Author: Chia-Lung Chien, Renteng Hou, Wen C. Hsi, Faraz Kalantari, Ganesh Narayanasamy, Zhong Su 👨🔬
Affiliation: University of Arkansas for Medical Sciences, Department of Radiation Oncology, University of Arkansas for Medical Sciences (UAMS) 🌍
Purpose:
PRDR is an effective re-irradiation treatment for patients who have already reached the tolerance dose to their organs at risk (OARs). While PRDR is implemented on TrueBeam by limiting the maximum instantaneous dose rate (MIDR) to 100 MU/min, Halcyon/Ethos operate at a fixed 800 MU/min with no option to adjust. This study demonstrates that Ethos can achieve a low instantaneous delivery rate in clinical mode and effectively deliver PRDR treatments.
Methods:
Fourteen patient cases (5 brain, 5 head and neck, and 4 spine) that were planned and delivered on TrueBeam were replanned on Ethos using VMAT with both single and double full-arc setups. Each 2 Gy fractional dose was delivered in 10 passes of 20 cGy. Plans were delivered in clinical QA mode and measured using the ArcCHECK phantom (Sun Nuclear Corporation, Melbourne, FL, USA). MIDR and delivery times were recorded, and IMRT QA was analyzed for one pass per plan using a 3%/2mm gamma threshold.
Results:
Double-arc plans achieved significantly lower MIDRs (66.9 ± 18.9 MU/min) than single-arc plans (124.2 ± 35.9 MU/min) (p < 0.001), with no differences across treatment sites (p = 0.303). Delivery times per arc were similar for single-arc (34.3 ± 4.7 s) and double-arc (34.9 ± 4.8 s) plans (p = 0.458). Single-arc plans achieved a 98.2% ± 1.4% passing rate, and double-arc plans achieved 97.7% ± 2.5%, outperforming TrueBeam PRDR plans (91.9% ± 10.1%; p = 0.008).
Conclusion:
PRDR radiotherapy was feasible on Ethos, particularly with double-arc plans, which maintained delivery rates below 100 MU/min. Ethos provided an efficient alternative for TrueBeam LINACs to deliver PRDR treatments, helping to distribute the clinical workload more effectively. Its low interleaf transmission improved QA passing rates by minimizing leakage noise, thereby boosting QA efficiency in the PRDR treatment workflow.