Air-gap correction in electron treatment planning

In electron treatment planning involving irregular surface contours the technique of shifting standard isodose curves to compensate for air gaps is unsatisfactory for oblique angles of incidence. For electron beams at normal incidence on a flat surface, earlier workers reported that the broad electron beam may be considered as the sum of many narrow beams, and that the standard isodose curves may be reproduced using the data for a narrow beam derived from a collimation experiment. A description is given of a simple alternative experiment to derive the necessary data, which include electrons scattered from the applicator walls. Computed plans using these data for electrons incident at 450 on a flat surface and for incidence on an irregular surface agree with experiment. The method is compatible with, and was designed for, a very limited computing facility. (author)