Calculation of dose profiles in homogeneous phantoms for irregular, partially attenuated, photon beams.

Measured and calculated dose profiles under partial attenuators which cover only part of the radiation beam are presented. The study was performed for x-ray beams generated with a 4-MV linear accelerator at a source-surface distance of 120 cm in a water phantom for lead attenuators of arbitrary shape but constant thickness. Dose correction factors, which are used to multiply the open beam data to predict doses under partial attenuators, depend strongly on attenuator parameters, such as its thickness, lateral dimensions, and distance from phantom or patient surface, in addition to depending on depths in the phantom. The dose correction factors are calculated with Clarkson sector integration techniques, and the results, in spite of the simplifying assumptions used in the algorithm, generally agree with measured data to within 3%. The calculational method therefore may be applied to general clinical situations in which partial attenuators are used.