Concepts of Dose Calculations Pt. 2

Question 1

Machine Output

Answer 1

Amount of radiation produced from the generated photons at a specified distance

Question 2

What is machine output measured in and whats the unit?

Answer 2

Monitor Units and cGy/MU

Question 3

Normalization Condition

Answer 3

Dose calculations that are based upon precision measurements taken at a specific energy, field size, depth, distance and machine

Question 4

What is Do?

Answer 4

Normalization Depth and

Question 5

SSD Calibration

Answer 5

SAD + Dmax, 10x10 field size, cGy/MU (Linac), cGy/Min (Co-60),

Question 6

SSD Calibration on Linac ( ? Distance + ____)

Answer 6

100cm + Dmax

Question 7

SSD Calibration on Co-60 ( ? Distance + ____)

Answer 7

80cm + Dmax

Question 8

Reference Dose (Kref, Dref, Kssd)

Answer 8

Calibrated dose rate for SSD Calibration

Question 9

What is SAD Calibration

Answer 9

Source to Axis Calibration

Question 10

SAD Calibration

Answer 10

100cm (Distance+Dmax), 10x10 field size, cGy/MU (Linac), cGy/Min (Co-60),

Question 11

Whats the advantage of using SAD calibration?

Answer 11

No adjustments are needed when set to 100cm on linac and 80cm on Co-60

Question 12

What is the dose calculation parameter for SSD?

Answer 12

Percentage Depth Dose (PDD)

Question 13

Treatment setup for SSD Calibration

Answer 13

Patient skins is set at a distance of 100cm from the source/target/isocenter of Linac or 80cm from Co-60

Question 14

What must the reference dose be set at to use PDD?

Answer 14

SAD+Dmax

Question 15

To use PDD, and you are given a different reference dose rate, what must be used?

Answer 15

Inverse square law

Question 16

What are the dose calculations parameters used for SAD?

Answer 16

Tissue-Air Ratio, Tissue-Maximum Ratio, Tissue Phantom Ratio

Question 17

Treatment setup for SAD Calibration

Answer 17

Patients tumor or midline is set at the isocenter at 100 cm from source of radiation target/source (80cm for Co-60)

Question 18

To use TMR,TAR and TPR, where must the reference dose rate be placed?

Answer 18

SAD

Question 19

To use TMR/TAR/TPR, and you are given a different reference dose rate, what must be used?

Answer 19

Inverse square law

Question 20

PDD

Answer 20

Percentage depth dose, percent of dose at a given depth compared to depth of maximum ionization

Question 21

As Field Size increases, PDD ____?

Answer 21

Increases

Question 22

As Depth increases, PDD ____?

Answer 22

Increases

Question 23

As Beam Energy increases, PDD ____?

Answer 23

Increases

Question 24

As SSD increases, PDD ____?

Answer 24

decreases

Question 25

What is the given dose?

Answer 25

Normalized doses to along the central axis that occur at Dmax

Question 26

What is the PDD Equation?

Answer 26

PDD= (Dd/Do) x 100

Dd = Dose at depth, Do = Dose maximum

Question 27

Dskin

Answer 27

Dose to Skin or Surface

Question 28

Skin-Sparing Effect

Answer 28

Dskin/Do x 100 < 100

Question 29

What happens to skin sparing as beam energy increases?

Answer 29

Skin-sparing decreases

Question 30

what happens to Dskin as field size increases?

Answer 30

Dskin increases as field size increases

Question 31

What occurs to Dmax as beam energy increases?

Answer 31

Dmax increases as Beam Energy increases

Question 32

What are the Dmax’s for Co-60, 4MV, 6MV, 10MV, 15MV and 18MV?

Answer 32

Co-60: 0.5cm
4MV: 1.2cm
6MV: 1.5cm
10MV: 2.5cm
15MV: 3.0cm
18MV: 3.3cm

Question 33

With Beam Energy <10MV, will Dmax be affected if field size is increased?

Answer 33

No

Question 34

With Beam Energy >10MV, will Dmax be affected if field size is increased?

Answer 34

Decreases due to electron contamination

Question 35

What two factors support that idea that PDD decreases as depth increases

Answer 35

1. Inverse Square

2. Exponential Attenuation