Monitor Unit Calculation Practice Problem

A patient is being treated with radiation therapy using a SSD of 103 cm and a rectangular field size of 9.7 cm by 10.8 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 17.8 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 = 17.8 \: cm, SSD = 100 \: cm)
9.7 0.999 0.999 44.0
9.9 1.000 1.000 44.2
10.2 1.001 1.000 44.3
12.0 1.008 1.004 45.1


MU