Deck 24: Photon Dosimetry Concepts and Calculations

Question 1

The equivalent square of rectangular field of 10 x 15 is:

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

Answer 1

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

Question 2

Dmax is defined as:

A) build-up region
B) depth of maximum equilibrium of occurs for photon beams
C) depth of dose deposited per unit of time
D) dose scatter to surface

Answer 2

B
Depth of maximum equilibrium,known as Dmax,is where the maximum dose is deposited for that beam.

Question 3

A wedge filter is used to alter the isodose distribution on an anterior chest field.The MU setting must be ________________ to account for the wedge filter in the field.

A) increased
B) decreased
C) unchanged

Answer 3

A
Anything in the path of the beam will attenuate the beam; therefore,more MU is needed to deliver the correct dose to the patient.

Question 4

Absorbed dose at a given depth divided by the dose in air dose at a fixed reference is the definition of:

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

Question 5

As the SSD increases,the TAR:

A) increases
B) decreases
C) remains the same
D) shows no effect

Question 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 7

As the field size increases,the PDD:

A) increase
B) decrease
C) remains the same
D) there is no effect

Question 8

A TAR at the depth of Dmax used to correct for scatter of dose is called the _________.

A) scatter-air ratio
B) tissue-phantom ratio
C) backscatter factor
D) Mayneord's F factor

Question 9

All of the following apply to "absorbed dose" except:

A) energy deposited at a point
B) measured in Gray
C) measured in air
D) can measure absorbed dose using ionization chambers

Question 10

Absorbed dose at depth x 100% = absorbed dose at Dmax is the definition of:

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

Question 11

As the depth increases,the PDD:

A) increases
B) decreases
C) remains the same
D) there is no effect

Question 12

The intensity of a radiation beam is measured at 10.0 mR/hr at a distance of 20 cm.What will be the intensity of this beam at 30.0 cm?

A) 22.5 mR/hr
B) 4.44 mR/hr
C) 6.67 mR/hr
D) 15.0 mR/hr

Question 13

As the energy increases,the TAR:

A) increases
B) decreases
C) remains the same
D) shows no effect

Question 14

As the energy increases,the PDD:

A) increases
B) decreases
C) remains the same
D) there is no effect

Question 15

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) 103
B) 97
C) 100
D) not enough information to determine

Question 16

As a field size increases from the standard established,the output factor for a treatment machine will _______________.

A) increase
B) decrease
C) remain the same

Question 17

Mayneord F factor is used in:

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 18

As the field size increases,the TAR:

A) increases
B) decreases
C) remains the same
D) there is no effect

Question 19

As the SSD increases,the PDD:

A) increases
B) decreases
C) remains the same
D) there is no effect

Question 20

The equivalent square of rectangular field of 20 x 10

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

Question 21

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 22

A patient is treated on a 6-MV accelerator; field size is 10 x 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 23

What is the patient separation if depth from the AP is 9 cm,and depth from PA is 11 cm?

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

Question 24

A patient is being treated on the 6-MV beam to an opposed lateral whole brain (midline).The collimator setting is 14 x 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 25

Adjacent fields that have an overlap at depth due to divergence are called:

A) separated fields
B) abutted fields
C) half beam blocking fields
D) geometric matching

Question 26

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

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

Question 27

A patient is being treated on the 18-MV beam to an AP/PA stomach (midline).The collimator setting is 12 x 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 is the dose delivered to the spinal cord?

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

Question 28

According to the text,methods of obtaining dose uniformity across field junctions include: 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 29

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

A) 56.0 %
B) 55.6 %
C) 53.7 %
D) 51.7 %

Question 30

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

A) 68.0 %
B) 64.1 %
C) 65.1 %
D) 62.8 %

Question 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 32

What is the patient's depth from the AP if the SAD is 100 cm and SSD is 91 cm?

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

Question 33

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

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

Question 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 x 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 35

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 36

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 x 18?

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

Question 37

A patient is treated on a 6-MV machine at 100 cm SSD.The collimator setting is 15 x 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 38

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 39

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