Monitor Unit Calculation Practice Problem
A patient is being treated with radiation therapy using a SAD of 100 cm and
a rectangular field size of 14.2 cm by 17.5 cm at the isocenter. 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 22.9 cm (at the isocenter), 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 = 22.9 \: cm, SSD = 100 \: cm) |
| 10.0 | 1.000 | 1.000 | 33.7 |
| 12.1 | 1.008 | 1.004 | 34.6 |
| 15.7 | 1.023 | 1.011 | 36.1 |
| 15.9 | 1.024 | 1.012 | 36.2 |
MU