Monitor Unit Calculation Practice Problem

A patient is being treated with radiation therapy using a SSD of 98 cm and a rectangular field size of 13.4 cm by 16.8 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 18.6 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 = 18.6 \: cm, SSD = 100 \: cm)
13.4 1.014 1.007 43.9
14.9 1.020 1.010 44.5
15.2 1.021 1.010 44.6
17.7 1.031 1.015 45.6


MU