Peripheral dose from a linear accelerator equipped with multileaf collimation

In radiation therapy, knowledge of the peripheral dose is important when anatomical structures with very low dose tolerances might be involved. Two of the major sources of peripheral dose, leakage from the linac head, and scatter from secondary collimators, depend strongly on the configuration of the linac head and therefore might be affected by the presence of a multileaf collimator (MLC). In this study, peripheral dose was measured at two depths and two field sizes for 6 and 18 MV photons from a linac with a MLC. The MLC was configured both with leaves fully retracted and with leaves positioned at the field edges defined by the secondary collimator jaws. Comparative measurements were also made for 6 MV photons from a linac without MLC. Peripheral dose was determined as a percentage of the central axis dose for the same energy, field size, and depth using diode detectors in solid phantom material. The data for the 6 MV without MLC agreed with those for the beam with MLC leaves retracted. For both energies at all depths and distances from the field edge, configuring the MLC leaves at the field edge yielded a reduction in peripheral dose of 6%-50% compared to MLC leaves fully retracted.