Deck 23: Photon Dosimetry Concepts and Calculations

Question 1

1)Which of the following does not apply to "absorbed dose?"

A) Energy deposited at a point
B) Measured in Gray
C) Measured in air
D) Can measure absorbed dose using ionization chambers

Answer 1

B
One Gray equals 1 Joule/kg.

Question 2

13)As the SSD increases, the PDD does what?

A) Increases
B) Decreases
C) Remains the same
D) No effect

Answer 2

A
As the SSD increases, the PDD increases, based on the Mayneord F factor.

Question 3

18)Which of the following is the equivalent square of rectangular field of 10 ´ 15?

A) 12 ´ 12
B) 3.6 ´ 3.6
C) 11.5 ´ 11.5
D) 13 ´ 13

Answer 3

A
The formula for equivalent square fields is (4 ´ area)÷ perimeter.

Question 4

6)The intensity of a radioactive beam is measured at a distance of 100 cm and found to be 250 mR/min.What will the intensity of this beam be at 105 cm?

A) 226.8 mR/min
B) 238.1 mR/min
C) 262.5 mR/min
D) 275.6 mR/min

Question 5

3)Which of the following defines Dmax?

A) Build-up region
B) Depth of maximum equilibrium
C) Depth of dose deposited per unit of time
D) Dose scatter to surface

Question 6

17)As the source-skin distance increases, what happens to the tissue-air ratio?

A) Increases
B) Decreases
C) Remains the same
D) No effect

Question 7

14)As the depth increases, what happens to the PDD?

A) Increases
B) Decreases
C) Remains the same
D) No effect

Question 8

19)Which of the following is the equivalent square of a rectangular field of 20 ´ 10?

A) 12 ´ 12
B) 15 ´ 15
C) 11.5 ´ 11.5
D) 13.3 ´ 13.3

Question 9

7)Absorbed dose at depth ´ 100% = absorbed dose at Dmax is the definition of which of the following?

A) TAR
B) TMR
C) TPR
D) PDD

Question 10

8)Absorbed dose at a given depth divided by the dose in air dose at a fixed reference is the definition of which of the following?

A) TAR
B) TMR
C) TPR
D) PDD

Question 11

5)The intensity of a radiation beam is measured at 10 mR/hr at a distance of 20 cm.What will be the intensity of this beam at 30 cm?

A) 22.5 mR/hr
B) 4.44 mR/hr
C) 6.67 mR/hr
D) 15 mR/hr

Question 12

11)As the field size increases, the PDD does what?

A) Increases
B) Decreases
C) Remains the same
D) No effect

Question 13

15)As the field size increases, the tissue-air ratio does what?

A) Increases
B) Decreases
C) Remains the same
D) No effect

Question 14

12)As the energy increases, what happens to the PDD?

A) Increases
B) Decreases
C) Remains the same
D) No effect

Question 15

21)A tray holding custom blocks is measured as 97 cGy, and the dose without the tray is 100 cGy.What is the tray transmission factor?

A) 1.03
B) 0.97
C) 1.00
D) Not enough information to determine

Question 16

20)A wedge filter is used to alter the isodose distribution on an anterior chest field.The monitor units setting must be ___________ to account for the wedge filter in the field.

A) increased
B) decreased
C) unchanged

Question 17

16)As the energy increases, the tissue-air ratio does which of the following?

A) Increases
B) Decreases
C) Remains the same
D) No effect

Question 18

9)What is a TAR at the depth of Dmax used to correct for scatter of dose called?

A) Scatter-air ratio
B) Tissue-phantom ratio
C) Backscatter factor
D) Mayneord's F factor

Question 19

4)Mayneord F factor is used in which of the following?

A) Isocentric setups when distance is increased
B) Isocentric setups when field size changes
C) SSD setups when field size changes
D) SSD setups when distance increases

Question 20

10)As a field size increases from the standard established, the output factor for a treatment machine will do what?

A) Increase
B) Decrease
C) Remain the same

Question 21

30)The physician prescribes the AP/PA field to be weighted 3:1 for the posterior field.How much dose is coming from the AP and the PA if total dose prescribed is 200 cGy?

A) AP = 50 cGy, PA = 150 cGy
B) AP = 75 cGy, PA = 125 cGy
C) AP = 150 cGy, PA = 50 cGy
D) AP = 175 cGy, PA = 25 cGy

Question 22

32)Calculate the PDD of depth (d)if the dose at Dmax is 250 cGy and the dose at d is 231 cGy.

A) 70%
B) 75%
C) 85.2%
D) 92.4%

Question 23

31)Calculate the PDD of depth (d)if the dose at Dmax is 180 cGy and the dose at d is 127 cGy.

