with photon energy.
4 Represents the probability per unit mass that photons will be scattered but not absorbed.
The mass attenuation coefficient for photons is the linear attenuation coefficient divided by the density and varies with photon energy. The units are cm**2/g. Photon attenuation may be due to absorption, scatter, or both, depending on the type of interaction.
Which type of nuclear radiation has the shortest range in tissue? (Assume equal energy.)
A Gamma rays
B Betas
C Neutrinos
D Alphas
E Neutrons
D. Alphas.
When protons interact with soft tissue, all of the following are true except
A LET increases toward the end of the proton track
B Protons have a finite range
C Protons undergo exponential attenuation
D The proton track ends in a "Bragg peak"
C. Photons are attenuated exponentially.
Which nuclei provide the best protection against neutrons?
A 210Pb
B 238U
3 109Ag
D 12C
E 1H
Neurotrons can bounce from heavier nuclei without significantly reducing their energy. Hydrogen not only slows neutrons down by sharing their energy but also binds slow neutrons by forming deuterium. This is used in radiation protection by itroducing concrete and polyethylene, both of which contain hydrogen.
Which of the following is not true regarding neutrons
A the threshold for neutron emission as a product of photon interaction is about 8MeV
B Neutrons are directly ionizing
C Neutron interactions with matter can cause gamma ray emission
D Neutron interactions with matter can cause proton emission
B.
Neutrons, like photons, do not have electrical charge and therefore do not ionize directly.
An x-ray film has an optical density (OD) of 1.5. This mean that
A The transmitted intensity is reduced by a factor of 1.5.
B The film is completely black.
C The film attenuates light by a factor of 10**1.5.
D 85% of the light from a viewbox will be transmitted.
C.
Attenuation is equal to 10**OD where OD is optical density.
A radioactive sample is counted many times, yielding a mean of 1000 counts. The most probably distribution is that 68% of the measurements fall between
A 990 and 1010 counts
B 968 and 1032 counts
C 936 and 1064 counts
D 900 and 1100 counts
E 800 and 1200 counts
B.
68% of the measurements fall with +/- of the mean.
The standard deviation = N**1/2.
= (1000)**1/2=32 counts
Concerning the Poisson distribution, all of the following are true except
A It is an approximation to the binomial distribution for small sample sizes.
B. it describes rare and random events.
C Radioactive decay as a function of time fits the Poisson distribution.
D The standard deviation increases as the number of measurements increases.
E The percent standard deviation decreases as the number of measurements increases.
A.
The Poisson distribution is an approximation to the binomial distribution for large samples and rare events such as radioactie decay.
Radioactive decay depnds only on whether the atoms disintegrate or not.
For N measurements =N**1/2= but % =N**1/2/N.
In a chi-square test, looking for a statistically significant difference between two experimental results, claims of such a different with a p value of 0.01
A means there is unquestionably a difference between the two results.
B Allows the experimenter a wider latitude of error than would a p value of 0.05.
C Means there is a 99% chance that the claim is true.
D Means there is a 99% change that the claim is incorrect.
C.
The "p-value" represents the probability of error in accepting the conclusion of the statistical analysis; ie, there is a 1% chance that the two results are not different.
Identify the prefix
A 10**-12
B 10**-6
C 10**6
D 10**9
E 10**12
Identify the prefix
A 10**-12 : pico
B 10**-6 : micro
C 10**6 ; mega
D 10**9 : giga
E 10**12 : tera
The most significant source of "man-made" radiation dose to the US population as a whole is from
A High-altitude air travel
B Television receivers and other consumer products
C Fallout from nuclear weapons exploded in the atmosphere
D Diagnostic radiological exams
E Nuclear reactor effluents
D. Millions of diagnostic x-ray procedures are performed each year. The next most significant source of radiation dose to the population as a whole is from nuclear medicine exams.
Concerning radiation induction of thyroid cancer, which of the following is true?
A Increase in thyroid cancer frequency is greater in men than in women
B Both benign and malignant thyroid tumors have increased in freq after x-ray exposure
C There is strong evidence of greater susceptibility of the infant thyroid for carcinogenesis compared to adults.
D An increase in the incidence of thyroid cancer exists only above thyroid doses of about 20 Gy.
E Evidence from children given thymic irradiation shows that more malignant thyroid cancers than benign lesions were induced from radiation to the thyroid.
B.
There is a greater increase in thyroid cancer freq per unit of x-ray dose in women than in men; parallel natural occurrence.
Poor evidenced of increased susceptibility of infant thyroids to radiocarcinogenesis compared to adults.
Incident is reduced above 20 Gy likely due to cell killing dominance.
The currently accepted mdoel of radiation dose versus effect used by the regulatory agencies in determining dose standards is
A Linear quadratic.
B Exponential.
C Gaussian.
D Linear, threshold.
E Linear, no threshold.
E.
Although there are valid arguments for the other models, the simple linear, n-threshold model is currently used by the regulators.
Perinatal death is most likely to occur as a reslt of irradiation in humans occuring in the gestational period of
A Implantation of the embryo.
B Major organogenesis 21-40 days)
C Second trimester
D Just before birth (30-36 weeks)
B.
In early organogenesis the organ buds consist of a few cells, and the loss of some of these can result in a major defect which may not be apparent during gestation, but after birth is too severe to permit independent life.
In radiation protection the enbryo/fetus is considered more vulnerable to radiation than an adult for all of the following reasons except
A In a given volume, more embryonic cells are proliferating than adult cells.
B In a given volume, more embryonic cells are differentiating than adult cells.
C An embryo consists of fewer cells, making the loss of cells by radiation injury otentially more damaging.
D The higher oxygen tension of the embryo/fetus results in a higher oxygen enhancement ratio (OER).
D.
It is questionable whether the fetis is in a hyperoxygenated state. Even if it is, there is little evidence that hyperoxygenation increases radiation damage.
Which of the following is true about film badges?
A Can measure total dose but cannot distinguish between high- and low-energy x-rays.
B Can measure exposures of 2mR.
C Are insensitive to heat.
D. Are used to determine exposure by measuring the optical density of the film
E. None of the above is true.
D.
Film badges cannot measure exposures below 10mR.
Placing filters over parts of the film allows one to testimate the proportion of dose due to x-rays in different energy ranges.
Heat, eg, exposure to intense sunlight, can cause film blackening.
If one lead apron attenuates 95% of an x-ray beam, two aprons will transmit approx ____%.
A 10
B 5
C 2.5
D 1.25
E 0.25
E.
The lead apron attenuates 95% and transmits 5%. Two aprons will transmit 0.05**2 = 0.0025 or 0.25%.
When calculating room shielding, the use factor U refers to
A The weekly dose at isocenter
B The fraction of operating time that the area in question is occupied.
C The fraction of operating time that the beam is directed toward the barrier
D The fraction of the workweek the machine is in operation.
C.
This is hard to extimate exactly so standard fractions such as 1/2, 1/4 are generally used.
Match the appropriate instrument to the procedure

