fun with physics Flashcards
the depth within the body that receives the same dose of radiation
isodose line
compensates for curves in the body
compensators
adjusts the beam profile to reduce the dose to more sensitive parts of the body
wedges
measurement made by passing a dosimeter across the beam
dose profile
measure of the side to side quality of the beam; usually required to be +/- 1-2% from one side of the beam to the other
symmetry
generally the max variation of the beam intensity in the central 80% of the beam; required to be +/- 2-3% across the beam
flatness
the TAR at max on the CAX of the beam is referred to as what?
Backscatter factor
the region between the patient’s skin and the max is referred to as what?
dose build up region
the absorbed dose at a given depth expressed as a % of the absorbed dose at a reference depth on the CAX of the field
PDD
PDD is dependent on what 5 variables?
energy, field size, SSD, depth, composition of the irradiated medium
does PDD increase or decrease with increased SSD
increase
does PDD increase or decrease with increasing field size
increase
does PDD increase or decrease with increased depth
decrease
what is dose in tissue/ dose at d-max in a phantom
TMR
what 3 factors is TMR dependant
depth, energy, field size
what is used to calculate changes in PDD
Mayneord’s Factor
what is dose in tissue/dose in air?
TAR
ratio of dose on the CAX at d-max to dose at the same point in air
BSF
BSF and __ at dmax are the same
TAR
is TMR dependent on SSD
no
Which of the following is true when measuring Percent Depth Dose (PDD)?
A. The Source to Chamber distance is constant.
B. The chamber to Axis distance is constant.
C. The Source to Surface distance is constant.
D. The chamber depth is constant.
C. the source to surface distance is constant
Which of the following is most often used to measure beam quality of a megavoltage photon beam?
A. PDD at 10 cm
B. TPR ratio at 20cm and 10 cm C. Half Value Layer
D. Depth where PDD is 50%
E. a and b only.
E. A and B only
at approximately what energy does backscatter factor become no longer meaningful?
10 MV
The reference depth for the Tissue Phantom Ratio, TPR, is: A. dmax. B. 5 cm. C. 10 cm. D. dependent on measurement choice
D. dependent on measurement choice
what is D-max
depth of the max dose
where are PDD 10 and TPR 20/10 measured
central axis
with TPR does the SSD change
yes
does scatter increase or decrease with field size
increase
with TAR, is the SCD constant? what about the measurement point?
with TAR the SCD and measurement point are constant
TAR varies with what 3 things
depth, field size, energy
dose at a point divided by the dose at d-max
depth dose
does depth dose increase or decrease with increasing field size
increase
will a shorter SSD make the depth dose go up or down
down
two measurements are made, one in phantom and one in air at the same time
TAR
a variation of TPR but the reference depth is usually taken to be D-max
TMR
a ratio of dose in a phantom divided by the dose in a phantom at a reference depth
TPR
what parameter is used to calculate the scatter dose component of a blocked field
scatter maximum ratio
Which of the following is true when measuring TPR?
A. The Source to Chamber distance is constant.
B. The chamber to surface distance is constant.
C. The Source to Surface distance is constant.
D. The chamber depth is constant.
A. the source to chamber distance is constant
what is penumbra?
area or region at the beams edge where the radiation. intensity falls to 0
when treating a patient with skin cancer and using electrons, which does the electron use, a scattering foil or a flattening filter?
scattering foil
