Monitor Unit Calculation Practice Problem
A patient is being treated with radiation therapy using a SAD of 100 cm and
a circular field with radius of 15.4 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 4.0 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 = 4.0 \: cm, SSD = 100 \: cm) |
| 10.0 | 1.000 | 1.000 | 91.3 |
| 26.2 | 1.065 | 1.032 | 92.3 |
| 27.3 | 1.069 | 1.035 | 92.3 |
| 27.7 | 1.071 | 1.035 | 92.4 |
MU