A Liquid scintillation counter.
B NaI well counter.
C Geiger-Muller (GM) counter.
D Thermoluminescent dosimeter (TLD).
E Ionization chamber survey meter.
Gamma ray sealed source wipe test: B A NaI well counter is an efficient device for masuring low=levels gammas.
Contamination survey of 99mTc: A GM counter has a fast response and the ability to detect low levels of gamma rays.
Radiation survey of a diagnostic x-ray installation: E An ionization chamber survey meter is capable to accurate x-ray dose rate measurements with minimal energy dependence.
Personnel monitoring: D The small size and relative energy independece of TLD make it useful as a personnel monitoring device.
"ALARA" stands for
A As long as reasonably allowable.
B As low as reasonsibly attainable.
C As low as reasonably achievable.
D As little as possible RadioActivity.
C.
alara is a basic tenet of radiation protection. Obviously, radiation levels could be reduced to negligible levels with huge amounts of shielding that would be prohibitively expensive and unwiedly. The ALARA concept seeks to strike a reasonable balance between safety and practicality.
Half value layer (HVL) =
HVL = 0.693 / LAC

LAC = linear attenuation coefficient
Mass attenuation coeffecient (MAC) =
MAC = LAC / d where d=density

units = cm**2/g
Standard deviation =
= N**0.5
Electron capture
Orbital electron is captured by the nucleus transforming a proton into a neutron.

0 A A
e + X --> Y + v + Q
-1 Z Z-1
Activity=

A=
Activity is the rate of decay of a radioactive material.

