Quantification of the role of lead foil in flattening filter free beam reference dosimetry

PURPOSE: To quantify the potential error in outputs for flattening filter free (FFF) beams associated with use of a lead foil in beam quality determination per the addendum protocol for TG‐51, we examined differences in measurements of the beam quality conversion factor k(Q) when using or not using lead foil. METHODS: Two FFF beams, a 6 MV FFF and a 10 MV FFF, were calibrated on eight Varian TrueBeams and two Elekta Versa HD linear accelerators (linacs) according to the TG‐51 addendum protocol by using Farmer ionization chambers [TN 30013 (PTW) and SNC600c (Sun Nuclear)] with traceable absorbed dose‐to‐water calibrations. In determining k(Q), the percentage depth‐dose at 10 cm [PDD(10)] was measured with 10×10 cm(2) field size at 100 cm source‐to‐surface distance (SSD). PDD(10) values were measured either with a 1 mm lead foil positioned in the path of the beam [%dd(10)(Pb)] or with omission of a lead foil [%dd(10)]. The %dd(10)x values were then calculated and the k(Q) factors determined by using the empirical fit equation in the TG‐51 addendum for the PTW 30013 chambers. A similar equation was used to calculate k(Q) for the SNC600c chamber, with the fitting parameters taken from a very recent Monte Carlo study. The differences in k(Q) factors were compared for with lead foil vs. without lead foil. RESULTS: Differences in %dd(10)x with lead foil and with omission of lead foil were 0.9 ± 0.2% for the 6 MV FFF beam and 0.6 ± 0.1% for the 10 MV FFF beam. Differences in k(Q) values with lead foil and with omission of lead foil were −0.1 ± 0.02% for the 6 MV FFF and −0.1 ± 0.01% for the 10 MV FFF beams. CONCLUSION: With evaluation of the lead foil role in determination of the k(Q) factor for FFF beams. Our results suggest that the omission of lead foil introduces approximately 0.1% of error for reference dosimetry of FFF beams on both TrueBeam and Versa platforms.