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Buildup region and depth of dose maximum of megavoltage x-ray beams.

The depth of dose maximum, dmax, of megavoltage x-ray beams was studied as a function of beam energy and field size for 6-, 10-, and 18-MV x-ray beams and field sizes ranging from 1 x 1 to 30 x 30 cm2. For a given beam energy, dmax increases rapidly with increasing field size at small fields, reaches a maximum around 5 x 5 cm2 and then gradually decreases with increasing field size for large fields. Monte Carlo simulations combined with measurements verified that the effect observed at small field sizes is caused by in-phantom scatter, while at large fields the effect is due to scatter contamination of the primary beam from the linac head. A comparison between the dmax behavior of flattened beams to that of unflattened beams indicates that the dmax decrease at large fields for flattened beams is caused mainly by contamination electrons which are produced in the flattening filter and further scattered by collimator jaws and air.