X-ray production

Question 1

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

Answer 1

Thermionic emission (heat)

Question 2

What is the heel effect?

Answer 2

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

Question 3

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

Answer 3

photon = electron energy

Question 4

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

Answer 4

all directions

> 100keV –> forward

Question 5

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

Answer 5

~1/3 Emax

Question 6

What determines the Emax of a photon beam?

Answer 6

KVP (voltage change between cathode and anode)

kVp = Emax

Question 7

What is the energy range of of orthovoltage therapy?

Answer 7

150-500kV

Question 8

Where does the max dose occur in orthovoltage therapy?

Answer 8

Skin surface and bone (because photoelectric effect)

Question 9

How and where are electrons accelerated towards the target?

Answer 9

Accelerated in the waveguide by microwaves traveling down tube

Question 10

Which waveguide design allows change in output energy?

Answer 10

Standing wave

Question 11

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

Answer 11

Lower energy

Larger penumbra

Question 12

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

Answer 12

5.26yrs

A declines ~ 1% monthly

Question 13

How is 60-Co produced?

Answer 13

Bombardment of non-radioactive Co with neutrons

Question 14

What are the decay products of 60-Co?

Answer 14

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

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

Question 15

What type of decay does 60-Co undergo?

Answer 15

B-

Question 16

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

Answer 16

1.25MeV

Question 17

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

Answer 17

240 cGy/min at 80cm SSD

Question 18

What is a delta ray?

Answer 18

Ejected electron with sufficient energy to produce a secondary ionization track

Question 19

What is fluence? What is energy fluence?

Answer 19

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

Question 20

What is intensity?

Answer 20

Intensity = energy fluence over time

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

Question 21

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

Answer 21

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

Question 22

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

Answer 22

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

Question 23

How does photon attenuation related to Z for

a) photoelectric effect
b) compton scatter

Answer 23

photoelectric effect - proportional to Z^3

compton scatter - independent of Z

Question 24

What is the threshold energy for pair production?

Answer 24

1.022MeV

Question 25

How does Z impact the probability of pair production?

Answer 25

Probability increases with increasing Z

Question 26

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

Answer 26

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

Question 27

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

Answer 27

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

Question 28

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

Answer 28

PE effect - Z^3 | Compton - electron density

Question 29

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

Answer 29

Most tissues/ elements - ~3 x 10^23 | Hydrogen - 6 x 10^23

Question 30

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

Answer 30

Compton

Question 31

When does coherent scattering occur?

Answer 31

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

Question 32

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

Answer 32

< 25keV

Question 33

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

Answer 33

25 keV - 10 MeV

Question 34

Why does the Bragg peak occur?

Answer 34

energy deposition of charged particles increases with decreasing velocity

Question 35

What is exposure?

Answer 35

measure of ionization produced in air by photons

Question 36

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

Answer 36

3MeV

Question 37

What are the units for exposure

Answer 37

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

Question 38

What is a free air ionization chamber used for?

Answer 38

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

Question 39

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

Answer 39

plane-parallel or extrapolation chamber

Question 40

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

Answer 40

plane-parallel or extrapolation chamber

Question 41

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

Answer 41

TLDs (thermoluminescent dosimeters)

Question 42

What are diodes useful for measuring?

Answer 42

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

Question 43

What advantage do TLDs have over diodes and MOSFETs?

Answer 43

do not require wires

Question 44

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

Answer 44

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

Question 45

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

Answer 45

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

Question 46

What is Gy equal to?

Answer 46

1 Gy = 1 J/kg

Question 47

What is kerma?

Answer 47

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

Question 48

How does kerma relate to exposure?

Answer 48

exposure = Kcol in air

Question 49

Where is the maximum kerma?

Answer 49

Kerma max occurs at the surface and decreases with depth

Question 50

What is the conversion factor from Roentgen to cGy?

Answer 50

0.876

Question 51

What are the limits of calculating absorbed dose from exposure?

Answer 51

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

Question 52

How is beam quality for an electron beam described?

Answer 52

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

Question 53

Why is LiF used for thermoluminescent dosimetry (TLD)

Answer 53

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

Question 54

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

Answer 54

orthovoltage - Do at surface (0cm) | all others Do = Dmax

Question 55

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

Answer 55

Dose buildup region becomes more pronounced with increasing energy

Question 56

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

Answer 56

Entrance - slightly increased dose due to backscatter | Transmission - slightly shielded/attenuated

Question 57

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

Answer 57

>20cm

Question 58

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

Answer 58

low energy beam --> greater lateral scatter | orthovoltage IDLs bulge outside the field

Question 59

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

Answer 59

Edge effect - greater biological damage to SQ tissues

Question 60

What is the treated volume?

Answer 60

volume enclosed by minimum target IDL line

Question 61

What is the irradiated volume?

Answer 61

volume of tissue receiving >50% of prescribed dose

Question 62

What is the modal target dose?

Answer 62

Dose that occurs most frequently within the tumor

Question 63

What is a hot spot?

Answer 63

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

Question 64

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

Answer 64

180 deg (backscatter) photons have the lowest energy

Question 65

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

Answer 65

Backscatter (180) = 250keV | Sidescatter (90) = 500keV

Question 66

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

Answer 66

1. 21 x Pb thickness | 3. 5% beam transmission

Question 67

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

Answer 67

1%

Question 68

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

Answer 68

<2% through | <3% between

Question 69

What is the ideal device to assess the surface dose?

Answer 69

Extrapolation chamber

Question 70

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

Answer 70

Depth dose in buildup region increases

Question 71

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

Answer 71

2 MeV/cm

Question 72

What is the ideal device for electron dosimetry?

Answer 72

Plane parallel chambers

Question 73

Where is electron beam energy specified?

Answer 73

at body surface

Question 74

What happens to surface dose as electron energy increases?

Answer 74

Surface does increases

Question 75

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

Answer 75

6 MeV ~0.5% | 15 MeV ~2%

Question 76

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

Answer 76

20% more Cerrobend ie Pb x 1.21

Question 77

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

Answer 77

Photon mode

Question 78

What is the f-factor?

Answer 78

Roentgen-to-rad conversion factor; 0.876 in air