Homework 8

Question 1

which of the following is true regarding electron beams?

a. they provide excellent skin sparing
b. they are most useful for treating superificial lesions
c. they are ideal for irregular external contours
d. they don’t require field shaping

Answer 1

b. they are most useful for treating superificial lesions

Question 2

when the kinetic energy of an electron is conserved (not converted to another form of energy), this type of collision is called

a. absorbed dose
b. elastic collision
c. radiative collision
d. bremsstrahlung

Answer 2

b. elastic collision

Question 3

the type of inelastic collision which occurs predominantly with high Z materials is called

a. excitation
b. bremsstrahlung
c. ionization
d. nuclear scatter

Answer 3

b. bremsstrahlung

Question 4

the rate of electron energy lost per unit path length corrected for density is call the

a. mass stopping power
b. ionization rate
c. percent depth dose

Answer 4

a. mass stopping power

Question 5

as a rule of thumb, approximately how much energy, in MeV, does an elecron lose per centimeter of travel?

Answer 5

2

Question 6

electron percent depth dose curves are characterized by which of the following?

a. high surface dose
b. rapid dose falloff after clinical depth
c. high dose gradient
d. all the answers are correct

Answer 6

d. all the answers are correct

Question 7

unlike photons, electrons lose energy with each interaction until they lose all of their energy. the depth at which this happens is called the

a. bremsstrahlung depth
b. therapeutic range
c. terminal depth
d. practical range

Answer 7

d. practical range

Question 8

match the depth with the approximate equation: D90

Answer 8

E(MeV)/4

Question 9

match the depth with the approximate equation: D80

Answer 9

E(MeV)/3

Question 10

match the depth with the approximate equation: D50

Answer 10

E(MeV0/2.33

Question 11

match the depth with the approximate equation: Rp

Answer 11

E(MeV)/2

Question 12

for higher energy electron beams, the isodose curves ________ for high dose levels but _________ for low dose levels

Answer 12

constrict

bulge out

Question 13

the measurement of the umbra based on an in-plane or cross-plane profile is called

a. uniformity index
b. symmetry
c. flatness

Answer 13

c. flatness

Question 14

trimmer bars or collimating cones are necessary for electron beams because

a. scatter in air broadens the beam
b. they define the uniformity index
c. they provide scatter to flatten the beam

Answer 14

a. scatter in air broadens the beam

Question 15

T/F: as the electron field size decreases, the dmax and the depth dose vurve both shift toward the surface

Answer 15

True

Question 16

how much of a margin, in cm, is required for the field size of an electron field in order to treat the target without including the penumbra?

Answer 16

2

Question 17

bolus is often used for electron beams in order to

a. increase the surface dose to the patient
b. reduce the beam penetration
c. even out (flatten) the surface
d. all the answers are correct

Answer 17

d. all the answers are correct

Question 18

high Z materialswhen used for internal shielding with electron treatments are often coated with tissue equivalent material because

a. it reduces electron backscatter
b. it reduces dose hot spots in the tissue
c. reduces the effect of scatted low energy xray
d. all of the above

Answer 18

d. all of the above