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Monitor Unit Calculation Practice Problem

A patient is being treated with radiation therapy using a SSD of 98 cm and a square field size of 8.5 cm2 at the surface. The SAD for this machine is 100 cm. 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 19.5 cm, 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 = 19.5 \: cm, SSD = 100 \: cm)
8.5 0.994 0.997 39.4
8.7 0.995 0.997 39.5
10.0 1.000 1.000 40.6
10.2 1.001 1.000 40.6


MU