ESTRO 2025 - Abstract Book

S328

Brachytherapy - Physics

ESTRO 2025

Results:

Step size errors (SSEs) require several dwells to be delivered before detection; larger ST measurement uncertainties mean more consecutive dwells are needed for error detection. SSEs can be detected with relative 2D measurements. For 2 mm SSEs, 5 dwell positions using the total uncertainty are needed before errors can be confidently detected, reducing to 3 dwells for a 3D relative ESSUT. For a 4 mm SSE, this changes to 4 dwells and 2 dwells respectively. Catheter swap error (CSE) detection is dependent on which catheters were incorrectly connected. If measurements show coronal deviations larger than the 2D relative uncertainty, it can be said with confidence that there is a CSE. If coronal deviations are within the 2D relative uncertainty, 3D deviations are compared to the 3D relative uncertainty as the catheters may be above each other. In both cases, the ESSUT is smaller than the total uncertainty, allowing for a more sensitive system. Conclusion: By utilising ESSUTs, the error sensitivity of ST-based treatment verification systems can be improved, and these concepts are generalisable to all ST systems. By identifying and minimising the largest uncertainty contributions the system can be optimised for clinical use. Uncertainties should still be reduced as anatomical dosimetry is dependent on absolute 3D measurements.

Keywords: Treatment verification, source tracking

References: [1] Smith RL, Hanlon M, Panettieri V, Millar JL, Matheson B, Haworth A, et al. An integrated system for clinical treatment verification of HDR prostate brachytherapy combining source tracking with pretreatment imaging. Brachytherapy 2018;17:111–21. https://doi.org/10.1016/j.brachy.2017.08.004.