Monitor Unit Calculation Practice Problem

A patient is being treated with radiation therapy using a SAD of 100 cm and a square field size of 8.9 cm2 at the isocenter. The medical linear accelerator is calibrated to deliver 1 cGy/MU at a depth t0 = dmax = 1.5 cm, using a setup SSD of 100 cm and a field size of 10 cm x 10 cm. What is the number of monitor units (MU) required to deliver an absorbed dose of 200 cGy to a depth of 10.5 cm (at the isocenter), on the central axis of the beam?

**Use the data provided below for square field sizes. (You will not need all the data provided, and some data may need to be derived using the F factor or the TMR/PDD conversion equation.)

Field Size (cm) S_c S_p PDD(d = 10.5 \: cm, SSD = 100 \: cm)
8.0 0.992 0.996 64.2
8.9 0.996 0.998 65.0
9.0 0.996 0.998 65.1
10.0 1.000 1.000 66.1


MU