Abstract
In the calculation of rectangular beam isodose profiles, three field-shaping constants are required in Cunningham's source equation. The method described uses photometric analysis of two field sizes at maximum build-up depth. For each field a collimator jaw transmission constant t is measured directly; a penumbra constant alpha is determined by fitting relative dose data obtained densitometrically from a radiographic beam cross-section image with a calibrated electro-optical image quantizer. For program IRREG, which calculates the doses at points in planes normal to beam axis, alpha and t can be determined as well for blocked mantle fields of irregular cross-section. For program BEAM, beam width at half-maximum and alpha in turn determine two constants beta and gamma which model beam dose profile as well as penumbra for any size beam. Values of the constants and the resulting dose distributions computed for several Co-60 and Linac units are presented; comparison of calculated with experimentally measured distributions are made and practical requirements for accuracy of the method discussed.
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