Dosimetric considerations for a multileaf collimator system.

A commercial linear accelerator with a factory-fitted multileaf collimator (MLC) was commissioned for clinical use. Measurements made of dosimetric parameters included central axis depth-dose, field-size factors, wedge factors, penumbra, and leaf leakage for the 6-MV and 15-MV photon beams available on this accelerator. The depth-dose characteristics, output factors, and transmission factors were similar to those reported in the literature for a machine by the same manufacturer with a standard treatment head. Because of scalloping, the effective penumbra for the MLC was 3 to 4 mm wider than that for the conventional collimator jaws. The output for the fields shaped by the MLC was generally lower than that for similar fields shaped with Lipowitz's metal (Cerrobend). The magnitude of the difference was field-size dependent and ranged from 0.5% to 4.5% for open shaped fields, increasing to 1% to 5% in the presence of wedges. Further analysis of this observation has shown it to be primarily due to differences in the scattered radiation from the collimator head.