Generating a Homogeneous Dose Distribution in the Junction Region between Two Adjacent Fields in Electron Beam Therapy

Treatment with megavoltage electron beams is ideal for irradiating superficial tumors because of their limited range in tissues. However, for electron treatment of extended areas, such as the chest wall, two or more adjacent fields can be used. Abutment of these fields may lead to significant dose in homogeneities in the junction region. The aim of this study is to offer a new method for generating a homogeneous dose distribution in the junction region between two adjacent fields in electron beam therapy. Several approaches have been proposed to solve the problem of ‘hot’ and ‘cold’ spots in the junction region between abutting electron fields. These techniques are based on beam-edge modifying devices or penumbra generators which act to broaden the electron beam penumbra, and thus facilitate field matching. But use of these devices is time consuming and design of the modifications to the applicators are generally applicator dependent. An idea which was originally proposed for matching two adjacent photon fields (with dose inhomogeneity of about 2%) is resurrected here. This method is based on the rotation of the gantry such that the adjacent fields have a common edge and the overlap region in treatment volume is eliminated. For this purpose, the effective source to surface distance (SSDeff) for the available electron beam energies (6, 9, 12 and 15 MeV) and applicators (cones) (6 × 6, 10 ×10, 15 ×15, 20 × 20 and 25 × 25 cm2) have been determined for a Varian 2100C linear accelerator. Using SSDeff, in respect to beam divergence, one can use the photon beam behavior for electron beams and achieve a uniform dose distribution in adjacent electron fields.Discussion and Compared to beam-edge modifying devices or penumbra generators that are usually time consuming to plan and set up, rotating the gantry to eliminate the overlap region is simple and applicable in the problem of abutting electron fields.