Matching photon and electron fields in the treatment of head and neck tumors

Radiotherapy of head and neck tumors frequently involves joining photon and electron fields. In such situations narrow penumbras combined with relatively small positioning errors can lead to significant "hot" and "cold" spots in the vicinity of the join-up. The objective of this work was to devise penumbra spreading techniques which lead to a relatively uniform dose distribution in the join-up region of these fields and which reduce the effect of positioning errors on dose uniformity. A stepped edge attenuator was used to obtain a wider penumbra for the 4-MV x-ray beam and a Lucite scatterer was used for the 10-MeV electron beam. The resulting composite beam profiles from these "modified" abutting photon and electron fields are provided and the effects of positioning errors on dose uniformity across the junction are illustrated. These profiles are compared with those resulting from "unmodified" adjacent electron and photon beams.