RT Treatment Planning 2

Question 1

For a fixed-SSD (SSD) treatment setup, which depth dose quantity is typically used in the monitor unit calculation?

Accepted Answer

Percent depth dose (PDD)

Answer

PDD is defined at a fixed SSD, so it is the natural quantity for SSD-based monitor unit calculations.

Question 2

A beam is calibrated at 100 cm. Approximately what fraction of the dose rate is delivered at 110 cm, based on the inverse square law?

Accepted Answer

0.826

Answer

(100/110)^2 = 0.826, so the dose rate drops to about 82.6%.

Question 3

What is the equivalent square of a 10 cm x 20 cm rectangular field using the area/perimeter (4A/P) method?

Accepted Answer

13.3 cm

Answer

4A/P = 4(200)/60 = 13.3 cm.

Question 4

Which dosimetric quantity is most commonly used for monitor unit calculations in an isocentric (SAD) setup?

Accepted Answer

Tissue-maximum ratio (TMR)

Answer

TMR does not depend on SSD, so it suits SAD setups where the point of calculation is at isocenter.

Question 5

The Mayneord F factor is used to:

Accepted Answer

Convert PDD from one SSD to another SSD

Answer

The Mayneord F factor approximately corrects PDD for a change in SSD by applying inverse-square terms.

RT - Radiation Therapist Certification Practice Test