Experimental characterization of the dosimetric leaf gap

Abstract

Transmission through the rounded leaf ends of Varian MLCs causes a discrepancy between the dosimetric and geometric field widths, defined as the dosimetric leaf gap (DLG). This parameter is necessary for Varian Eclipse dose calculations to accurately model field modulation. Despite the results of several studies noting the effect of incorrect DLG measurements on dose calculations, the dependency of DLG on factors such as SSD, depth of measurement, beam energy, dose rate, and ionization chamber used for the measurement has not been thoroughly investigated. This study measures DLG using an integral dose (sliding gap) method to characterize these dependencies. Results show that DLG is insensitive to variations in SSD (following correction for geometric projection), depth of measurement, dose rate, and ionization chamber, while an expected increase is observed with higher beam energy. Periodic DLG quality assurance measurements should be accurate within 1.31% ± 0.09% with consistent measurement conditions.