ESTRO 2025 - Abstract Book

S325

Brachytherapy - Physics

ESTRO 2025

1 Radiation Oncology, Calvary Mater Newcastle, Newcastle, Australia. 2 Medical Physics, Cambridge University Hospitals NHS Foundation Trust, Cambridge, United Kingdom. 3 Radiation Oncology, University of Washington, Seattle, USA Purpose/Objective: An increasing discrepancy between the measured reference air kerma rate (RAKR) of high dose rate (HDR) Ir-192 sources and that reported by the manufacturer on the source certificate was observed at a hospital in Australia. This study aimed to determine whether the drift recorded in a single clinic was a local anomaly or more widespread. Material/Methods: A questionnaire was sent to physicists at brachytherapy centres across Australia, New Zealand and the UK to determine the prevalence of the issue, and dependent factors. The questionnaire gathered details on HDR Ir-192 source model and origin, well chamber model, well chamber calibration provider, and each departments’ history of the discrepancies in measured to manufacturer source certificate RAKR. Results: 26 centres provided data including calibration history ranging from 2 to 13 years (mean 7.3 years). The manufacturer of all sources included in this analysis was Curium (formally Mallinkrodt). Respondents had their well chambers calibrated at three standards laboratories, National Physical Laboratory (NPL), UK, University of Wisconsin Accredited Dosimetry Calibration Laboratory (UWADCL), USA and Physikalisch Technische Werkstätten (PTW), Germany. One site cross-calibrated with a thimble ionisation chamber, calibrated at the Australian Radiation Protection and Nuclear Safety Agency (APRANSA). Table 1 shows the mean discrepancy between measured and source certificate RAKR values in surveyed sites. Figure 1 shows the percentage difference between measures and source certificate RAKR values. Sites with well chambers calibrated at NPL or UWADCL showed an increase in discrepancy between measured and source certificate RAKR, approximately 0.2% per year on average from 2015 to present. In 2024 all measurements were higher than the calibration certificate. PTW is an accredited secondary standard dosimetry calibration, traceable to Physikalisch-Technische Bundesanstalt (PTB). Four centres had well chamber calibrations traceable to PTB, these sites did not show the same upward trend in measured compared to source certificate RAKR. Notably, Curium source certificate values are also traceable to PTB. PTB reduced their secondary standard calibration factor by 0.34% in 2018 and by 0.83% in 2023 [1], resulting in a reduction in the stated RAKR values of 1.17% compared to pre-2018.