Purpose: Evaluate the plan quality, treatment efficiency and accuracy of single-isocenter volumetric modulated arc therapy (VMAT) of double-vertebral segment SBRT on Halcyon Linac.
Methods: In-house multi-target end-to-end phantom tests and independent dose verification using MD Anderson’s single-isocenter/multi-target thorax phantom (spine/lung targets) was completed. Six clinical patients with two-vertebral segments on thoracic and/or lumbar spine were replanned on Halcyon Linac with 6MV-FFF beam using a single-isocenter between vertebral segments. Three full arcs with 0o and ±10o collimator angles and advanced Acuros-based dose engine for heterogeneity corrections were used. Prescription was 35 Gy in 5 fractions to each vertebral-segment, synchronously. Halcyon VMAT-SBRT plans were retrospectively created on Truebeam Linac with a 6MV-FFF beam using identical planning geometry and optimization objectives. Plan quality metrics were evaluated per NRG-BR002 protocol. Treatment delivery parameters were recorded for both plans.
Results: Phantom measurements showed acceptable spatial accuracy (<1mm within 5 cm from isocenter) of CBCT-guided Halcyon SBRT treatments. MD Anderson’s phantom irradiation credentialing results met IROC requirements. Mean isocenter-to-tumor center distance was 3.3±0.6cm (range, 2.4–4.3cm). Mean combined PTV was 57.3±31.3cc (range, 20.1–99.9cc). Both Halcyon and Truebeam VMAT-SBRT plans met NRG-BR002 criteria and show similar CI, HI, GI, D2cm and maximal dose to adjacent OAR including dose to partial cord. Total monitor units, modulation, and overall treatment time were lower with Halcyon plans by 130 MU, 0.2, 3.8 min, on average, with similar beam-on time. Portal-dosimetry patient-specific QA on Halcyon showed a high pass rate of 99.6±0.3% for 2%/2 mm clinical gamma passing criteria.
Conclusion: SBRT treatment of double-vertebral segment via single-isocenter VMAT on Halcyon for select patients, is feasible and dosimetrically comparable to Truebeam Linac. Faster treatment delivery (≤ 10 min) of double-vertebral segment SBRT on Halcyon can improve patient tolerance due to back pains, potentially reduce intra-fraction motion errors, improve patient throughput and clinical efficiency.
Funding Support, Disclosures, and Conflict of Interest: This work is partially supported by VARIAN Medical Systems.