Determination of calibration reference values -1as detailed in the IPEM 2020 high-energy photon code of practice

Background. In 2020, the Institute of Physics and Engineering in Medicine (IPEM) published an updated code of practice (COP) for high-energy photon therapy dosimetry [Eaton DJet al2020 IPEM code of practice for high-energy photon therapy dosimetry based on the NPL absorbed dose calibration servicePhysics in Medicine & Biology65195006], with a further update published in 2023 [IPEM 2023Policy Statement IPEM Recommendations on the Implementation of Codes of PracticeInstitue of Physics and Engineering in Medicine]. The 2020 COP provided an option to calibrate isocentrically at 10 cm deep to a reference value for each energy that is less than 1 cGy M^-1U^-1, to give approximately 1 cGy/MU at the depth of dose maximum (d_max). These reference values can relate the machine calibration at d_maxin a fixed source-to-surface distance (SSD) setup, to the machine calibration in an isocentric setup at 10 cm deep. This option was provided to give consistency in the number of monitor units (MU) with dose in cGy in the updated reference conditions, to those typical of fixed SSD conditions, with minimal adjustment to linac output.Purpose.The aim of this study was to determine such reference values for beam energies 6MV, 15MV, 6FFF and 10FFF and their respective tissue phantom ratios (TPR_20,10) 0.630-0.763.Methods.Reference values for clinical energies 6MV, 15MV, 6FFF and 10FFF were determined over multiple Varian TrueBeam linacs, field ionisation chambers and electrometers and from historical quality control (QC) records over 11 years.Results. Reference values were determined as 0.799, 0.929, 0.761 and 0.855 for 6MV, 15MV, 6FFF and 10FFF respectively. The variation in these data (1 standard deviation, [SD]) was <0.004, demonstrating the stability of these reference values in a single centre study. Modelling of reference values using treatment planning system (TPS) data also demonstrated small differences compared to measured data of 0.06 ± 0.29 %. The relative standard uncertainty in these reference values was determined as 0.38 %. Adding this uncertainty in quadrature to the published uncertainties for isocentric calibration published in the 2020 COP contributes an additional 0.06 %.Conclusions. Published reference values for clinical range of TPRs may aid centres implementing this option of the 2020 COP to confirm their data, with the shift away from a reference value of unity. Understanding the magnitude of the uncertainty in these reference values shows the impact of dose calibration at a reference position e.g. d_max, different to a dose planned and delivered isocentrically and can help ascertain the risk/benefit of changing machine calibration and measurement conditions. Similarly, given the magnitude of the uncertainty in these reference values, incorporating such a factor to convert from one calibration setup condition to another could be used to assist in peer-to-peer audit against the 2020 COP.