Dosimetric effects of matching 6 MV photon and electron fields in the treatment of head and neck cancers

The purpose of this study is to evaluate the dosimetry across the junction between an enface 9 MeV electron field and 6 MV parallel opposing photon fields for two photon configurations: (1) laterally opposed divergent beam set-up and (2) laterally opposed half-beam (non-divergent) set-up using asymmetric collimator jaws. In this study, film dosimetry technique was performed to measure dose profiles at depths of 1, 2, and 3 cm in the junction of the matching photon and electron fields. In order to investigate the changes in the dose distributions due to set-up uncertainties, dose profiles were measured at these depths using no gap, 2 and 4 mm overlaps and gaps between the photon and electron fields. A 2 mm gap resulted in approximately +15% and +20% hot spots in the photon field at 1 and 3 cm depths, respectively, for divergent photon beams. However, at 2 cm depth, an approximately +30% hot spot and -10%-10% cold spot occurred in the junction region. Four millimeter overlap and gap resulted in an unacceptable dose inhomogeneity in the junction. As a result of this study, the magnitudes of hot and cold spots might be clinically acceptable for 2 mm gap between photon and electron fields.