Treatment Planning Flashcards

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

Answer: Percent depth dose (PDD)

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

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?

Answer: 0.826

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

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

Answer: 13.3 cm

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

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

Answer: Tissue-maximum ratio (TMR)

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

The Mayneord F factor is used to:

Answer: Convert PDD from one SSD to another SSD

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

For two beams with a hinge angle of 90 degrees, what is the ideal wedge angle?

Answer: 45 degrees

Wedge angle = 90 - (hinge angle / 2) = 90 - 45 = 45 degrees.

When matching two adjacent fields on the skin, the gap is calculated to make the fields meet:

Answer: At a chosen depth inside the patient

Skin gaps let divergent beam edges meet at a chosen depth, which avoids overlapping hot spots over critical structures.