Verification of representative data for output factors of SRS cones utilizing IAEA TRS 483 recommendations

Abstract

Measurements of small fields continue to be a clinical challenge despite the recent work done to identify their characteristics. Due to this challenge, many physicists use representative data supplied by their vendors to verify their own measurements for small field output factors. However, with recent guidelines being released in IAEA TRS 483, the question remains if this representative data provides an accurate representation for small field dosimetry. A Sun Nuclear EDGE detector, PTW 60012 stereotactic diode, and GafChromic EBT3 films were used to measure the output factor for a set of Varian SRS cones (4 mm–17.5 mm diameters) on a TrueBeam linear accelerator. The measured output factors were then compared to the Varian provided SRS representative data. IAEA TRS 483 recommendations for measuring small field output factors were applied and the impact of those recommendations were examined. The EDGE detector showed good agreement with the representative data when correction factors were not applied (0.01%–1.64% difference) but the PTW 60012 diode showed larger deviation (0.61%–3.35% difference). The EBT3 film showed the largest difference with the representative data (0.66%–9.19%). After application of IAEA TRS 483 detector specific correction factors the output factors measured by the diodes showed good agreement with the EBT3 film for 6MV (<1.8% difference) but showed a large deviation with the representative data (up to 9% difference). The 6FFF energy output factors agreed between the EDGE, the PTW 60012, and EBT3 Film. This work shows that the use of uncorrected representative data on the Truebeam can lead to a significant over estimation of the SRS cone output factors.