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Rad Onc > X-ray production > Flashcards

X-ray production Flashcards

1
Q

What is the process by which electrons are emitted from the cathode?

A

Thermionic emission (heat)

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2
Q

What is the heel effect?

A

The energy of the photons at the cathod cathode end of the beam is higher

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3
Q

How does the energy a Brehmsstrahlung photon relate to that of the incident electron?

A

photon = electron energy

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4
Q

In what direction are Brehmsstrahlung x-rays emitted when the incident electron is <100keV?

A

all directions

> 100keV –> forward

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5
Q

What is the average x-ray energy in a photon beam?

A

~1/3 Emax

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6
Q

What determines the Emax of a photon beam?

A

KVP (voltage change between cathode and anode)

kVp = Emax

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7
Q

What is the energy range of of orthovoltage therapy?

A

150-500kV

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8
Q

Where does the max dose occur in orthovoltage therapy?

A

Skin surface and bone (because photoelectric effect)

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9
Q

How and where are electrons accelerated towards the target?

A

Accelerated in the waveguide by microwaves traveling down tube

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10
Q

Which waveguide design allows change in output energy?

A

Standing wave

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11
Q

What are the features of a cobalt beam compared to a linac beam in terms of energy and penumbra?

A

Lower energy

Larger penumbra

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12
Q

What is the half-life of 60-Co, and how much does its’ activity decline monthly?

A

5.26yrs

A declines ~ 1% monthly

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13
Q

How is 60-Co produced?

A

Bombardment of non-radioactive Co with neutrons

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14
Q

What are the decay products of 60-Co?

A

60-Ni + B- particle + two g-rays

B- particle = 0.32MeV
g-rays = 1.17 & 1.33 MeV

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15
Q

What type of decay does 60-Co undergo?

A

B-

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16
Q

What is the average energy of 60-Co g-rays?

A

1.25MeV

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17
Q

What is the dose rate in free space of 60-Co?

A

240 cGy/min at 80cm SSD

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18
Q

What is a delta ray?

A

Ejected electron with sufficient energy to produce a secondary ionization track

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19
Q

What is fluence? What is energy fluence?

A

Fluence = # photons entering an imaginary sphere of cross sectional area a
Energy fluence = sum of all photon energies that enter the sphere

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20
Q

What is intensity?

A

Intensity = energy fluence over time

where energy fluence = sum of all photon energies that enter the sphere

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21
Q

T/F: Coherent scattering of photons refers to a change in direction without energy loss.

A

TRUE: Energy and wavelength of incident photon are retained, only thing that changes is direction of travel

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22
Q

What is the energy of an electron ejected by the photo electric effect?

A

hv (of incident photon) - Eb (electron binding energy)

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23
Q

How does photon attenuation related to Z for

a) photoelectric effect
b) compton scatter

A

photoelectric effect - proportional to Z^3

compton scatter - independent of Z

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24
Q

What is the threshold energy for pair production?

A

1.022MeV

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25
Q

How does Z impact the probability of pair production?

A

Probability increases with increasing Z

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26
Q

What happens to the linear attenuation coefficient (u) as incident photon beam energy increases?

A

u decreases in photoelectric effect range as energy increases, then stabilizes in Compton range

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27
Q

What are do absorption edges and for which interaction do they occur?

A

Absorption edges are “jumps” on graph of photoelectric effect mass attenuation coefficient, correspond to electron binding energies of inner electron shells (L, K)

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28
Q

What characteristic of the absorbing material determines the mass attenuation coefficient for photoelectric effect? For Compton effect?

A

PE effect - Z^3

Compton - electron density

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29
Q

What is the approximate electron density of most elements/tissues? For which element does this differ?

A

Most tissues/ elements - ~3 x 10^23

Hydrogen - 6 x 10^23

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30
Q

Attenuation for what type of interaction depends on electron density of the absorbing material?

A

Compton

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31
Q

When does coherent scattering occur?

A

low photon energy (<10keV) and high Z material

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32
Q

At what energy range does the photoelectric effect predominate in soft tissue?

A

< 25keV

33
Q

At what energy range does the Compton effect predominate in soft tissue?

A

25 keV - 10 MeV

34
Q

Why does the Bragg peak occur?

A

energy deposition of charged particles increases with decreasing velocity

35
Q

What is exposure?

A

measure of ionization produced in air by photons

36
Q

Exposure is not useful above ___ MeV due to negation of charged particle equilibrium

A

3MeV

37
Q

What are the units for exposure

A

Coulomb/kg

1 Roentgen = 2.58x10^-4 C/kg air

38
Q

What is a free air ionization chamber used for?

A

Reference standard, measures exposure according to definition - only used in calibration

39
Q

What type of ionization chamber can be used to measure dose at phantom surface?

