Room: Exhibit Hall | Forum 7
Purpose: Beam data for an empirical algorithm is determine to calculate the dose per MU accurately for a Halcyon flattening-filter free (FFF) photon beam, which can be used for secondary MU check.
Methods: A kernel-based algorithm based on five parameters 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 â€“ 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.
Results: 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 28 cm and depth between 0 and 30 cm to a max and standard deviations of 1.9% and 1.0%, respectively, for Halcyon 6x FFF. The value of b is 0.022 from fitting the POAR. The resulting phantom scatter parameters are consistent with those obtained from MC simulation. Benchmark points are established on and off-axis to check the calculated dose per MU at 23 points and they generally agrees to within 3% except for regions of dose disequilibrium and off-axis points.
Conclusion: Halcyon photon beam has lower photon energy than Varian Truebeam for the same 6xfff photons. The D/MU based on the beam data agrees with measurement within 3% except for regions of dose disequilibrium and off-axis points where up to 4.1% difference are observed.