A = A0 e**-delta(t)

where
A=activity remaining at time t
A0=original activity (lambda N0)
where
N0=initial number of radioactive atoms
lambda = 0.693/T(1/2)
Units of activity

And their conversion
Curie (Ci) and SI unit Bequerel (Bq)

1 Ci = 3.7EE10 disintegrations/sec (DPS) = 3.7EE10 Bq

1 Bq = 1 dps = 2.70EE-11 Ci
Decay constant lambda =
Decay constant lambda =

0.693 / T(1/2)
The line of stability
Graph of number of neutrons to number of protons in the nucleus of stable atoms.
For elements with low atomic number (Z), the line of stability is located where there are equal numbers of neutrons and protons.
For elements with high Z, the lnie of stability is located where there are more neutrons than protons.
Above or below this line, the nuclei are unstable and decay with radioactive emissions.
Atoms with excess neutrons are created . . .
in a nuclear reactor where there are huge fluxes of neutrons to bombard the stable atoms.
Atoms with excessprotons are created . . .
in particle accelerators such as linear accelerators and cyclotrons which bombard stable nuclei with charged particles.
Z
Atomic number = number of proton in the nucleus
N
Neutron number = number of neutrons inthe nucleus
A
Mass number - sum of neutrons plus protons in the nucleus = Z+N
Isotope with examples
IsotoPe for proton - same Z, diff N & A

11 12 13
C, C, C
6 6 6
Isotone with examples
IsotoNe for neutron - same N, diff Z & A

12 13 14
C, N, O
6 7 8
Isobar with examples
IsobAr for atomic number - same A, diff Z & N

14 14 14
C, N, O
6 7 8
Isomer with examples
IsomEr - everything: same Z, N, & A

99m 99
Tc, Tc
43 43
Mass defect
The loss in mass that occurs when neutrons and protons are combined in the nucleus.
The lost mass is converted into energy to hold the nucleus together.
E=mc**2=energy gained from loss of mass.
E=0.511MeV for an electron and 931MeV for a neutron or proton.
This is equal to 1 amu.
Typically the nuclear binding energy is between 1 and 8 MeV per particle (nucleon) with an average of about 2-4 MeV per nucleon.
Fast moving electrons transfer energy to water and cellular molecules in discrete events whose average energy is
A 200 eV
B 20 eV
C 2 eV
D 60 eV
E 1000 eV
D The most frequent energy transfer event by fast moving electrons is called a spur and involves about 60 eV of energy transfer on average.
Multiple-damaged sites or clustered lesions describe which one of the following radiation-induced lesions.
A A complex of DNA breaks associated with nuclear membrane fragments.
B. A DS break formed by two independent electron events.
C. A group of several chemical alterations in a ~10 nm diameter volume of chromatin produced by blobs or trackends.
D. A DNA lesion that includes at least one protein-DNA crosslink.
E. A dominant DNA lesion produced selectively in S-phase.
Clustered lesions are thought to result from high-energy and local energy depositions of 300-800 eV.
Consider an object with an electron density different from water sitting inside a water phantom as shown below.

If th object's electron density is greater than water, the hot spots and the cold spots would be at which points?
Hot spots
- A due to backscatter
- B due to sidescatter
Cold spots
- C & D due to decreased transmission
A patient is being treated with 12 MeV electrons with 1 cm of bolus on the skin surface using a 10x10 cone with no cutout at 100 cm SSD to the bolus. Calculate the number of MUs to deliver 200 cGy at a depth of 3 cm from the skin surface.
MU = Dose/(%dd * Se * EDF * OF)
= 200 cGy / (80% * 1 * 1 * 1)
= 250 MU

Note depth is 3 cm but there is also a 1-cm bolus. At 4-cm depth, the rule thumb tells us the dose is 80% (R80).
A patient is being treated with 12 MeV electrons with 1 cm of bolus on the skin surface using a 15 x 15 cone with a 10 x 10 cutout (giving 1.03 cGy/MU) at 45 degrees olbiquity (OF=0.80) at 110 cm SSD to the bolus (EDF=0.75). Calculate the number of MUs to deliver 200 cGy at a depth of 4 cm from the surface of the bolus.
MU=Dose / (%dd * Se * EDF * OF)
= 200 cGy / (80% * 1.03 cGy/MU * 0.80 * 0.75)
= 405 MU
A patient is being treated with 9 MeV electrons with 1 cm of bolus on the skin surface. An internal lead shield is to be placed at a depth 3 cm from the skin surface.
What is the energy of the electon beam striking the lead shield?
Ed = Eo (1- d/Rp)

