ESTRO 2025 - Abstract Book

S327

Brachytherapy - Physics

ESTRO 2025

Conclusion: There appears to be an increasing discrepancy between measured Ir-192 HDR source strength compared to the manufactures certificate for centres not traceable to PTB. This may be related to changes made to calibration factors at PTB (-1.17%). However, the changes observed within the clinics (1.86% for NPL, 1.71% for UWADCL) since 2015 exceed these values, suggesting further investigation is warranted.

Keywords: Ir-192, calibration, RAKR

References: [1] C Kessler et al Key comparison BIPM.RI(I)-K8 of high dose-rate 192Ir brachytherapy standards for reference air kerma rate of the PTB and the BIPM 2024 Metrologia 61 06002

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Digital Poster One size certainly does not fit all: using variable uncertainty budgets for optimised source tracking detection thresholds in HDR brachytherapy Maximilian D P Hanlon 1,2 , Jordan Wallace 2 , Ryan L Smith 1,3,2 , Rick D Franich 2,1 1 Alfred Health Radiation Oncology, The Alfred, Melbourne, Australia. 2 School of Science, RMIT University, Melbourne, Australia. 3 Central Clinical School, Monash University, Melbourne, Australia Purpose/Objective: Source tracking (ST) is a valuable approach to treatment verification for HDR brachytherapy. The measurement of dwells during treatment allows delivery confirmation, error detection, or calculation of delivered dosimetry. Detecting errors allows for adaptation to recover the planned dose. Error detection is dependent on the ST measurement uncertainty. Errors smaller than uncertainties cannot be confidently detected. Reducing measurement uncertainties increases system sensitivity and robustness. Material/Methods: A flat-panel x-ray detector (FPD) is mounted in a modified treatment couch-top, with a ceiling-mounted x-ray unit for pre-treatment imaging. [1] Registration between anatomical (CT) and FPD (measurement) spaces is achieved via fiducial markers, and radiopaque markers used to localise the implant. An example ST uncertainty budget is shown in Figure 1. Not all errors are sensitive to all uncertainties, e.g. some can be detected with relative ST only. By separating uncertainty components into categories, Error Signature Specific Uncertainty Thresholds (ESSUTs) can be made for each error type. Expanded (k=2) uncertainties are used, increasing specificity. Two delivery errors are discussed: incorrect dwell step size (afterloader or TPS error), and incorrect indexer length (wrong transfer tube or incorrect length in TPS).