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Monitor Unit Calculation Practice Problem

A patient is being treated with radiation therapy using a SAD of 100 cm and a circular field with radius of 9.2 cm 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 9.2 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 = 9.2 \: cm, SSD = 100 \: cm)
10.0 1.000 1.000 71.1
14.8 1.019 1.010 72.0
16.3 1.025 1.013 72.3
16.5 1.026 1.013 72.3


MU