SU-E-T-523: Modeling Beam Data for Flattening Filter Free (FFF) Photon Beams

For flattening-filter free (FFF) photon beams, conventional algorithm based on equivalent-square to calculate dose per MU is invalid because of the non-uniform profile. In this study, an empirical algorithm is developed to calculate the dose accurately, which can be used for secondary MU check for IMRT using FFF beams.

A kernel-based algorithm based on three parameters (a0, w0, d0) is used to quantify the phantom scatter characteristics of the photon beam. The model is modified to quantify the shape of the FFF at off-axis locations by fitting the primary off-axis ratio (POAR) by a linear function 1 - br, where b is a constant and r is the radial distance. The resulting parameters are used in a kernel-based dose calculation algorithm for dose calculation.

It is found that the proposed model can fit the product of the fractional depth doses (FDD) and phantom scatter factors (Sp) for field sizes between 2 and 40 cm and depth between 0 and 40 cm to a max and standard deviations of 1.7% and 0.01% and 1.8% and 0.01%, respectively, for 6 and 10 MV FFF beams. The value of b is 0.025 and 0.0323 for 6 MV and 10 MV photons, respectively, from fitting the POAR. The resulting phantom scatter parameters are consistent with those obtained from MC simulation. If the slope is not taken into account (b = 0), then the model cannot fit the central-axis Sp*FDD accurately and resulted in a maximum error of 3% and 4% for 6 and 10 MV, respectively.

We have demonstrated that the shape of POAR from FFF beam will impact on the dose calculation on the central-axis. Conventional equivalent square law concept will not be applicable for dose calculation for FFF beams.