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Monitor Unit Calculation Practice Problem

A patient is being treated with radiation therapy using a SSD of 101 cm and a rectangular field size of 12.6 cm by 12.9 cm 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 5.3 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 = 5.3 \: cm, SSD = 100 \: cm)
12.6 1.010 1.005 86.4
12.6 1.010 1.005 86.4
12.7 1.011 1.005 86.4
13.4 1.014 1.007 86.4


MU