(*-*) Dosimetric Calculation Factors

Question 1

dosimetric calculation factors are the factors that get included in ___ calculations

Answer 1

MU

Question 2

what dosimetric calculation factor occurs in treatment head with collimating jaws and flattening filter

Answer 2

collimator scatter (Sc)

Question 3

what dosimetric calculation factor occurs in patient or phantom

Answer 3

phantom scatter (Sp)

Question 4

what dosimetric calculation factor contains both collimator scatter and phantom scatter

Answer 4

total scatter factor (Scp)

Question 5

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

Answer 5

TAR (Tissue Air Ratio)

Question 6

what dosimetric calculation factor is used for low energy beams?

Answer 6

backscatter factor (BSF)

Question 7

what dosimetric calculation factor is used for high energy beams above 4 MV?

Answer 7

peak scatter factor (PSF)

Question 8

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

Answer 8

TPR (Tissue Phantom Ratio)

Question 9

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

Answer 9

TMR (Tissue Maximum Ratio)

Question 10

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

Answer 10

Off Axis Ratio

Question 11

Larger field sizes would have MORE scatter, so more or less MU would be needed?

Answer 11

LESS MU - to compensate for added dose

Question 12

Smaller field sizes would have LESS scatter, so more or less MU would be needed?

Answer 12

MORE MU - to compensate for loss of dose

Question 13

scatter factors of collimator scatter (Sc) are based on a __x___ cm treatment field size at 100 SAD

Answer 13

10 x 10 cm field size at 100 SAD

Question 14

concept check - if you have a large field size, with a lot of scatter, what can be adjusted to compensate?

Answer 14

lower the MU

Question 15

phantom scatter (Sp) scatter varies depending on treatment ____

Answer 15

volume

Question 16

phantom scatter (Sp) ___can/cannot___ be directly measured

Answer 16

cannot!!!!

Question 17

as field size increases, what happens to phantom scatter (Sp) - does it INCREASE, DECREASE, STAY THE SAME ?

Answer 17

increases (however not Nappi says “…although it cannot be directly measured”)

Question 18

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.

Answer 18

Scp (Total Scatter Factor)

Question 19

TAR (Tissue Air Ratio) is independent of ______

Answer 19

SSD

Question 20

TAR is dependent on what (3)

Answer 20

depth, energy, field size

Question 21

as energy and field size increase, what happens to TAR?

Answer 21

TAR would increase

Question 22

as depth increases, what would happen to TAR?

Answer 22

TAR would decrease as depth goes past dmax due to beam attenuation

Question 23

both _______ and ________ represent TAR at the reference depth of dmax along the central axis for their respective high/low energies

Answer 23

BSF (Backscatter Factor) and PSF (Peak Scatter Factor)
*BSF - low energy, PSF - high energy

Question 24

both BSF and PSF compare the dose rate in ______ to dose rate within a _______ at dmax

Answer 24

both BSF and PSF compare the dose rate in a free space, to dose rate within a phantom at dmax

Question 25

unlike TAR which uses air, _____ measures within a phantom

Answer 25

TPR (Tissue Phantom Ratio)

Question 26

what dosimetric calculation factor does not measure in air and is used for calculations of higher beam energies?

Answer 26

TPR (Tissue Phantom Ratio)

Question 27

what is formula for SAD MU?

Answer 27

(prescribed dose) / (TMR) (other given factors)

Question 28

to find SSD MU, we use what formula?

Answer 28

PDD
(prescribed dose) / (PDD) (other given factors)

Question 29

during/for an Off Axis Ratio, what is happening to the central axis?

Answer 29

it is blocked by MLCs or jaws - ex. half beam block technique

Question 30

beam shaping devices may reduce the _______

Answer 30

dose rate; devices between the beam and patient may attenuate some beam and reduce the dose rate… to compensate we would increase the MU… but we need to know by how much. Thus, we use transmission factors!

Question 31

what is formula for wedge factor?

Answer 31

= dose with wedge / dose without wedge

Question 32

what is formula for tray factor?

Answer 32

= dose with tray / dose without tray

Question 33

wedge factor and tray factor are _____________ factors

Answer 33

transmission

Question 34

____________ describe the percent of radiation that passes through a certain beam shaping device

Answer 34

transmission factors = answer ..
- such as wedge and tray factor

Question 35

we use what when SSD is greater than 100 cm?

Answer 35

extended distances

Question 36

_________ compensates for the loss of dose that comes with greater distances

Answer 36

extended distances

Question 37

when treating distances larger or greater than 100, that is considered an ______ ______

Answer 37

extended distance

Question 38

what does ISF stand for?

Answer 38

Inverse Square Factor

Question 39

what is formula for ISF?

Answer 39

= (calibrated SSD + depth / extended SSD + depth)^2

Question 40

calibrated SSD tends to be what?

Answer 40

100

Question 41

MU of an extended distance equals what? - what is formula?

Answer 41

= (prescribed dose) / (ISF) (other given factors)

Question 42

_______ compensates for loss of dose that comes with greater distances

Answer 42

ISF (inverse square factor)