ESTRO 2023 - Abstract Book

S221

Saturday 13 May

ESTRO 2023

iCycle automated planning was applied to 30 HNC patients who were previously treated with IMPT with 7000 cGy prescription dose to the primary tumor (CTV7000) and 5425 cGy dose prescription to the elective volumes (CTV5425). The iCycle wish-list was configured for automated generation of four-beam robust IMPT plans in line with clinical requirements. For robust target coverage optimization, 21 scenarios with 3 mm setup error and ±3% density uncertainty were included for the clinical target volumes (CTVs), similar to the clinical treatment plans. A comparison between the automated iCycle IMPT plans and clinical IMPT plans was performed based on OAR doses and (robust) target coverage. Robust target coverage was assessed in the voxel-wise minimum (vw-min) dose constructed from the robust scenarios, similar to evaluation in clinical practice, with D98%>95% criterion. A Wilcoxon signed rank test was performed to assess if there was a statistically significant difference (p<0.05) between the iCycle and clinical plans. Results For the nominal scenario, the D98 in all clinical and iCycle plans fulfilled the D98>95% criterion for both CTVs. The D98 in the iCycle plans was on average (± SD) comparable in both CTVs compared to the clinical plans (CTV7000: 6892±14 cGy vs. 6889±25 cGy, CTV5425: 5453±29 cGy vs. 5378±19 cGy for the iCycle and clinical plans respectively). In the vw-min dose, the D98 for both CTVs fulfilled the D98>95% criterion in 29/30 iCycle plans, while in the clinical plans this criterion was fulfilled in 27/30 patients. For the vw-min dose, the D98 was on average comparable for both CTVs in the iCycle plans as compared to the clinical plans (CTV7000: 6677±14 cGy vs. 6697±40 cGy, CTV5425: 5254±19 cGy vs. 5197±28 cGy, for the iCycle and clinical plans respectively). The average mean dose to all evaluated OARs was lower in the iCycle plans compared to the clinical plans, see figure 1. This difference was statistically significant for all evaluated OARs, with p<0.05.

Conclusion iCycle was able to automatically produce acceptable HNC IMPT plans with comparable (robust) target coverage D98 and statistically significant lower mean doses to OARs.

OC-0286 Analysis of Performance and Failure Modes of the IROC Proton Liver Phantom H. Mehrens 1 , P. Taylor 1 , P. Alvarez 1 , S. Kry 1 1 MD Anderson Cancer Center, Radiation Physics, Houston, USA

Purpose or Objective Radiotherapy facilities participating in national clinical trials in the United States must successfully irradiate credentialing phantoms from the Imaging and Radiation Oncology Core (IROC). This work analyzes trends in institutional performance and failure modes for IROC’s proton liver phantom. Materials and Methods The proton liver phantom includes two moving target volumes. Results of 66 phantom irradiations from 28 institutions between 2015-2020 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 percent 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, treatment technique, motion management technique, number of isocenters, and superior- inferior extent (in cm) of the 90% TPS isodose line for both targets. In addition, failures were categorized by failure mode. Results The overall average pass rate was only 52%, although the average TLD ratio has slightly improved with time. As the of the treatment field increased the pass rate actually decreased (p < 0.01). Lower pass rates were observed for Mevion machines, scattered irradiation techniques and gating and 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 direction of motion (superior-inferior), and 3) range error. Only 44% of failures has the same failure modes between the two targets.

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