The physics behind the use of a negatively weighted beam to calculate the dose distribution under a midline block in computerized treatment planning systems is reviewed. To correctly reproduce the dose under the block, it is necessary and sufficient to know the relative dose at one reference depth on the central axis under the block. If the relative weight of the negative beam is then adjusted to produce agreement at that depth, good agreement can be obtained throughout. Comparisons between calculated and measured dose distributions under a midline block are presented for two therapy beams with 4MV and 6MV nominal energies.
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