Measurement and application of the optimum value of head scatter correction factors in Radcalc for 6MV photon beams from varian linear accelerators.

To determine the optimum value of head scatter correction factor ([Formula: see text]) used in Radcalc software. The head scatter factors for a selection of multi-leaf collimator fields were measured on a Varian TrueBeam Edge and TrueBeam linear accelerators using an ionisation chamber in a mini-phantom. Radcalc calculated the head scatter values for the same fields. Radcalc calculates head scatter as [Formula: see text]. The head scatter value from Radcalc was recorded when the value of [Formula: see text] was set to 0 or 1. An optimum value of [Formula: see text] was obtained by minimising the sum of the differences between measured and calculated. The optimum values of [Formula: see text], for each linear accelerator type, were applied to clinical patient volume modulated arc therapy calculations. Minimising the summed differences yielded optimum values of [Formula: see text] of 0.149 and 0.276 for the TrueBeam Edge and the TrueBeam datasets respectively. Applying these values to 100 clinical patient volume modulated arc therapy plans, for each linear accelerator type, reduced the mean difference between the primary calculation and the independent check from 0.55 ± 0.95% (µ ± σ) to -0.11 ± 0.85% for the TrueBeam Edge and from 0.79 ± 1.16% to 0.24 ± 0.90% for the TrueBeam plans compared to when a generic Sc value of 0.675 was used. Using optimal values of [Formula: see text]in Radcalc, determined by measurement, reduced the mean monitor unit difference when compared to the primary calculation of a treatment planning system compared to using the standard value of 0.675.