Question 1

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.

Accepted Answer

Use the formula: Gap = (13/2 x 5/100)x (35/2 x 5/100)

Question 2

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?

Accepted Answer

The equivalent square is 10.9.Using interpolation,the PDD is 83.18.

Question 3

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

Accepted Answer

There is a depth of 9 cm from the AP and a depth of 11 cm from the PA for a total separation of 20 cm.

Question 4

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?

Accepted Answer

First the equivalent square must be determined.A 14 x 18 field equivalent square is 15.75.The entrance and exit dose must be calculated.The PDD must be determined at Dmax (entrance dose)and the exit dose.

Question 5

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

Accepted Answer

Abutting fields match on the skin surface,and unless the divergence is eliminated,there would be overlap at depth.

Question 6

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

Accepted Answer

Using the ratio formula to calculate dose at various points,you can also determine the PDD.
250 cGy = 231 cGy
100% x

Question 7

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?

Accepted Answer

First the PDD for 12 cm,4 cm,and 20 cm must be determined.The 12 cm represents the midline structure where the physician has prescribed 100 cGy per field.The 4 cm represents the spinal cord depth from the PA,and the 20 cm represents the cord depth from the AP.The dose to 4 cm and 20 cm must be determined and then added together.

Question 8

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

Accepted Answer

Dosimetric isodose matching is accomplished by using the treatment planning computers to determine hot and cold spots and viewing isodose curves.A junction shift moves the edges of the adjacent fields.Over the course of treatment,the hot or cold spot is feathered.Half beam blocking is blocking along the central ray of the beam where there is no divergence,thereby eliminating divergence at depth.Geometric matching mathematically calculates the divergence of fields to match at a point in depth.

Question 9

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

Accepted Answer

Using the PDD 18-MV beam data table,locate the equivalent square of 12 x 12 at a depth of 19 cm,and then look up the value at those parameters.

Question 10

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

Accepted Answer

Using the PDD 6-MV beam data table,locate the equivalent square of 5 x 5 at a depth of 10 cm,and then look up the value at those parameters.

Question 11

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

Accepted Answer

Using the ratio formula to calculate dose at various points,you can also determine the PDD.
180 cGy = 127 cGy
100% x

Question 12

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

Accepted Answer

If the SAD is 100 cm and the SSD is 91 cm,then 100 - 91 = 9 cm.

Question 13

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

Accepted Answer

The PDD for the 18-MV beam at a depth of 3.5 cm is 100% at any field size because this is Dmax.

Question 14

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?

Accepted Answer

First find the equivalent square using Sterling's formula.A 10 x 13 equivalent square is 11.3.Look up the PDD on the 18-MV chart (Table 24-7)and interpolate to get a PDD of 80.03.

Question 15

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

Accepted Answer

If the SAD is 100 cm and the SSD is 89 cm,then 100 - 89 = 11 cm.

Question 16

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?

Accepted Answer

First find the equivalent square using Sterling's formula.A 15 x 18 equivalent square is 16.36.Look up the PDD on the 6-MV chart (Table 24-6)and interpolate to get a PDD of 70.26 %.

Question 17

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.

Accepted Answer

The equivalent square is 15 cm.The daily dose is 300 cGy/day.The PDD for a 6-MV beam at 3 cm for 15-cm equivalent square is 95.2%.The RDR is 0.993 cGy/MU.
MU setting = --------------------------

Question 18

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.

Accepted Answer

Use the formula: Gap = (21/2 x 13/100)x (37/2 x 13/100)

Question 19

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?

Accepted Answer

Add the sum of weighting (3 + 1 = 4).Then divide the dose with the total number of parts (200 cGy/4).Multiply the total number with your answer (50 x 1= 50 cGy AP dose)(50 x 3 = 150 cGy PA dose).

Quiz 24: Photon Dosimetry Concepts and Calculations

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?

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

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?

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

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

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?

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

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

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

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

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

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

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?

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

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?

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.

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?

Quiz 24: Photon Dosimetry Concepts and Calculations

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?

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

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?

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

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

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?

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

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

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

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

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

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

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?

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

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?

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.

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?

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