SU‐E‐T‐536: Is BJR Supplement #25 Recommendation for Megavoltage Energy Independent Scatter Factor Still Valid for Flattening Filter Free Photon Beams?

Purpose: In the process of measuring and validating fundamental dosimetry data prior to the clinical use of a treatment unit, it is prudent to compare measurements with previously published equivalent data. During the commissioning of an accelerator with flattening filter free (FFF) photon beams (Varian True Beam 6 MV FFF and 10 MV FFF) we compared measured Phantom Scatter Factors (Sp) with the Normalized Peak Scatter Factors (NPSFs) from the British Journal of Radiology Supplement 25 (BJR #25). The purpose of this work was to determine whether the energy independent NPSFs BJR#25 are valid for comparison with FFF photon beams. Methods: All measurements were performed using a Varian TrueBeam linear accelerator with photon energies of 6‐MV, 6‐MV FFF and 10‐MV FFF modes. For all measurements, a Scanditronix CC04 ionization chamber was used. Both water and in‐air measurements were made to obtain NPSFs normalized to the 10 × 10 cm 2 field size. For measurements in water, the chamber was positioned at 100 cm source‐to‐axis distance at the depth of dose maximum. For in‐air measurements, the chamber was positioned at 100 cm source‐to‐axis distance with appropriate build‐up cap. From BJR #25, NPSFs were obtained for comparison with the measurements. Results: The NPSF agreement between the 6‐MV and 6‐MV FFF with the BJR#25 were all within ±0.5%. The agreement ranged from 0.996 to 1.004 and 0.995 to 1.002 for 6‐MV and 6‐MV FFF, respectively. We also found that 10‐MV FFF showed very similar trend. Conclusions: The scatter factors reported in BJR #25 are valid for comparison for 6‐MV, 6‐MV FFF, and 10‐MV. Additional investigation is needed to further understand the dosimetric characteristics of FFF mode.