Analysis of Performance and Failure Modes of the IROC Proton Liver Phantom

Abstract

Purpose: To analyze trends in institutional performance and failure modes for the Imaging and Radiation Oncology Core's (IROC's) proton liver phantom.

Materials and methods: Results of 66 phantom irradiations from 28 institutions between 2015 and 2020 were retrospectively analyzed. Univariate analysis and random forest models were used to associate irradiation conditions with phantom results. Phantom results included pass/fail classification, average thermoluminescent dosimeter (TLD) ratio of both targets, and percentage of pixels passing gamma of both targets. The following categories were evaluated in terms of how they predicted these outcomes: irradiation year, treatment planning system (TPS), TPS algorithm, treatment machine, number of irradiations, treatment technique, motion management technique, number of isocenters, and superior-inferior extent (in cm) of the 90% TPS isodose line for primary target 1 (PTV1) and primary target 2 (PTV2). In addition, failures were categorized by failure mode.

Results: Average pass rate was approximately 52% and average TLD ratio for both targets had slightly improved. As the treatment field increased to cover the target, the pass rate statistically significantly fell. Lower pass rates were observed for Mevion machines, scattered irradiation techniques, and gating and internal target volume (ITV) motion management techniques. Overall, the accuracy of the random forest modeling of the phantom results was approximately 73% ± 14%. The most important predictor was the superior-inferior extent for both targets and irradiation year. Three failure modes dominated the failures of the phantom: (1) systematic underdosing, (2) poor localization in the superior-inferior direction, and (3) range error. Only 44% of failures have similar failure modes between the 2 targets.

Conclusion: Improvement of the proton liver phantom has been observed; however, the pass rate remains the lowest among all IROC phantoms. Through various analysis techniques, range uncertainty, motion management, and underdosing are the main culprits of failures of the proton liver phantom. Clinically, careful consideration of the influences of liver proton therapy is needed to improve phantom performance and patient outcome.

Keywords: IROC; liver; phantoms; quality assurance.

Figure 1.

(a) Pass rates (solid orange) and number of irradiations (dashed blue) for the proton liver phantom through the years with an overall average pass rate of 51.5%. (b) Average TLD ratios and gamma pass rate values for the proton liver phantom for both targets through the years (solid blue = PTV1 TLD ratio, solid orange = PTV2 TLD ratio, dashed red = PTV1 gamma pass rate, dashed green = PTV2 gamma pass rate). Passing rates have fluctuated through the data cohort’s timeframe but recently (2019-22) the average has increased to approximately 74%. The TLD ratios and gamma pass rates have shown increase during the cohort’s timeframe; however, only PTV2 TLD ratio was statically significant. Abbreviations: GPR, gamma pass rate; PTV1, primary target 1; PTV2, primary target 2; TLD, thermoluminescent dosimeter.

Figure 2.

(a) The number of phantoms and their respective pass rates by irradiation number. The first and second irradiation by clinics tend to have a similar pass rate of approximately 50%, with the third irradiation and fourth irradiation having an increase of 5% and 15%, respectively. More than 1 analysis can be provided for an irradiation, that is, the clinic submits separate dose calculation algorithms for the same irradiation. (b) Number of phantoms and pass rate (blue = PTV1, orange = PTV2) of each target according to the superior-inferior extent of the TPS 90% isodose line. The larger the extent of the TPS target, the more likely that the phantom will fail. (c) Percentage of failing phantoms broken down by failure modes. The graph represents the combination category separated into its constituents. A total of 32 failures were analyzed regardless if they failed PTV1, PTV2, or both, hence why there is a no failure category, which implies that the phantom only failed one of the PTVs. The most common failure modes are systematic underdose, localization dose error in the motion direction (superior-inferior), and range error. Abbreviations: PTV1, primary target 1; PTV2, primary target 2; RLSP, relative linear stopping power; TPS, treatment planning system.