X-ray production Flashcards
What is the process by which electrons are emitted from the cathode?
Thermionic emission (heat)
What is the heel effect?
The energy of the photons at the cathod cathode end of the beam is higher
How does the energy a Brehmsstrahlung photon relate to that of the incident electron?
photon = electron energy
In what direction are Brehmsstrahlung x-rays emitted when the incident electron is <100keV?
all directions
> 100keV –> forward
What is the average x-ray energy in a photon beam?
~1/3 Emax
What determines the Emax of a photon beam?
KVP (voltage change between cathode and anode)
kVp = Emax
What is the energy range of of orthovoltage therapy?
150-500kV
Where does the max dose occur in orthovoltage therapy?
Skin surface and bone (because photoelectric effect)
How and where are electrons accelerated towards the target?
Accelerated in the waveguide by microwaves traveling down tube
Which waveguide design allows change in output energy?
Standing wave
What are the features of a cobalt beam compared to a linac beam in terms of energy and penumbra?
Lower energy
Larger penumbra
What is the half-life of 60-Co, and how much does its’ activity decline monthly?
5.26yrs
A declines ~ 1% monthly
How is 60-Co produced?
Bombardment of non-radioactive Co with neutrons
What are the decay products of 60-Co?
60-Ni + B- particle + two g-rays
B- particle = 0.32MeV
g-rays = 1.17 & 1.33 MeV
What type of decay does 60-Co undergo?
B-
What is the average energy of 60-Co g-rays?
1.25MeV
What is the dose rate in free space of 60-Co?
240 cGy/min at 80cm SSD
What is a delta ray?
Ejected electron with sufficient energy to produce a secondary ionization track
What is fluence? What is energy fluence?
Fluence = # photons entering an imaginary sphere of cross sectional area a
Energy fluence = sum of all photon energies that enter the sphere
What is intensity?
Intensity = energy fluence over time
where energy fluence = sum of all photon energies that enter the sphere
T/F: Coherent scattering of photons refers to a change in direction without energy loss.
TRUE: Energy and wavelength of incident photon are retained, only thing that changes is direction of travel
What is the energy of an electron ejected by the photo electric effect?
hv (of incident photon) - Eb (electron binding energy)
How does photon attenuation related to Z for
a) photoelectric effect
b) compton scatter
photoelectric effect - proportional to Z^3
compton scatter - independent of Z
What is the threshold energy for pair production?
1.022MeV
How does Z impact the probability of pair production?
Probability increases with increasing Z
What happens to the linear attenuation coefficient (u) as incident photon beam energy increases?
u decreases in photoelectric effect range as energy increases, then stabilizes in Compton range
What are do absorption edges and for which interaction do they occur?
Absorption edges are “jumps” on graph of photoelectric effect mass attenuation coefficient, correspond to electron binding energies of inner electron shells (L, K)
What characteristic of the absorbing material determines the mass attenuation coefficient for photoelectric effect? For Compton effect?
PE effect - Z^3
Compton - electron density
What is the approximate electron density of most elements/tissues? For which element does this differ?
Most tissues/ elements - ~3 x 10^23
Hydrogen - 6 x 10^23
Attenuation for what type of interaction depends on electron density of the absorbing material?
Compton
When does coherent scattering occur?
low photon energy (<10keV) and high Z material
At what energy range does the photoelectric effect predominate in soft tissue?
< 25keV
At what energy range does the Compton effect predominate in soft tissue?
25 keV - 10 MeV
Why does the Bragg peak occur?
energy deposition of charged particles increases with decreasing velocity
What is exposure?
measure of ionization produced in air by photons
Exposure is not useful above ___ MeV due to negation of charged particle equilibrium
3MeV
What are the units for exposure
Coulomb/kg
1 Roentgen = 2.58x10^-4 C/kg air
What is a free air ionization chamber used for?
Reference standard, measures exposure according to definition - only used in calibration
What type of ionization chamber can be used to measure dose at phantom surface?
plane-parallel or extrapolation chamber
What type of ionization chamber can be used to measure electron beams?
plane-parallel or extrapolation chamber
What type of detector is used for personnel monitoring for radiation exposure?
