Purpose: Evaluate the plan quality, delivery efficiency, and accuracy of VMAT liver SBRT on 6MV-FFF O-ring Halcyon.
Methods: Eight liver cancer patients received VMAT-SBRT, delivering 30-50 Gy in 5 fractions, prescribed to 80% isodose line on Halcyon. Mean PTV volume was 167.4±70.88 cc (range, 71.7-304.1cc). VMAT liver SBRT plans were generated with full RTOG-1112 compliance, advanced AcurosXB algorithm for heterogeneity corrections with Halcyon couch and SBRT board insert. Patients were treated every other day using 2-4 partial/full arc geometry with 6X-FFF beams including fast 15-second kV-iCBCT imaging for set up corrections. For comparison, clinical Halcyon plans were retrospectively reoptimized on TrueBeam Linac with identical geometry and optimization parameters. Plan quality, organs-at-risk (OAR) and treatment delivery parameters were compared after identical target normalization. For clinical Halcyon plans, quality assurance and couch corrections results are reported.
Results: Halcyon and TrueBeam plans showed similar conformity index, total monitor units, modulation factors and maximal dose fall-off at 2cm away from the PTV. Mean PTV dose was 106.0±2.4% and 105.9±1.8% of the prescribed dose for Halcyon and TrueBeam, respectively. RTOG volumetric doses for OAR were similar: 8.7Gy vs 8.3 Gy (0.5cc of duodenum); 13.1Gy vs 12.9Gy (0.5cc of stomach); 9.6 Gy vs 9.9 Gy (0.5cc of small bowel) and 4.4 Gy vs 4.1 Gy (30cc of heart) for Halcyon vs TrueBeam plans. Halcyon pre-treatment portal-dosimetry QA result was 98.7%, on average for 2%/2mm gamma passing criteria. The 3D radial couch correction was 0.65±0.47cm (0.41–0.95cm). Mean beam-on time was 3.4±1.2min (2.0–5.7min). Overall treatment time on Halcyon was <15 min.
Conclusion: For select patients, SBRT of liver cancer on Halcyon is safe, feasible, efficient and dosimetrically similar to TrueBeam Linac. Faster overall liver SBRT treatment on Halcyon may increase patient compliance, reduce intra-fraction movement and improve clinic workflow. Clinical follow up of these patients is underway.
Funding Support, Disclosures, and Conflict of Interest: This work is partially supported by Varian Medical Systems.