An investigation of equivalent square formulas

Abstract

Head scatter factors can be determined either experimentally or using published formulas. Two available formulas are the 4A/P method and a formalism presented by Vadash and Bjärngard¹, of which the second accounts for collimator exchange effect. It was the aim of this study to compare the accuracy of these formulas in calculating head scatter factors for a Siemens Oncor linear accelerator. Head scatter factors were measured using a mini-phantom, and equivalent squares were then determined by fitting to the square field data. These were compared with calculated equivalent squares using the Vadash formalism, and a parameter was found that minimised differences between calculated and measured data. The Vadash and 4A/P formulae were then used to calculate head scatter factors for a range of field sizes, and compared relative to measured data. It was found that the Vadash formalism improved the accuracy of calculated head scatter factors by an average 0.5% for Siemens Primus and Oncor linear accelerators across all field sizes and energies, when compared with the 4A/P method.