Evaluation of a total scalp electron irradiation technique.

A dosimetric evaluation of a total scalp electron-beam irradiation technique that uses six stationary fields was performed. The initial treatment plan specified a) that there be a 3-mm gap between abutted fields and b) that the field junctions be shifted 1 cm after 50% of the prescribed dose had been delivered. Dosimetric measurements were made at the scalp surface, scalp-skull interface, and the skull-brain interface in an anthropomorphic head phantom using both film and thermoluminescent dosimeters (TLD-100). The measurements showed that the initial technique yields areas of increased and decreased dose ranging from -50% to +70% in the region of the field junctions. To reduce regions of nonuniform dose, the treatment protocol was changed by eliminating the gap between the coronal borders of abutted fields and by increasing the field shift from 1 cm to 2 cm for all borders. Subsequent measurements showed that these changes in treatment protocol resulted in a significantly more uniform dose to the scalp and decreased variation of doses near field junctions (-10% to +50%).