Quantifying clinical severity of physics errors in high-dose rate prostate brachytherapy using simulations

PURPOSE: To quantitatively evaluate through automated simulations the clinical significance of potential high-dose rate (HDR) prostate brachytherapy (HDRPB) physics errors selected from our internal failure-modes and effect analysis (FMEA). METHODS AND MATERIALS: A list of failure modes was compiled and scored independently by 8 brachytherapy physicists on a one-to-ten scale for severity (S), occurrence (O), and detectability (D), with risk priority number (RPN) = SxOxD. Variability of RPNs across observers (standard deviation/average) was calculated. Six idealized HDRPB plans were generated, and error simulations were performed: single (N= 1722) and systematic (N=126) catheter shifts (craniocaudal; -1cm:1 cm); single catheter digitization errors (tip and connector needle-tips displaced independently in random directions; 0.1 cm:0.5cm; N=44,318); and swaps (two catheters swapped during digitization or connection; N= 528). The deviations due to each error in prostate D90%, urethra D20%, and rectum D1 cm(3) were analyzed using two thresholds: 5-20% (possible clinical impact) and >20% (potentially reportable events). RESULTS: Twenty-nine relevant failure modes were described. Overall, RPNs ranged from 6 to 108 (average +/- 1 standard deviation, 46 +/- 23), with responder variability ranging from 19% to 184% (average 75% +/- 30%). Potentially reportable events were observed in the simulations for systematic shifts >0.4cm for prostate and digitization errors >0.3 cm for the urethra and >0.4cm for rectum. Possible clinical impact was observed for catheter swaps (all organs), systematic shifts >0.2cm for prostate and >0.4cm for rectum, and digitization errors >0.2cm for prostate and >0.1cm for urethra and rectum. CONCLUSIONS: A high variability in RPN scores was observed. Systematic simulations can provide insight in the severity scoring of multiple failure modes, supplementing typical FMEA approaches. (C) 2021 American Brachytherapy Society. Published by Elsevier Inc. All rights reserved.