Characteristics of Mevatron 77 15-MV photon beam

The characteristics of 15-MV photon beam of Mevatron 77 have been examined with respect to clinical parameters. Some of the characteristics of this 15-MV photon beam are different from those of similar machines used in radiotherapy. The tissue-maximum ratio values have been determined experimentally for SAD 100 cm and percentage depth dose values have been determined from tissue-maximum ratio data. It is found that the experimentally determined PDD values for SSD 80, 100, and 120 cm are within +/- 1% of the calculated data. The scatter-maximum ratio data have been calculated from the experimentally determined zero area attenuation coefficient and tissue-maximum ratio data and are presented as function of depth and radii of circular fields. The relative dose factors have been determined both for square and rectangular fields. The relative dose factor for rectangular fields depends on the two pairs of jaws differently and the maximum difference is about 1.5%. The flatness and symmetry are within the specifications set by the manufacturer.