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 Bjärngard, 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.
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