DOSIMETRY UPDATED EXAM QUESTIONS AND
CORRECT ANSWERS
Two reasons why water is specifically used for phantoms? - CORRECT ANSWERS 1. Water
closely approximates radiation absorption and scatter
2. The availability (and affordability) of water
Solid dry phantoms have the same_____as water. - CORRECT ANSWERS Effective atomic
number
Human shaped phantoms are called what? - CORRECT ANSWERS Anthropomorphic
phantoms
Two examples of anthropomorphic phantoms: - CORRECT ANSWERS 1. pixy
2. Rando
Traditionally where is dose distribution measured? - CORRECT ANSWERS Along the central
axis
Anything that is not measure of the CA is referred to as what? - CORRECT ANSWERS off
axis
Define normalizing: - CORRECT ANSWERS Taking the highest dose and making it the 100%
line
Define percent depth dose (PDD): - CORRECT ANSWERS the quotient (ratio), expressed as
a percentage, of the absorbed dose at a depth compared to the dose at a normalized fixed
depth (d max)
, When are PDD tables traditionally used? - CORRECT ANSWERS With SSD techniques
(usually Co60)
Define SSD technique: - CORRECT ANSWERS Where the treatment SSD= machine SAD
Formula for PDD: - CORRECT ANSWERS Dose and depth/ d max *100
What 4 factors affect PDD: - CORRECT ANSWERS 1. Increased energy= increase PDD
2. Increased field size=Increased PDD
3. Increased distance (SSD)= increase PDD
4. Increased Depth= Decrease PDD
Define dose build up region: - CORRECT ANSWERS The region between the skin surface
and the point of maximum dose.
Where would the dose build up region be on a orthovoltage machine? - CORRECT
ANSWERS There will not be a dose build up region due to the fact that there is not any
cause for forward scatter at the skin surface.
Dose build up region gives rise to what is clinically known as what effect? - CORRECT
ANSWERS Skin sparing effect
What does the effect of oblique incidence do to surface dose? - CORRECT ANSWERS skin
doses increase with an increased angle of incidence
CORRECT ANSWERS
Two reasons why water is specifically used for phantoms? - CORRECT ANSWERS 1. Water
closely approximates radiation absorption and scatter
2. The availability (and affordability) of water
Solid dry phantoms have the same_____as water. - CORRECT ANSWERS Effective atomic
number
Human shaped phantoms are called what? - CORRECT ANSWERS Anthropomorphic
phantoms
Two examples of anthropomorphic phantoms: - CORRECT ANSWERS 1. pixy
2. Rando
Traditionally where is dose distribution measured? - CORRECT ANSWERS Along the central
axis
Anything that is not measure of the CA is referred to as what? - CORRECT ANSWERS off
axis
Define normalizing: - CORRECT ANSWERS Taking the highest dose and making it the 100%
line
Define percent depth dose (PDD): - CORRECT ANSWERS the quotient (ratio), expressed as
a percentage, of the absorbed dose at a depth compared to the dose at a normalized fixed
depth (d max)
, When are PDD tables traditionally used? - CORRECT ANSWERS With SSD techniques
(usually Co60)
Define SSD technique: - CORRECT ANSWERS Where the treatment SSD= machine SAD
Formula for PDD: - CORRECT ANSWERS Dose and depth/ d max *100
What 4 factors affect PDD: - CORRECT ANSWERS 1. Increased energy= increase PDD
2. Increased field size=Increased PDD
3. Increased distance (SSD)= increase PDD
4. Increased Depth= Decrease PDD
Define dose build up region: - CORRECT ANSWERS The region between the skin surface
and the point of maximum dose.
Where would the dose build up region be on a orthovoltage machine? - CORRECT
ANSWERS There will not be a dose build up region due to the fact that there is not any
cause for forward scatter at the skin surface.
Dose build up region gives rise to what is clinically known as what effect? - CORRECT
ANSWERS Skin sparing effect
What does the effect of oblique incidence do to surface dose? - CORRECT ANSWERS skin
doses increase with an increased angle of incidence