= 9 MeV (1 - 4cm/4.5cm)

= 1 MeV
A patient is being treated with 9 MeV electrons with 1 cm of bolus on the skin surface. An internal lead shield is to be placed at a depth 3 cm from the skin surface.
What is the thickness of lead required to absorb the electrons?
Pb(thickness) = (0.505 mm/MeV) (1 MeV)
= 0.505 mm Pb
In treating with electrons using a bolus (1 MeV), assume the electron backscatter factor is 1.5. This means the dose at the skin on the "upstream" surface of the lead is 50% above the prescribed dose.
How much wax would need to be wrapped around the lead to reduce the dose to only 5% above the precribed dose?
EBF = 1.5
RBI = 5%/50% = 0.1 --> 4 mm wax
Exposure
Ionization per unit mass of air produced by PHOTONS
Unit: Roentgen
SI unit: C/kg air
Absorbed dose
A measure of the biologically significant effects produced by ionizing radiation
Mean energy imparted by ionizing radiation to a mass
Applies to both charged and uncharged particles and all energies (differs from exposure)
Old unit: rad
- Radiation Absorbed Dose
- Absorption of 100 ergs of energy per gram of absorbing material
SI unit: Gray = 1 J/kg = 100 rad
KERMA
Kinetic Energy Released in the Medium

Unit: J/kg (same as dose)
SI unit: Gray
Radionuclide: Au-198

Half-life:

Energy:

HLV mm Pb:
Radionuclide: Au-198

Half-life: 2.7 d

Energy: 0.412 MeV

HLV mm Pb: 2.5
Radionuclide: Co-60

Half-life:

Energy:

HLV mm Pb:
Radionuclide: Co-60

Half-life: 5.26 yr

Energy: 1.17, 1.33

HLV mm Pb: 11
Radionuclide: Cs-137

Half-life:

Energy:

HLV mm Pb:
Radionuclide: Cs-137

Half-life: 30 yr

Energy: 0.662 MeV

HLV mm Pb: 5.5
Radionuclide: I-125

Half-life:

Energy:

HLV mm Pb:
Radionuclide: I-125

Half-life: 60 d

Energy: 0.028 MeV

HLV mm Pb: 0.025
Radionuclide: Pd-103

Half-life:

Energy:

HLV mm Pb:
Radionuclide: Pd-103

Half-life: 17 d

Energy: 0.021 MeV

HLV mm Pb: 0.008
Radionuclide: Ra-226

Half-life:

Energy:

HLV mm Pb:
Radionuclide: Ra-226

Half-life: 1622 yr

Energy: 0.047-2.45 (avg 0.83 MeV)
(per mg instead of mCi)

HLV mm Pb: 12
Radionuclide: Rn-222

Half-life:

Energy:

HLV mm Pb:
Radionuclide: Rn-222

Half-life: 3.83 d

Energy: 0.047-2.45 (avg 0.83 MeV)

HLV mm Pb: 12
Annual dose limits

Occupational EDE whole body

Occupational EDE lens

Occupational EDE skin, hands, feet

Occupational EDE pregnant
Annual dose limits

Occupational EDE whole body: 50 mSv (5 rem)/yr

Occupational EDE lens: 150 mSv (15rem)/yr

Occupational EDE skin, hands, feet: 500 mSv (50rem)/yr

Occupational EDE pregnant: 0.5 mSv (0.05 rem)/mo
General public EDE with
freq/continuous exposure

General public EDE with
infreq exposure
General public EDE with
freq/continuous exposure: 1 mSv (0.1 rem)/yr

General public EDE with
infreq exposure: 5 mSv (0.5 rem)/yr
1 rem = ? Sv
1 rem - 0.01 Sv
LD-50
Dose of any agent or material that causes mortality of 50% in experimental group.
Hematopoietic syndrome
Dose:
Tx:
Hematopoietic syndrome
Dose: 3-8 Gy
Tx: Abx, transfusions, BMT > 10 Gy
Some may be saved
GI syndrome
Dose:
Death:
GI syndrome
Dose: 5-12 Gy
Death: 3-10d
Cerebrovascular syndrome
Dose:
Death:
Cerebrovascular syndrome
Dose: ~100 Gy
Death: 1-3d
Backround radiation in SF

