DOSIMETRY FINAL EXAM QUESTIONS AND
CORRECT ANSWERS
Cobalt 60 - CORRECT ANSWERS dose rate measured in cGy/min
dose rate considered constant for short periods of time
Linac - CORRECT ANSWERS dose rate measured in cGy/MU
dose rate changes constantly. Shut off by monitor ionization chamber once predetermined dose
is given.
PDD - CORRECT ANSWERS Percent dose depth
For SSD setups
ratio of absorbed dose at given depth to the absorbed dose at a fixed reference depth of DMAX.
Dose @ depth/Dose @ DMAX
PDD depends on... - CORRECT ANSWERS increased energy = more penetrating (increased
PDD)
increased depth = attenuation through matter (decreased PDD)
increased field size = more scatter (increased PDD)
increased SSD = increased PDD (ISL)
TAR - CORRECT ANSWERS Tissue Air Ratio
Independent of SSD
ratio of absorbed dose at given depth in tissue to absorbed dose at same point in air
Calculated two measurements at same point in space
Dose in tissue/Dose in air
, TAR depends on... - CORRECT ANSWERS increased energy = increased TAR
increased field size = increased TAR
increased depth = decreased TAR
When depth is at DMAX, TAR is called... - CORRECT ANSWERS backscatter factor
TPR - CORRECT ANSWERS Tissue Phantom Ratio
absorbed dose at given depth in phantom to absorbed dose at same point at a reference depth
in phantom
Dose @ depth/Dose @ reference depth
TPR depends on... - CORRECT ANSWERS reference depth:
deeper reference depth = greater TPR
Scatter - CORRECT ANSWERS Backscatter: radiation deflected back toward patient
Most absorbed dose received by patient results from collisions of scattered electrons produced
when primary photon interacts with collimator
BSF - CORRECT ANSWERS Backscatter Factor
when using blocks: ratio of dose rate with scattering medium to dose rate at same point without
scattering medium at level of max equilibrium (DMAX)
PSF for megavoltage units
***Percent change to dose due to change in field size. Readings at DMAX and compare to dose
in air.
SAR - CORRECT ANSWERS Scatter Air Ratio
CORRECT ANSWERS
Cobalt 60 - CORRECT ANSWERS dose rate measured in cGy/min
dose rate considered constant for short periods of time
Linac - CORRECT ANSWERS dose rate measured in cGy/MU
dose rate changes constantly. Shut off by monitor ionization chamber once predetermined dose
is given.
PDD - CORRECT ANSWERS Percent dose depth
For SSD setups
ratio of absorbed dose at given depth to the absorbed dose at a fixed reference depth of DMAX.
Dose @ depth/Dose @ DMAX
PDD depends on... - CORRECT ANSWERS increased energy = more penetrating (increased
PDD)
increased depth = attenuation through matter (decreased PDD)
increased field size = more scatter (increased PDD)
increased SSD = increased PDD (ISL)
TAR - CORRECT ANSWERS Tissue Air Ratio
Independent of SSD
ratio of absorbed dose at given depth in tissue to absorbed dose at same point in air
Calculated two measurements at same point in space
Dose in tissue/Dose in air
, TAR depends on... - CORRECT ANSWERS increased energy = increased TAR
increased field size = increased TAR
increased depth = decreased TAR
When depth is at DMAX, TAR is called... - CORRECT ANSWERS backscatter factor
TPR - CORRECT ANSWERS Tissue Phantom Ratio
absorbed dose at given depth in phantom to absorbed dose at same point at a reference depth
in phantom
Dose @ depth/Dose @ reference depth
TPR depends on... - CORRECT ANSWERS reference depth:
deeper reference depth = greater TPR
Scatter - CORRECT ANSWERS Backscatter: radiation deflected back toward patient
Most absorbed dose received by patient results from collisions of scattered electrons produced
when primary photon interacts with collimator
BSF - CORRECT ANSWERS Backscatter Factor
when using blocks: ratio of dose rate with scattering medium to dose rate at same point without
scattering medium at level of max equilibrium (DMAX)
PSF for megavoltage units
***Percent change to dose due to change in field size. Readings at DMAX and compare to dose
in air.
SAR - CORRECT ANSWERS Scatter Air Ratio
