(*-*) Dosimetric Calculation Factors Flashcards
dosimetric calculation factors are the factors that get included in ___ calculations
MU
what dosimetric calculation factor occurs in treatment head with collimating jaws and flattening filter
collimator scatter (Sc)
what dosimetric calculation factor occurs in patient or phantom
phantom scatter (Sp)
what dosimetric calculation factor contains both collimator scatter and phantom scatter
total scatter factor (Scp)
what dosimetric calculation factor is the ratio of an absorbed dose at a depth within the phantom at a given distance compared to the absorbed dose in a free space at the same distance
TAR (Tissue Air Ratio)
what dosimetric calculation factor is used for low energy beams?
backscatter factor (BSF)
what dosimetric calculation factor is used for high energy beams above 4 MV?
peak scatter factor (PSF)
what dosimetric calculation factor is the ratio of the dose at a given depth in a phantom, to the dose rate at the same source-point distance at a reference depth
TPR (Tissue Phantom Ratio)
what dosimetric calculation factor is the ratio of the dose rate at a specified depth in a phantom, to the dose rate at dmax at the same distance from the radiation source
TMR (Tissue Maximum Ratio)
what dosimetric calculation factor is the ratio of the dose rate at a point off-axis to the dose rate at a point on the central axis at the same distance from the source
Off Axis Ratio
Larger field sizes would have MORE scatter, so more or less MU would be needed?
LESS MU - to compensate for added dose
Smaller field sizes would have LESS scatter, so more or less MU would be needed?
MORE MU - to compensate for loss of dose
scatter factors of collimator scatter (Sc) are based on a __x___ cm treatment field size at 100 SAD
10 x 10 cm field size at 100 SAD
concept check - if you have a large field size, with a lot of scatter, what can be adjusted to compensate?
lower the MU
phantom scatter (Sp) scatter varies depending on treatment ____
volume
phantom scatter (Sp) ___can/cannot___ be directly measured
cannot!!!!
as field size increases, what happens to phantom scatter (Sp) - does it INCREASE, DECREASE, STAY THE SAME ?
increases (however not Nappi says “…although it cannot be directly measured”)
helpful hint - if you are given all 3 (Sc, Sp, and Scp) in a MU calc problem, you can use just ________ in denominator, as it contains all.
Scp (Total Scatter Factor)
TAR (Tissue Air Ratio) is independent of ______
SSD
TAR is dependent on what (3)
depth, energy, field size
as energy and field size increase, what happens to TAR?
TAR would increase
as depth increases, what would happen to TAR?
TAR would decrease as depth goes past dmax due to beam attenuation
both _______ and ________ represent TAR at the reference depth of dmax along the central axis for their respective high/low energies
BSF (Backscatter Factor) and PSF (Peak Scatter Factor)
*BSF - low energy, PSF - high energy
both BSF and PSF compare the dose rate in ______ to dose rate within a _______ at dmax
both BSF and PSF compare the dose rate in a free space, to dose rate within a phantom at dmax