A) 47%
B) 70.6%
C) 56%
D) 75%

Question 24

28)A patient is treated on a 6-MV accelerator; field size is 10 ´ 12 cm and depth of tumor is 6 cm.What is the PDD?

A) 83.02
B) 83.18
C) 83.0
D) 83.2

Question 25

25)Find the PDD at 10 cm of a 6-MV beam of radiation that has a 5 ´ 5 cm treatment field.

A) 68%
B) 64.1%
C) 65.1%
D) 62.8%

Question 26

29)A patient is treated on a 6-MV machine at 100 cm SSD.The collimator setting is 15 ´ 15.There is no blocking used for this treatment.The prescription states that a dose of 3000 cGy is to be delivered to a depth of 3 cm in 10 fractions using a posterior treatment field using the nonisocentric method of calculation.The patient central axis separation is 20 cm.What is the MU setting for the treatment? Reference dose rate (RDR)for a 15 cm equivalent square is 0.993 cGy/MU.

A) 315
B) 321
C) 319
D) 317

Question 27

27)Find the PDD at 3.5 cm of an 18-MV beam of radiation that has a 23 ´ 15 cm treatment field.

A) 99.8%
B) 98%
C) 100%
D) 98.8%

Question 28

40)A patient is being treated on the 18-MV beam to an AP/PA stomach (midline).The collimator setting is 12 ´ 12 with no blocks.The dose is 4400 cGy/22 fractions.The separation is 24 cm.The cord is 4 cm from the PA.What dose is delivered to the spinal cord?

A) 206.4 cGy
B) 180 cGy
C) 220 cGy
D) 133.7 cGy

Question 29

38)Methods of obtaining dose uniformity across field junctions include which of the following?
I)Dosimetric isodose matching
II)Separated fields
III)Junction shift
IV)Half beam blocking
V)Geometric matching

A) I, II, III, and V
B) I, III, IV, and V
C) II, III, IV, and V
D) III, IV, and V

Question 30

37)Which of the following are adjacent fields that match on the surface and have an overlap at depth due to divergence?

A) Separated fields
B) Abutted fields
C) Half beam blocking fields
D) Geometric matching

Question 31

39)A patient is being treated on the 6-MV beam to an opposed lateral whole brain (midline).The collimator setting is 14 ´ 18 with no blocks.The dose is 4000 cGy/20 fractions.The separation is 22 cm.What is the dose delivered to Dmax?

A) 200 cGy
B) 212 cGy
C) 150 cGy
D) 61.9 cGy

Question 32

35)Find the skin gap necessary to match two symmetric ports at midline that are 21 and 37 cm in length.Assume the midline depth is 13 cm and both ports are treated at 100 cm SAD.

A) 1.53
B) 3.6
C) 2.5
D) 3.77

Question 33

24)What is the depth from the PA if the SAD is 100 cm and the SSD is 89 cm?

A) 8 cm
B) 9 cm
C) 10 cm
D) 11 cm

Question 34

26)Find the PDD at 19 cm of an 18-MV beam of radiation that has a 12 ´ 12 cm treatment field.

A) 56%
B) 55.6%
C) 53.7%
D) 51.7%

Question 35

34)A patient is being treated on the 18-MV beam to a PA spine at a depth of 10 cm.The collimator setting is 10 ´ 13 with no blocks.The dose is 300 cGy per fraction.What is the dose delivered to Dmax?

A) 375 cGy
B) 240 cGy
C) 400 cGy
D) 300 cGy

Question 36

36)A patient is treated using two superiorly/inferiorly adjacent fields.The collimator setting for the superior field is 10 cm wide by 13 cm long.The collimator setting for the inferior field is 20 cm wide by 35 cm long.Both fields are treated at 100 cm SSD.Calculate the gap at the skin surface if the fields abut at a depth of 5 cm.

A) 1.5
B) 1.2
C) 2.0
D) 1.0

Question 37

23)What is the patient's depth from the AP if the source-axis distance (SAD)is 100 cm and source-skin distance (SSD)is 91 cm?

A) 8 cm
B) 9 cm
C) 10 cm
D) 11 cm

Question 38

22)What is the patient separation if depth from the anterior posterior (AP)is 9 cm and depth from posterior anterior (PA)is 11 cm?

A) 18 cm
B) 19 cm
C) 20 cm
D) 21 cm

Question 39

33)Using the 6-MV PDD chart, what is the dose at 10 cm if the dose at Dmax is 300 cGy per day on a 6-MV beam for a field size of 15 ´ 18?

A) 205 cGy
B) 210 cGy
C) 215 cGy
D) 225 cGy