Dosimetry 1 5-6 to 5-12
Normalizing depth for TPR: (2)
-Any depth can be chosen for TPR (normalization depth), but 5 cm is the usual choice. -The Dmax depth would certainly be simple.5cm is the usual choice as , for energies greater than 20 MV (20-45 MV), dmax depth actually changes with field size and SSD*
3 uses for SAR data:
-calculates scatter dose separately from primary dose -used for calculating the scattered component in a medium -most commonly used for irregular field calculations
Factors affecting BSF (2)
1. Energy 2. Field size
Factors affecting SAR: (3)
1. Energy (direct) 2. Field size (direct) 3. Depth (inverse)
Factors affecting TAR (3)
1. Energy (direct) 2. Field size (direct) 3. Depth (inverse)
Factors that effect TMR (3)
1. Energy (direct) 2. Field size (direct) 3. Depth (inverse)
Other Co60 "need to know" data 50% dose line with 10x10 at 80 cm SSD:
12cm deep in tissue
TAR first introduced in ___ as Tumor air ratio
1953
TAR and SAR may not be suitable for energies exceeding ___ due to the fact that measurements are taken in air (limitations of the ion chambers and unit "R")
3 MeV
BSF formula
BSF= Dmax dose/Dfs (dose in free space)
What does BSF stand for?
Backscatter Factor
who first proposed SAR as a quantity?
Clarkson
What does CSF stand for?
Collimator Scatter Factor
Average TAR Method:
Consists of taking TAR's at various regular intervals (usually 20 degrees apart) along the arc and making calcs based on the average of those TAR values.
Who developed SAR as an quantity?
Cunningham
what is another name for Collimator Scatter Factor
FSOF field size output factor
what is (SMR)
Scatter-maximum ratio
T/F SAR is similar to TAR but calculated off the CA.
T
T/F TMR compares dose at dmax to the dose at other areas, so dmax dose is the reference dose it's exactly like TPR except the "reference depth" is always dmax instead of 5cm).
T
___ is still best method for rotational calcs
TAR
Why is TAR better for rotational calculations?
TAR lets you not have to worry about moving SSD since everything is measured at iso
What does TAR mean
TISSUE AIR RATIO
______ is the most commonly used dose table
TMR
TMR formula
TMR= dose in tissue/dose in phantom (at Dmax)
TPR formula
TPR=dose in tissue/dose in phantom
Irregular field: Def:
Technically, any field other than a square, rectangle or circle. But, clinically, the field has to have extensive blocking, making the treated field a very irregular shape.
define TAR
The ratio of dose at a given point in a phantom to the dose in free space (air) at the same point.
What does TMR stand for?
Tissue Maximum Ratio
what is (TPR)
Tissue Phantom Ratio
, by far the most important factor affecting dmax depth is ____________
beam energy
Scatter dose-def-
dose separate from the primary beam
What factor has the most effect on output factor?
field size
BSF has____ relationship with energy
inverse
what is SAR used for?
it is used to give a way to calculate how much scatter is taking away from dose
2 examples of irregular fields
mantle and Inverted Y
TMR limits measurements to the _____, not air.
phantom
TPR works much like TAR but limits ALL measurements to a ___, there are no measurements in air at all (same goes for TMR)
phantom
TAR was intended originally for ___
rotational calcs
what does SAR stand for
scatter air ratio
Why does distance not affect tar?
the distance isn't always constant
Effective primary dose-def-t
the dose due to the primary photons as well as those scattered photons from the collimation system (like dose at depth minus phantom scatter)
. Tissue Phantom Ratio (TPR) Def:
the ratio of the dose at a given point in a phantom to the dose at a fixed reference depth (usually 5cm).
Tissue Maximum Ratio TMR Def:
the ratio of the dose at a given point in the phantom to the dose at the same point at the dmax depth.
BACKSCATTER FACTOR (BSF) Def:
the ratio of the dose on the CA at the dmax depth* to the dose at the same point in free space.
SAR Def:
the ratio of the scattered dose at a given point in the phantom to the dose in free space at the same point. This is for calculating doses to off-axis points (points off the central axis).
Scatter-maximum ratio (SMR) Def:
the ratio of the scattered dose at a given point in the phantom to the effective primary dose at the same point at the point of Dmax.
Why don't we use BSF for our hand calcs?
we are dealing with higher energies so we have forward scatter
