A simple method for matching electron beams in radiotherapy.

Many solutions have been proposed to solve the problem of 'hot' and 'cold' spots in the junction between abutting electron fields. Although some of these methods have proved satisfactory, the designs of the modifications to the applicators are generally applicator dependent and involve measuring data for individual fields. An idea which was originally proposed as a solution to a different problem is resurrected here because it happens to solve the beam-matching problem and because it is very simple to apply. The idea is to cover the end of the electron applicator completely with a slab of plastic, called a 'spoiler'. Formulae are proposed and tested for estimating the penumbra broadening and the extent of over- or underdosing at field junctions. The technique is applicator independent, and no extra beam-data measurements need be made for computer treatment planning of matching fields. The price paid for such simplicity is that consideration needs to be given to leading-off the other borders of the treatment fields because of the increased penumbra widths.