Dose from routine trip SF-NY flight
2-2.5 mSv per year

<0.06 mSv
Radiophysics: SI Units

Length
Mass
Time
Electric current
Thermodynamic temp
Amount of substance
Luminous intensity
Radiophysics: SI Units

Length - Meter
Mass - Kilogram
Time - Second
Electric current - Ampere
Thermodynamic temp - Kelvin
Amount of substance - Mole
Luminous intensity - Candela
Physics: Characteristic radiation
Physics: Characteristic radiation

The photon emitted during the transition of an electron between two shells with a specific energy
Physics: Auger electron
Physics: Auger electron

When a photon produced by a transition in inner shells interacts with an outer electron that is knocked out of the atom. The ejected electron is an Auger electron.
Physics: Activity of a radioactive sample
Physics: Activity of a radioactive sample

The rate of decay of radioactive atoms

1 Bq = 1 disintegration / sec (dps)
1 MBq = 10E**6 dps
1 Ci = 3.7 x 10**10 dps
Physics: Average life of a radioactive material
Physics: Average life of a radioactive material

Tav = 1/λ = T1/2 / 0.693 = 1.44 t1/2
Physics: Radioactive equilibrium
Physics: Radioactive equilibrium

Transient
- Occurs when t1/2 parent > t1/2 daughter
- After transient eq is reached
-- Daughter activity > parent
-- Daughter activity decreases with t1/2 = t1/2 of longer lived parent
- Used for production of technetium from parent 99Mo
Physics: Radioactive equilibrium
Physics: Radioactive equilibrium

Secular
- t1/2 parent >>> t1/2 daughter
- Occurs in a closed environment
- After secular eq is reached
-- Daughter activity = parent activity
-- Daughter activity decays with t1/2 of parent
-- 222Rn from 226Ra
-- 90Sr in secular eq with 90Y for intraophthalmic irradiation
Physics: Electromagnetic radiation types
Physics: Electromagnetic radiation types

Light waves
Radiowaves
Microwaves
Ultraviolet rays
X-rays
Gamma rays
Physics: Electromagnetic radiation by size
Physics: Electromagnetic radiation by size (shortest to longer wavelength)

X-rays
Gamma rays
Visible light (blue...red)
Microwaves
Radiowaves
Physics: EM equivalences
Physics: EM equivalences

E = hν = hc/λ

c = λν
Physics: X-ray production
Physics: X-ray production

Cathode is heated
Bleeds off electrons
Electrons accelerate to anode
Produce x-rays upon impinging the anode by bremmstrahlung

Characteristic x-rays produced also as bombarding electrons eject inner electron and outer electrons fall inward to fill shell
Physics: X-rays vs gamma rays

Similarities

Differences
Physics: X-rays vs gamma rays
Similarities
- Attenuated by matter
-- Photoelectric interaction
-- Pair production
-- Photonuclear disintegration
- Scattering
-- Compton interaction (incoherent)
-- Rayleigh scattering (coherent)
Differences
- X-rays
-- Polyenergetic
-- Determined by peak voltage and filtration
- Gamma rays - Monoenergetic
Physics: Linear attentuation coefficient μ
Physics: Linear attentuation coefficient μ

μ=0.693/HVL

Depends on density and atomic number of attenuating material as well as energy of photon beam
Physics: Photoelectric interaction
Physics: Photoelectric interaction- Incoming photon interacts with an inner shell electron
- Photon is completely absorbed
- Inner shell electron is ejected out of the atom
KE of ejected electron =
hν - Ebinding
Energy of photon must exceeding Ebinding for interaction to occur

Dominant mode for energies < 30 keV (x-rays < 100 kVp)

Proportional Z**3/E**3
Physics: Compton interaction
Physics: Compton interaction

- Incoming photon interacts with a loosely bound or free electron of the attenuator
- Incoming photon is scattered with reduced energy
- Electron recoils with KE equal to diff in energy between incoming and scattered photon
- Dominant for 30 keV to 30 MeV → RT
- Independent of atomic number
- Proportional to 1/E
Physics: Compton interaction

Higher energy vs lower energy
Physics: Compton interaction

Higher energy vs lower energy

- At MeV recoil electron gains most of the energy of the incoming photon
- At lower energies photon retains most of its energy; recoil electron gets on a fraction of the photon's energy
Physics: Compton scattering directions
Physics: Compton scattering directions

