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In some treatment protocols different parts of a target volume need to be irradiated with different doses. This can be achieved with a four-beam irradiation technique and dose rate modifying absorbers. The
calculation of the transmission and thickness of the absorbers are based on the unmodified doses from single beam at a calculation point. Corrections are done by a modified graphic beam-zone method to consider the influence of the absorbers on scattered radiation. Phantom measurements show good agreement with the calculated values. The modification method is applicable without 3D planning systems but requires a small number of beams to limit the calculation and manufacture efforts.
