A method for producing uniform dose distributions in the junction regions of large hinge angle electron fields.

PURPOSE: The planning problems presented by abutting electron fields are well recognized. Junctioning electron fields with a large hinge angle compounds the problems because of the creation of closely situated high dose and low-dose regions. METHODS AND MATERIALS: The technique involving a compensated superficial x-ray (SXR) field to treat the junction region between electron fields was developed and used in a particular clinical case (treatment of a squamous cell carcinoma of the forehead/scalp). The superficial x-ray beam parameters were chosen and the compensator was designed to make the SXR field complementary to the electron fields.
RESULTS: Application of a compensated SXR field eliminated low dose zones in the junction region and reduced high dose zones to 110%. In the clinical case discussed, the high-dose areas due to the SXR field would not appear because of increased attenuation of the soft X-rays in bone.
CONCLUSION: The technique proposed produces uniform dose distribution up to 3 cm deep and can be considered as an additional tool for dealing with electron field junctioning problems.