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61 Cards in this Set

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Define and calculate the 1st and 2nd half-value layers.
You can find the HVL but you need to know what μ is HVL = .693 / μ
Usually can be found on a chart or got o the 50% line. This is your first HVL. To find the second HVL got to the 25% line. Take the 2nd HVL and subtract if from the 1st HVL. The answer is your 2nd HVL
Calculate the homogeneity coefficient (identify homogeneous versus heterogeneous beams).
Homogeneity Coeff = first HVL / second HVL
(FYI, the closer it is to 1, the more homogenous it is)
Homogenous beams – beams made up of a single energy
Heterogeneous beams – beam consisting of many different energies
Know what combination filters contain and the proper order for their arrangement.
Thoraeus filter- (toward machine) Tin, Copper, Aluminum (closest to patient)
Highest atomic # closest to x-ray target and lowest closest to patient (al)
Know the difference between voltage divider and sphere-gap method of determining kvp.
Voltage divider- resistance tower is placed across the high-tension leads
Sphere-Gap Method- Each high-voltage lead is connected to a polished metallic sphere by a cable adapter, distance between the two spheres is reduced until an electric spark passes between them, by knowing the critical distance, the peak voltage can be calculated
Know the three types of indirect measurement of KVp
Fluorescence Method
Attenuation Method
Penetrameter
Using materials of several different K absorption edges, can calibrate the kVp dial of the machine
Fluorescence Method
Comparing transmission through two materials with x-ray absorptions that change differently with photon energy
Penetrameter
Based on: the slope of the transmission curve of an x-ray beam at high filtration depends on the peak kilovoltage
Attenuation Method
Define the effective energy of an x-ray beam.
Energy of photons in a monoenergetic beam which is attenuated at the same rate as the radiation in question, determined by finding the energy of monoenergetic photons which have the same u as the given beam
Attenuation curve for a given material is characterized by __________
the linear attenuation coefficient (u).
the effective energy of a heterogeneous beam varies with the _________
absorber thickness
What is the practical method of determining the megavoltage beam energy.
measuring % depth dose distribution, tissue-air ratios, or tissue-maximum ratios and comparing them with published data
This applies only to x and gamma radiations, measure of ionization in air only, cannot be used for photon energies above 3 MeV
Exposure
TRhis is the quantity of radiation for all types of ionizing radiation, Measure of the biologically significant effects produced by ionizing radiation
Absorbed Dose-
formula for %DD
Mean energy imparted by ionizing radiation (dE) divided by material of mass(dm)
Kenetic Energy Released in a Medium
Kerma
Explain the relationship between exposure and kerma.
Exposure is the ionization equivalent of the collision kerma in air
Explain the relationship between absorbed dose and kerma.
When a broad beam of photons enters a medium, kerma is maximum at the surface and decreases with depth
Used to calculate dose directly from ion chamber measurements in a medium
Bragg-Gray cavity theory
This is the energy loss by electrons per unit path length of a material
stopping power
Describe the materials that phantoms are composed of.
Water, polystyrene, or acrylic phantoms
Explain three reasons why LiF is the most commonly used TLD in radiation therapy.
LiF has an effective atomic number 8.2 compared with 7.4 for soft tissue. Under electronic equilibrium conditions, the ratio of absorbed dose in the 2 media will be the same as the ratio of their mass energy absorption coefficients. Also may be able to use the Bragg-Gray relationship if the dimensions of the dosimeter are smaller than the ranges of the electrons crossing the dosimeter.
Describe the basic principle of Bragg-Gray cavity theory and how it is implemented in the TG-21 protocol.
Bragg-Gray cavity theory may be used without restrictions to calculate dose directly from ion chamber measurements in a medium.
The ionization produced in a gas-filled cavity placed in a medium is related to the energy absorbed in the surrounding medium.
The Bragg-Gray cavity theory is used in TG-21 for absorbed dose determination to a medium using a heterogenous chamber, that is, a chamber with wall material different from the surrounding medium.
