The Concept of Modified Scatter-Phantom Ratio and its Application to Irregular Field Dosimetry at 8MV

Abstract

SUMMARY The concept of Modified Scatter‐Phantom Ratio has been applied to the calculation of central axis depth dose for irregular fields. The results are in good agreement with data obtained by direct measurement. Comparisons between the scatter summation method and the ‘equivalent square’ procedure suggest the former is superior. However, the errors in the ‘equivalent square’ method are small, and may be compensated by the simplicity of performing the calculation. With the parallel between MSPR and SAR established, the calculation procedure may be extended to points off the central axis along the lines suggested by Cundiff et al, 1973, in mantle techniques. ABSTRACT Modified Scatter‐Phantom Ratios are introduced as a formal extension of the concept of Scatter‐Air Ratios for beam energies greater than 2MV. Central Axis depth dose measurements for rectangular and irregular fields at 8MV are compared with calculations based on summation of primary and scattered radiation. Comparisons are also made with calculations using the equivalent square formula pointed out by Worthley(1966).