Author: Yunfeng Cui, Joseph P. Kowalski, Qiuwen Wu, Yibo Xie 👨🔬
Affiliation: Duke University, Duke University Medical Center 🌍
Purpose: A Varian Ethos/Halcyon linac was recently commissioned in our clinic and was used to treat head and neck cancer patients. The purpose of this study is to compare treatment plan quality and delivery efficiency between the Ethos and conventional Truebeam linac.
Methods: A group of 12 patients previously treated on Truebeam with a Millennium MLC using 6MV photon were retrospectively selected. Treatment regimens consisted of two sequential phases of 44 Gy (primary) and 26 Gy (boost) delivered in 2 Gy fractions. All plans utilized VMAT for delivery and included 3-4 arcs for the primary and 2-3 arcs for the boost. Planning was performed in Eclipse V16 with no changes to optimization objectives or beam design between TrueBeam and Ethos plans. Same plan normalization was also chosen to maintain PTV coverage. The standard plan evaluation dosimetric indices for OARs included D0.1cc (equivalent to Dmax) for brainstem, cord+5mm, and mandibles; Dmedian for larynx, oral cavity, pharynx, parotid glands, and submandibular glands. The total MUs are also compared.
Results: Target coverage and isodose distributions between Ethos and TrueBeam plans were equivalent. Although minor OAR dose differences were noted, none were statistically significant. No significant difference was found in MUs, either. Treatment time on Ethos was significantly shorter due to the combined effects of higher dose rate, and faster gantry and MLC motion, prompting the inclusion of additional arcs. No immediate improvement to plan quality was noted, however it is hypothesized that these characteristics will permit additional parameter tuning capable of achieving improved results.
Conclusion: Treatment plans for head and neck cancers on Ethos are dosimetrically and clinically equivalent to those on Truebeam, but can be treated more efficiently. Fine-tuning Ethos specific parameters may offer further improvement in plan quality.