Grenz Ray unit: what is energy
less than 20 kV
Kilovoltage unit: what is energy
up to 300kVp
Contact therapy: what is energy and filtration
40-50 kV with a filter of .5-1 mm AL
Superficial Therapy: energy and filtraiton
50-150 kV usually 1-6 mm Al filters
Orthovoltage Therapy: energy and filtration
150-500 kV, 1-4 mmCu
Supervoltage therapy: energy and filtration
500-100kV mm of aluminum
Megavoltage: energy
1MV or greater
Van de Graaf: energy
2MV-10MV
Proton Beams- energy
150-250MeV
Dmax of 4 MV
1cm
damx of 6 mV
1.5 cm
Dmax of 8 MV
2 cm
Dmax of 10 MV
2.5 cm
Dmax of 15 MV
3 cm
Dmax of 18 MV
3.5 cm
Dmax of 24 MV
4 cm
what is the HVL for 4 MV
1.6 cm Pb
what is the HVL for 10 MV
1.7 cm Pb
Co-60 HVL=
1.1-1.2 cm
Orthovoltage HVL-
2.2mm Cu
Superficial HVL=
1-8 mm Al
Van de Graaff HVL=
7.45mm Pb and 12.5mm Cu
Betatron HVL=
14mm Pb
Explain three reasons why LiF is the most commonly used TLD in radiation therapy.
LiF has an effective atomic number 8.2 compared with 7.4 for soft tissue. Under electronic equilibrium conditions, the ratio of absorbed dose in the 2 media will be the same as the ratio of their mass energy absorption coefficients. Also may be able to use the Bragg-Gray relationship if the dimensions of the dosimeter are smaller than the ranges of the electrons crossing the dosimeter.
Know how to correct an ionization chamber’s reading for non-standard temperature and pressure, for timer error and for stem effect.
PT,P= (760/P)(273.2+T/295.2)
Given the TG-21 or TG-51 protocol worksheet explain how to calculate the corrected Gy/MU.
TG - 21 is older. They are both correction factors for the machine. You want the machine to run at 1cGy per MU.
Is a basic method of determining absorbed dose in a medium. It based on the principle that the energy absorbed in a medium from radiation appears ultimately as heat energy while a small amount may appear in the form of a chemical change. This results in a small increase in temperature of the absorbing medium, which, if measured accurately, can be related to the energy absorbed per unit mass or the absorbed dose
calorimetry
Name the types of chemical dosimetry to measure absorbed dose.
Ferrous sulfate or the Fricke dosimeter. The G value is the number of molecules produced per 100eV of energy absorbed and the ferric ion concentration is determined by spectrophotometry of the dosimeter solution.
List the types of solid state methods for the dosimetry of ionizing radiation.
Integrating type dosimeters (TLDs, radiophotoluminescent glasses, optical density-type dosimeters such as glass and film) and electrical conductivity dosimeters (semiconductor junction detectors, induced conductivity in insulating materials.) Most used are TLDs, diodes and film.
ionization chambers operate in region .....
region 2
proportional counter operate in region ......
region 3
GM counters operate in region ....
region 4
In which of the 5 region, the voltage is not great enough to prevent ion recombination, so many ions are not counted. No detectors operate in this region
region 1
In which of the 5 regions, the voltage is now great enough to collect all ions created in the chamber (hence the plateau). This region is called the saturation region. Cutie pies, Victoreen condenser re-meters, and other thimble chambers operate in this region. The currents produced are low, so amplification of the signal is usually necessary. A separate device (electrometer) measures the charge collected by the chamber.
region 2
In which of 5 regions, the voltage is now great enough to accelerate the primary ions produced to high velocities that they begin creating secondary ions on their own (which are also collected); this proceed is called gas amplication (gains of up to 1000 are possible). This region is called the particle’s passage are proportional to its deposited energy; it is thus possible to distinguish between particle types. Proportional counters operate in this region.
region 3
In hich of the 5 regions, the voltage is now great enough to cause every gas molecule to become ionized (due to primary and secondary ionization). An ionizing particle will produce current of constant size, regardless of its intitial enegery; thus, individual particles can be detected, but they cannot be distinguished one from another. This region is called Geiger region because Geiger counters operate in this region.
region 4
In which of the 5 regions, the voltage is so high that the gas breaks down, producing a continuous discharge. No detectors operate in this region.
region 5
What are 4 thingd that solid dry phantoms have
same effective atomic number, number of electrons per gram, mass density of water, and same electron density as water