Deck 26: Electron Beams in Radiation Therapy

Question 1

Interactions with which of the following is not a cause of photon contamination of an electron beam?

A) scattering foils
B) collimators
C) air
D) patient

Answer 1

D
The dose in this end region is composed of bremsstrahlung-produced x-ray contamination from the interaction of the electron beam with scattering foils,ionization chambers,collimators,and the air between the patient and the treatment unit.

Question 2

What is the depth of the 90% isodose line of a 9-MeV electron beam?

A) 2.25 cm
B) 3 cm
C) 4.5 cm
D) 9 cm

Answer 2

A
The depth of the 90% isodose curve may be found by dividing the energy of the electron beam in MeV by 4.

Question 3

What will be the energy of an electron beam at a depth of 4 cm that had a surface energy of 16 MeV?

A) 4 MeV
B) 12 MeV
C) 14 MeV
D) 16 MeV

Answer 3

B
The second of these relationships deals with the reduction of the energy of an electron beam as it moves through matter.The energy of an electron beam at a given depth in water may be approximated based on the following relationship.The rate at which an electron beam loses energy is approximately 2 MeV/cm in water.For example,an electron beam with an energy of 10 MeV incident on the phantom surface will have an energy of 6 MeV at a depth of 2 cm in water.

Question 4

Which electron beam energy is most likely to underdose at the surface?

A) 9 MeV
B) 12 MeV
C) 16 MeV
D) 20 MeV

Question 5

Which processes are responsible for electron interactions with matter?
I)collisional processes
II)radiation processes
III)gradient processes

A) I and II
B) I and III
C) II and III
D) I,II,and III

Question 6

When created using a linear accelerator,electrons strike a target to produce a uniform beam before exiting the machine.

Question 7

A partial bolus should never be used with electrons.

Question 8

A 12-MeV electron beam lacks skin sparing.

Question 9

What effect will an increase in the nominal energy of an electron beam have on the depth of the 80% isodose line?

A) increase
B) decrease
C) remain the same
D) dose dependence

Question 10

Unlike photons,electrons have a mass and a charge.

Question 11

What are electrons scattered with enough energy to cause further ionization and excitations in other atoms?

A) alpha particles
B) beta particles
C) delta rays
D) gamma rays

Question 12

What is the practical range of a 25-MeV electron beam?

A) 12.5 cm
B) 12.5 mm
C) 50 cm
D) 50 mm

Question 13

Which process(es)are responsible for the majority of energy loss for electron energy ranges useful in radiation therapy?
I)collisional processes
II)radiation processes
III)gradient processes

A) I only
B) II only
C) III only
D) I and II

Question 14

What is the depth of the 80% isodose line of a 15-MeV electron beam?

A) 3 cm
B) 3.75 cm
C) 5 cm
D) 7.5 cm

Question 15

Which electron beam's diameter (field size)is most likely to have an effect on a 20-MeV beam's surface dose and percent depth dose?

A) 5 cm
B) 10 cm
C) 15 cm
D) 20 cm

Question 16

Photons and electrons of similar energy have the same probability of interacting with matter.

Question 17

Which technique is most likely to result in photon contamination of the electron beam?

A) use of an electron gun
B) use of a bending magnet
C) use of a scattering foil
D) use of a scanning beam

Question 18

Given a mean energy of an electron beam is 6 MeV,what is the depth of the 50% isodose line?

A) 2.5 mm
B) 14.4 mm
C) 2.5 cm
D) 14.4 cm

Question 19

Large tissue heterogeneities and small tissue heterogeneities have the same effect on the dose distribution of electron beams.

Question 20

Which treatment energy for electrons is most likely to use a scanning beam technique rather than a scattering foil?

A) 4 MeV
B) 12 MeV
C) 20 MeV
D) 25 MeV

Question 21

What effect will an increase in the energy of an electron beam have on the lateral bulge of the 80% isodose line?

A) increase
B) decrease
C) remain the same
D) dose dependence

Question 22

According to the AAPM Task Group 25 report,what distance is considered an extended SSD for electron beams?

A) greater than 100 cm SSD
B) greater than 110 cm SSD
C) greater than 115 cm SSD
D) greater than 130 cm SSD

Question 23

What is the danger to tissue in front of an internal shield when used with electron beams therapy?

A) photon contamination
B) underdosing
C) electron backscatter
D) infection

Question 24

What thickness of Lipowitz alloy is necessary to adequately shield a 9-MeV electron beam?

A) 3 cm
B) 3.6 cm
C) 4.5 cm
D) 5.4 cm

Question 25

What thickness of lead is necessary to adequately shield a 9-MeV electron beam?

A) 3 cm
B) 3.6 cm
C) 4.5 cm
D) 5.4 cm

Question 26

Which of the following is not an application of bolus when used with electron beams?

A) increase surface dose
B) tissue compensator
C) shape isodose distributions
D) increase the effective energy at depth

Question 27

Which process(es)are responsible for the majority of energy loss for electron energy ranges useful in radiation therapy?
I)effective point source method
II)virtual source method

A) I only
B) II only
C) I and II
D) neither I nor II