Monitor Unit Calculation Practice Problem
A patient is being treated with radiation therapy using a SSD of 105 cm and
a square field size of 8.5 cm2 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 8.3 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 = 8.3 \: cm, SSD = 100 \: cm) |
| 8.1 | 0.992 | 0.996 | 72.8 |
| 8.5 | 0.994 | 0.997 | 73.1 |
| 9.2 | 0.997 | 0.998 | 73.7 |
| 10.0 | 1.000 | 1.000 | 74.4 |
MU