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:0.5 cm; 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 D1cm3 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 >0.4 cm for prostate and digitization errors >0.3 cm for the urethra and >0.4 cm for rectum. Possible clinical impact was observed for catheter swaps (all organs), systematic shifts >0.2 cm for prostate and >0.4 cm for rectum, and digitization errors >0.2 cm for prostate and >0.1 cm 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.

Impact of use of optical surface imaging on initial patient setup for stereotactic body radiotherapy treatments.

PURPOSE: To evaluate the effectiveness of surface image guidance (SG) for pre-imaging setup of stereotactic body radiotherapy (SBRT) patients, and to investigate the impact of SG reference surface selection on this process. METHODS AND MATERIALS: 284 SBRT fractions (SG-SBRT = 113, non-SG-SBRT = 171) were retrospectively evaluated. Differences between initial (pre-imaging) and treatment couch positions were extracted from the record-and-verify system and compared for the two groups. Rotational setup discrepancies were also computed. The utility of orthogonal kVs in reducing CBCT shifts in the SG-SBRT/non-SG-SBRT groups was also calculated. Additionally, the number of CBCTs acquired for setup was recorded and the average for each cohort was compared. These data served to evaluate the effectiveness of surface imaging in pre-imaging patient positioning and its potential impact on the necessity of including orthogonal kVs for setup. Since reference surface selection can affect SG setup, daily surface reproducibility was estimated by comparing camera-acquired surface references (VRT surface) at each fraction to the external surface of the planning CT (DICOM surface) and to the VRT surface from the previous fraction. RESULTS: The reduction in all initial-to-treatment translation/rotation differences when using SG-SBRT was statistically significant (Rank-Sum test, α = 0.05). Orthogonal kV imaging kept CBCT shifts below reimaging thresholds in 19%/51% of fractions for SG-SBRT/non-SG-SBRT cohorts. Differences in average number of CBCTs acquired were not statistically significant. The reference surface study found no statistically significant differences between the use of DICOM or VRT surfaces. CONCLUSIONS: SG-SBRT improved pre-imaging treatment setup compared to in-room laser localization alone. It decreased the necessity of orthogonal kV imaging prior to CBCT but did not affect the average number of CBCTs acquired for setup. The selection of reference surface did not have a significant impact on initial patient positioning.

Technical Note: The design, construction, and evaluation of a liquid-based single phantom solution for TG128 brachytherapy ultrasound QA.

PURPOSE: Since the publication of the AAPM TG128 report for the quality assurance (QA) of prostate brachytherapy ultrasound systems, no commercially available phantoms have been developed which satisfy all of the task group recommendations. Current solid phantoms require a separate user-implemented setup using a container with liquid medium to evaluate the alignment between the needle template and the electronic grid, a test of geometric accuracy with critical implications in dosimetric quality. Utilizing a 3D printer, we constructed a cost-effective, liquid-based phantom that provides a complete TG128 solution which improves the efficiency of brachytherapy ultrasound QA. METHODS: The TG128 report was used to guide the design process of the liquid-based phantom. The needle template and electronic grid alignment setup served as the foundation with specific components developed to integrate all remaining tests. Water was chosen as the liquid medium, with speed of sound adjusted to 1,540 m/s via salinity per the task group recommendations. The proof of concept was evaluated by comparing the time stamps labeled on QA images between the liquid-based phantom and a commercially available one for both a new and experienced user. RESULTS: A TG128 QA trial run demonstrated that all recommended tests can be completed with the single phantom setup. Evaluation of the time data revealed a total QA duration of 45 min (average of two trials) with the liquid-based phantom, compared to 70 and 90 min with the commercial phantom for a new and experienced user. CONCLUSIONS: The liquid-based phantom is specifically designed to satisfy the recommendations of the TG128 report. The incorporation of 3D printing allows simple design modifications to adapt the phantom on-the-fly if needed. The resulting product improves the efficiency of brachytherapy ultrasound QA by eliminating the need for multiple phantom setups.