Deck 25: Electron Beams in Radiation Therapy

Question 1

5)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

Answer 1

C
A drawback of the use of scattering foils is the production of bremsstrahlung contamination by the electron beam's interaction with the scattering foil.

Question 2

16)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

Answer 2

B
With increasing electron beam energy, the ballooning of the isodose lines decreases.

Question 3

13)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

Answer 3

A
There is little effect of field size on both surface dose and percent depth dose of electron beams, provided the fields are of sufficient size.The rule is that the electron beam's diameter (field size)in centimeters should not be less than the practical range.

Question 4

18)Which process is 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

Question 5

20)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 6

10)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 7

6)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 8

14)Which electron beam energy is most likely to under-dose at the surface?

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

Question 9

3)Which processes 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 10

7)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

Question 11

2)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

12)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

Question 13

15)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 14

17)According to the Association of Physicists in Medicine (AAPM)Task Group 25 report, what distance is considered an extended source-skin distance (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 15

4)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 16

9)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

Question 17

8)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 18

1)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 19

19)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 20

11)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 21

24)As the mean energy of an electron beam increases, the _________________________.

A) surface dose decreases
B) surface dose increases
C) percent depth dose decreases
D) practical range depth decreases

Question 22

22)Surface dose values for electron beams in the 6- to 20-MeV range vary from __________.

A) 100% to 105%
B) 95% to 100%
C) 80% to 90%
D) 70% to 80%

Question 23

25)Which of the following is one very important consideration during the use of internal shields when administering electron therapy?

A) Shielding materials become radioactive
B) Dose to tissues directly in front of the shield is increased
C) Internal shields need to be secured by suturing
D) Shielding material thickness is found by MeV/3

Question 24

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

A) Photon contamination
B) Under dosing
C) Electron backscatter
D) Infection

Question 25

23)To produce a therapeutically useful electron beam, the stream of electrons leaving the electron gun must contact the ___________ before exiting the gantry head.

A) target
B) scattering foil
C) flattening filter
D) mirror and bulb assembly