With high-energy photons, Compton-scattered photons travel mostly at small angles relative to the incoming photons → lateral or side scattering is relatively small compared to orthovoltage or lower energy photon beams
Physics: Pair production
Physics: Pair production

- Incoming photon is converted to an electron and a positron near the nuclear charge of an atom
- At least 1.022 MeV is required
- Proportional to E of photon and Z of attenuator
- Generally not significant for MeV beams used in RT
Physics: Triplet production
Physics: Triplet production

- Pair production near an orbital electron of an atom
- Two electrons and a positron come out of the atom
- 2.044 MeV is required
Physics: Color by increasing wavelength
Physics: Color by increasing wavelength

Violet 425
Blue 475
Green 535
Yellow 585
Orange 610
Red 700
Physics: EM spectrum by decreased frequency
Physics: EM spectrum by decreased frequency

Cosmic rays 10EE22
Gamma rays 10EE20
X-rays 10EE18
UV light 10EE16
Visible light 10EE14
Infrared light 100EE12
Radar 10EE10
Television and FM radio 10EE8
Shortwave radio 10EE7
AM radio 10EE6
Sound 10EE5-10EE2
Subsonic 10EE1-0
Physics: Luminance
Physics: Luminance
Physics: 2008-288
Which of the following combinations is most appropriate for a T1 MRI sequence?
TR---- TE----
A 3000ms 80ms
B 500ms 5ms
C 30ms 15ms
D 3ms 1000ms
Physics: 2008-288
Which of the following combinations is most appropriate for a T1 MRI sequence?
TR---- TE----
B 500ms 5ms
Physics: TR & TE for the following MR sequences

Proton density
T1
T2
Physics: TR & TE for the following MR sequences

Proton density TR 3000 ms / TE 15 ms
T1 TR 500ms / TE 15ms
T2 TR 3000 ms / TE 90 ms
Electrons: In-/Elastic Interactions
A With atomic electrons
B With nuclei
Electrons: Interactions
A With atomic electrons
Inelastic - Collisional energy loss
Elastic - Less prominent
B With nuclei
Inelastic - Bremsstrahlung (radiative energy loss)
Elastic - Multiple Coulomb scattering (direction change)
Electrons: Interaction
- Electron-nuclear interaction
- Electron-nuclear interactions
Electrons: Interaction
- Electron-nuclear interaction
--Excitation
--Ionization
- Electron-nuclear interactions
--Coulomb scattering
--Bremsstrahlung
Electrons: Excitation
Electrons: Excitation
- An atom or orbital electron absorbs and reradiates energy
- No atomic electron is ejected
Electrons: Ionization
Electrons: Ionization
- An orbital electron is ejected from an atom by gaining energy
- Creates an electron-ion pair
Electrons - Bremsstrahlung
Electrons - Bremsstrahlung
Yield
= radiative/collisional
= TZ/800
Probability proportional to Z2
Electrons: Rate of energy loss

If E>1 MeV. . .
Radiative loss increases with?

