Dose calculations for arbitrarily shaped electron beams.

A method for the calculation of absorbed dose distributions of arbitrarily shaped electron beams is described. Isodose distributions and the output factor of a newly designed treatment field can be predicted with good accuracy, without the need for any dose measurement in the actual field. Two different Gaussian pencil beams are used as building elements for the treatment beams of each electron energy. The dose distributions of the pencil beams are derived from measurements of broad beam dose distributions; in this way the influence of electrons scattered by the applicator walls is taken into account. The contribution to the dose by electrons scattered from a high Z metal frame which defines the treatment field contour is calculated separately and added. This calculation is based on experimentally derived data. The method has been tested for electron beams with 6, 10, 14 and 20 MeV nominal energy. The distance between calculated and measured isodose lines with values between 90 and 10 per cent of the maximum dose did not exceed a limit of 0.3 cm. The difference between calculated and measured output factors remained within 2 per cent.