[Dose calculation for very small irregular electron beam fields. 1. Dose calculation for the central beam using a simple field zone method].

Very small electron beams show considerable reduction of the dose output factor, the therapeutically relevant range, the practical range and the therapeutically relevant field size, compared to broad beams. The first three effects were measured on the central beam axis for different quadratic field sizes and are presented in the first part of this publication. A simple field zone method for calculation of irregular shaped electron beams was developed from the different depth dose curves. The method allows the determination of the maximum dose and the 80% range with sufficient accuracy for practical purposes. The examination of the reduction of the therapeutically relevant field size and the consequences following thereof with respect to the adequate electron therapy of a small irregular target volume will be published in a second part.