Rate of energy loss ~
Electrons: Rate of energy loss

If E>1 MeV, collisional loss is independent of energy

Radiative loss increases with energy

Rate of energy loss ~ 2MeV/cm
Electrons: Variable defs
D(s)
D(x)
R(n)
R(100)
R(p)
R(t)
Electrons: Variable defs
D(s)=surface dose (% max)
D(x)=Brem dose (% max)
R(n)=depth where relative dose is n% of max dose
- Can be proximal or distal relative to R(100)
- Typical: R(90), R(80), R(10)
R(100)=depth of max CAX dose
R(p)
- Practical range
- Depth where tangent to PDD curve at R(50) intercepts extrapolated brem tail
R(t) - Therapeutic depth
- Typically R(90) or R(80)
Electrons: Rules of thumb
-Units: [E]=?, [R]=?
-For therapeutic energies
--Stopping power is . . .
--S/p=
E(p,z)=
R(p)=
R(90)=
R(80)=
Electrons: Rules of thumb
-Units: [E]=MeV, [R]=cm
-For therapeutic energies
--Stopping power is constant
--S/p=2 MeVcm2/g
E(p,z)=
R(p)=E(p,0)/2
R(90)=E(p,0)/4
R(80)=E(p,0)/3
Electrons: Shielding
-Lead
-Cerrobend
Electrons: Shielding
-Lead: t(pb)[mm]=E(p,0)[MeV]/2 + 1mm
-Cerrobend: 1.2t(Pb)
Electrons: Chars of pencil beam
-
-
Electrons: Chars of pencil beam
- Finite range due to continuous energy loss
- Lateral spread due to multiple Coulomb scattering
Electrons: Depth Dose
Electrons: Depth Dose
-Surface dose increases with E
-Dmax and R90 decrease with field size
-Brem tail increases with E
-Scanning beam (no scattering foil) has lowest brem
Electrons: Beam profile specs
-Def ref plane
-Flatness
-Symmetry
Electrons: Beam profile specs
-Def ref plane
--Parallel to surface of phantom
--Perpendicular to CAX
--Depth of plane at distal R95
-Flatness
--For 10x10 or larger, consider 2cm within geometric field edge
--Variation in dose relative to CAX
---Should not exceed =/- 5%
---Ideally +/- 3% within ref plane
-Symmetry
--Cross beam dose profile in ref plane
---+/- 2%
---At any pair of points situated symmetrically with respect to CAX
Electrons: Beam profiles
-Beam edge maintains?
-Increased penumbra with
--?
--?
Increased CAX dose with?
Electrons: Beam profiles
-Beam edge maintains "tear drop" shape of pencil beam
-Increased penumbra with
--Depth
--SSD
Increased CAX dose with SSD
Electrons: Obliquity
Electrons: Obliquity
Theta=angle of incident electron beam relative to line normal to surface
As theta increases
-D(s) increases
-R(100) decreases (max dose closer to surface)
-Absolute max dose increases
-R(90) decreases
-Penetration depth of beam increases
In practice: keep theta < 40 degrees
Electrons: Air gap & obliquity
-Change in OF with air gap --With small air gap
-Virtual source
-Obliquity causes dmax and d90
Electrons: Air gap & obliquity
-Change in OF with air gap does not follow invsq law
-With small air gap, ISL can be used if a virtual source position is derived from measurements
-Virtual source is generally closer than the x-ray target
-Obliquity causes dmax and d90 to shift towards surface
Electrons: Surface irregs

Depression
Electrons: Surface irregs

Depression (ear canal)
-Tissue lateral to depression
--Scatters medially toward depression
--No lateral scatter from depression to balance it
-Results
--Hot spot just distal to depression
--Cold spots lateral to hot spot
Electrons: Surface irregs

Protrusion (nose)
-Scatters laterally out
-No tissue on either side of protrusion to laterally scatter dose back in
-Results
--Cold spot just distal to protrusion
--Hot spot lateral to cold spot
Electrons: Surface irregs

Protrusion (nose)
-Scatters laterally out
-No tissue on either side of protrusion to laterally scatter dose back in
-Results
--Cold spot just distal to protrusion
--Hot spot lateral to cold spot
Electrons: Abutted fields
-Considerable scatter from collimator edges makes matching a challenge
-Four possible methods
Electrons: Abutted fields

Four possible methods
1 Place the geometric edges of each beam in perfect alignment - difficult to achieve
2 Separate the fields - may increase cold spot
3 Use small poystyrene or aluminum wedges at the edge of one or both beams
4 Electron arc delivery
Electrons: Virtual source
Electrons: Virtual source

The intersection of phantom-surface electrons' momentum vectors backprojected toward the source
Electrons: Virtual SSD
Electrons: Virtual SSD
-Can be determined experimentally
-InvSq calc using SSDvir valid only for large field sizes
-For smaller field sizes, SSDvir underestimates decrease in dose due to loss of side-scatter equilibrium
Charged particles: Rest mass & charge
Charged particles

Rest mass (Mev) & charge
Alpha 3727/+2
Deuteron 1876/+1
Neutron 940/0
Proton 938/+1
Pion 140/+/-1
Muon 106/+1
Positron 0.511/+1
Electron 0.511/-1
Protons: Rate of energy loss
Protons: Rate of energy loss
Proportional to square of particle charge
Inversely proportional to square of its velocity
As particle slows down
-Rate of energy loss increases
-Absorbed dose to medium increases
Dose deposition
-Increases slowly with depth
-Then very sharply near end of range
Peak near end of range is Bragg peak
Heavy charged particles
Heavy charged particles

