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Scatter Factor and Dose Per MU Verification for FFF Fields


A Sharikova

A Sharikova*, T Zhu, Univ Pennsylvania, Philadelphia, PA

SU-E-T-321 Sunday 3:00PM - 6:00PM Room: Exhibit Hall

Purpose:
To calculate scatter factor (SF) for flattening filter free (FFF) fields using pencil-beam convolution and equivalent square methods, and determine the accuracy of the predicted dose per MU in both cases by comparison with experimental data.

Methods:
Scatter factor and dose per MU were calculated for 6xFFF and 10xFFF linac Varian TrueBeam at the benchmark points using two methods. The equivalent square method assumed constant SF based on the field size. In the convolution method, the off-axis scatter factor values were found by calculating a convolution of an extrapolated scatter kernel, based on measured data, with primary off-axis ratio (POAR) values. Essentially, the equivalent square method assumed a flat beam profile, while the convolution method took into account the actual shape of the beam.

Results:
The normalized scatter factor values were up to 25% different between two methods. Since the convolution method took into account that FFF fields are non-uniform, it was superior for the accurate FFF beam dose calculation.

Conclusion:
This work indicates the importance of precise scatter factor determination to accurately predict D/MU for FFF beams due to non-uniform beam profile. The implication for secondary MU calculation will be discussed.

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