TLDs (thermoluminescent dosimeters)
What are diodes useful for measuring?
patient dosimetry
depth dose scans/beam profile
NOT useful for calibration
What advantage do TLDs have over diodes and MOSFETs?
do not require wires
What does MOSFET stand for, and what advantage do MOSFETs have over diodes?
Metal oxide semiconductor field effect transistor - can be used to measure low doses
What is the effective energy of a photon beam produced from a linac?
The energy of photons in a mono energetic beam that would have the same HVL as the given beam
What is Gy equal to?
1 Gy = 1 J/kg
What is kerma?
kinetic energy released in the medium
ie the sum of all the initial kinetic energies of electrons liberated by photons in mass
How does kerma relate to exposure?
exposure = Kcol in air
Where is the maximum kerma?
Kerma max occurs at the surface and decreases with depth
What is the conversion factor from Roentgen to cGy?
0.876
What are the limits of calculating absorbed dose from exposure?
Cannot be used >3MeV
Cannot be used when electronic equilibrium does not exist
Cannot be applied to particle radiation
How is beam quality for an electron beam described?
R50 - depth in water at which %dd is 50% for a broad beam
Why is LiF used for thermoluminescent dosimetry (TLD)
has effective atomic # 8.2, very close to soft tissues (7/4)
What is the reference dose for orthovoltage? for all other energies?
orthovoltage - Do at surface (0cm)
all others Do = Dmax
What happens to the initial dose buildup region as beam energy increases?
Dose buildup region becomes more pronounced with increasing energy
For energies <10MV, what happens to the dose in tissue surrounding bone, on the entrance and transmission side of the beam?
Entrance - slightly increased dose due to backscatter
Transmission - slightly shielded/attenuated
To preserve skin sparing for MV beams, how far from the skin must a compensator be placed?
> 20cm
What causes the “bulging” observed in the IDLs for an orthovoltage beam?
low energy beam –> greater lateral scatter
orthovoltage IDLs bulge outside the field
What is the result of treating with one of a pair of parallel opposed fields per day, as opposed to both fields per day?
Edge effect - greater biological damage to SQ tissues
What is the treated volume?
volume enclosed by minimum target IDL line
What is the irradiated volume?
volume of tissue receiving >50% of prescribed dose
What is the modal target dose?
Dose that occurs most frequently within the tumor
What is a hot spot?
Area outside the target receiving a higher dose than specified target dose
In compton interactions, the angle at which the scattered photon has the lowest energy is always ___?
180 deg (backscatter) photons have the lowest energy
As a rule of thumb, what are the scattered photon energies at 180 and 90 deg for Compton interactions?
Backscatter (180) = 250keV
Sidescatter (90) = 500keV
What is the minimum thickness of Cerrobend for a custom block (compared to lead)? What is the beam transmission through this?
- 21 x Pb thickness
3. 5% beam transmission
How much of the primary beam is allowed transmitted through the x and y jaws?
1%
What is the allowed primary beam transmission through the MLC leaves (ie interleaf)? Between the leaves (interleaf)?
<2% through
<3% between
What is the ideal device to assess the surface dose?
Extrapolation chamber
As field size increases, what happens to dose in the buildup region?
Depth dose in buildup region increases
What is the rate of energy loss for electrons (>1MeV) in water?
2 MeV/cm
What is the ideal device for electron dosimetry?
Plane parallel chambers
Where is electron beam energy specified?
at body surface
What happens to surface dose as electron energy increases?
Surface does increases
How much x-ray contamination is found within a 6MeV electron beam? a 15 MeV beam?
6 MeV ~0.5%
15 MeV ~2%
The thickness of Cerrobend required compared to Pb for blocking is..?
20% more Cerrobend
ie Pb x 1.21
When is beam current highest (electron mode or photon mode)?
Photon mode
What is the f-factor?
Roentgen-to-rad conversion factor; 0.876 in air
