Monitor Unit Calculation Practice Problem
A patient is being treated with radiation therapy using a SSD of 103 cm and
a circular field with radius of 11.4 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 28.3 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 = 28.3 \: cm, SSD = 100 \: cm) |
| 11.4 | 1.006 | 1.003 | 25.4 |
| 19.6 | 1.038 | 1.019 | 28.4 |
| 20.2 | 1.041 | 1.020 | 28.6 |
| 25.8 | 1.063 | 1.032 | 30.7 |
MU