Stopping power S =

Bragg peak
- Concentrates dose inside target volume
- Electrons also have Bragg peak but smeared due to multiple Coulomb scattering
Protons
-Therapeutic range
-Production
-Advantage
Protons
-Therapeutic range: 150-250 MeV
-Production: Cyclotron or linac
-Advantage
--Narrow beam with mag field
--Bragg peak
Protons: Problem #1
Protons: Problem #1 Too narrow a peak
Special equipment used to combine protons of various energies to broaden Bragg peak to match the thickness of individual targets
Protons: Problem #2
Protons: Problem #2
Requires customized dose-shaping deivces to block proton beam outside a specified safety margin
Pions
-Mass
-Charge
-Production
Pions
-Mass: 139 MeV
-Charge: +1, -1, 0
-Production
--Cyclotron/linac -> proton beam
--Impinges graphite target -> +/-/0 pions produced
--Beam line with magnets collects negative pions
--Contamination: 4% electrons, 10% muons
Heavy charged particles
Heavy charged particles

Stopping power S =

Bragg peak
- Concentrates dose inside target volume
- Electrons also have Bragg peak but smeared due to multiple Coulomb scattering
Protons
-Therapeutic range
-Production
-Advantage
Protons
-Therapeutic range: 150-250 MeV
-Production: Cyclotron or linac
-Advantage
--Narrow beam with mag field
--Bragg peak
Protons: Problem #1
Protons: Problem #1 Too narrow a peak
Special equipment used to combine protons of various energies to broaden Bragg peak to match the thickness of individual targets
Protons: Problem #2
Protons: Problem #2
Requires customized dose-shaping deivces to block protom beam outside a specified safety margin
Pions
-Mass
-Charge
-Production
Pions
-Mass: 139 MeV
-Charge: +1, -1, 0
-Production
--Cyclotron/linac -> proton beam
--Impinges graphite target -> +/-/0 pions produced
--Beam line with magnets collects negative pions
--Contamination: 4% electrons, 10% muons
QA
-Def
-Reduce uncertainties
QA
-Def-Execute all planned and systematic actions to provide reasonable confidence level that patients will receive prescribed/intended treatments
-Reduce uncertainty in
--Positioning
--Target and critical structure delineation
--Planning
--Delivery
-Reduce uncertainties
QA: CT Performance Parameters
QA: CT Performance Parameters
Alignment of gantry lasers: Daily
Other laser QA: Monthly
Table vert/long: Monthly
Table index/position: Annually
Gantry tilt: Annually
Scan localization: Annually
Radiation profile width: Annually
Sensitivity provide width: Semiannually
Generator tests: After replacement
QA: CT Image Performance Eval
QA: CT Image Performance Eval
CT number
- Daily for water
- Monthly 4-5 diff materials
- Annually for ED phantom
Image noise: Daily
Field uniformity
- Daily x or y
- Monthly both
ED to CT number conv: Annually
Spatial resolution: Annually
Contrast resolution: Annually
QA: MLC tx delivery techniques
- Step-and-shoot technique
- Dynamic technique
QA: MLC tx delivery techniques
- Step-and-shoot technique
--Beam hold between shots
- Dynamic technique
--Beam on continuously
HDR Prostate selection criteria
1
2
3
4
5
HDR Prostate selection criteria
1 PSA > 10
2 Gleason 7-10
3 Bulky T2a or >=T2b
4 4 cores and/or bilobar dz
5 Neg met workup
Advantages of HDR v LDR brachy
1
2
3
4
5
6
7
8
Advantages of HDR v LDR brachy
1 Elimination of exposure to personnel
2 Treatment as outpatient
3 More stable positioning during treatment
4 Possibility of holding normal tissue away from applicator
5 Smaller applicators more comfortable
6 Tailor dose distribution
7 Immediate treatment after adjustment in OR
8 Better documentation
SRS floor stand system

Accuracy
Compared with GammaKnife
Positioning
QA
SRS floor stand system

Accuracy: 0.2 mm
Compared with GammaKnife: equiv
Positioning: Isocentric
QA: Annual
Radionuclide: 192Ir

Half-life:

Energy:

HLV mm Pb:
Radionuclide: 192Ir

Half-life: 74 days

Energy: 0.136-1.06 (avg 0.38 MeV)

HLV mm Pb: 2.5