A

plane-parallel or extrapolation chamber

40
Q

What type of ionization chamber can be used to measure electron beams?

A

plane-parallel or extrapolation chamber

41
Q

What type of detector is used for personnel monitoring for radiation exposure?

A

TLDs (thermoluminescent dosimeters)

42
Q

What are diodes useful for measuring?

A

patient dosimetry
depth dose scans/beam profile
NOT useful for calibration

43
Q

What advantage do TLDs have over diodes and MOSFETs?

A

do not require wires

44
Q

What does MOSFET stand for, and what advantage do MOSFETs have over diodes?

A

Metal oxide semiconductor field effect transistor - can be used to measure low doses

45
Q

What is the effective energy of a photon beam produced from a linac?

A

The energy of photons in a mono energetic beam that would have the same HVL as the given beam

46
Q

What is Gy equal to?

A

1 Gy = 1 J/kg

47
Q

What is kerma?

A

kinetic energy released in the medium

ie the sum of all the initial kinetic energies of electrons liberated by photons in mass

48
Q

How does kerma relate to exposure?

A

exposure = Kcol in air

49
Q

Where is the maximum kerma?

A

Kerma max occurs at the surface and decreases with depth

50
Q

What is the conversion factor from Roentgen to cGy?

A

0.876

51
Q

What are the limits of calculating absorbed dose from exposure?

A

Cannot be used >3MeV
Cannot be used when electronic equilibrium does not exist
Cannot be applied to particle radiation

52
Q

How is beam quality for an electron beam described?

A

R50 - depth in water at which %dd is 50% for a broad beam

53
Q

Why is LiF used for thermoluminescent dosimetry (TLD)

A

has effective atomic # 8.2, very close to soft tissues (7/4)

54
Q

What is the reference dose for orthovoltage? for all other energies?

A

orthovoltage - Do at surface (0cm)

all others Do = Dmax

55
Q

What happens to the initial dose buildup region as beam energy increases?

A

Dose buildup region becomes more pronounced with increasing energy

56
Q

For energies <10MV, what happens to the dose in tissue surrounding bone, on the entrance and transmission side of the beam?

A

Entrance - slightly increased dose due to backscatter

Transmission - slightly shielded/attenuated

57
Q

To preserve skin sparing for MV beams, how far from the skin must a compensator be placed?

A

> 20cm

58
Q

What causes the “bulging” observed in the IDLs for an orthovoltage beam?

A

low energy beam –> greater lateral scatter

orthovoltage IDLs bulge outside the field

59
Q

What is the result of treating with one of a pair of parallel opposed fields per day, as opposed to both fields per day?

A

Edge effect - greater biological damage to SQ tissues

60
Q

What is the treated volume?

A

volume enclosed by minimum target IDL line

61
Q

What is the irradiated volume?

A

volume of tissue receiving >50% of prescribed dose

62
Q

What is the modal target dose?

A

Dose that occurs most frequently within the tumor

63
Q

What is a hot spot?

A

Area outside the target receiving a higher dose than specified target dose

64
Q

In compton interactions, the angle at which the scattered photon has the lowest energy is always ___?

A

180 deg (backscatter) photons have the lowest energy

65
Q

As a rule of thumb, what are the scattered photon energies at 180 and 90 deg for Compton interactions?

A

Backscatter (180) = 250keV

Sidescatter (90) = 500keV

66
Q

What is the minimum thickness of Cerrobend for a custom block (compared to lead)? What is the beam transmission through this?

A
  1. 21 x Pb thickness

3. 5% beam transmission

67
Q

How much of the primary beam is allowed transmitted through the x and y jaws?

A

1%

68
Q

What is the allowed primary beam transmission through the MLC leaves (ie interleaf)? Between the leaves (interleaf)?

A

<2% through

<3% between

69
Q

What is the ideal device to assess the surface dose?

A

Extrapolation chamber

70
Q

As field size increases, what happens to dose in the buildup region?

A

Depth dose in buildup region increases

71
Q

What is the rate of energy loss for electrons (>1MeV) in water?

A

2 MeV/cm

72
Q

What is the ideal device for electron dosimetry?

A

Plane parallel chambers

73
Q

Where is electron beam energy specified?

A

at body surface

74
Q

What happens to surface dose as electron energy increases?

A

Surface does increases

75
Q

How much x-ray contamination is found within a 6MeV electron beam? a 15 MeV beam?

A

6 MeV ~0.5%

15 MeV ~2%

76
Q

The thickness of Cerrobend required compared to Pb for blocking is..?

A

20% more Cerrobend

ie Pb x 1.21

77
Q

When is beam current highest (electron mode or photon mode)?

A

Photon mode

78
Q

What is the f-factor?

A

Roentgen-to-rad conversion factor; 0.876 in air

Rad Onc (6 decks)

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