Purpose: A 28 cm² field size at isocenter is a limitation of the ring-mounted Halcyon V2.0 linac. Field size is extendable up to 36 cm utilizing dual-isocenter and junctional auto-feathering option during VMAT optimization. We report initial clinical experience using Halcyon dual-isocenter VMAT, enabling EF treatment for complex gynecological cancers.
Methods: Five complex gynecologic cancer patients received EF irradiation with dual-isocenter VMAT, delivering 45 Gy in 25 fractions incorporating a 55 Gy simultaneous integrated boost (SIB) to PET-avid lymph nodes (LNs) on Halcyon. Two plans were generated with 4 full arcs (2 per isocenter) and three plans with 8 full arcs (4 per isocenter); different collimator settings were used for auto-feathering. Dual-isocenters were separated by 8 cm in the craniocaudal (CC) direction and the daily CBCT imaging isocenter was placed between them. We report plan quality, quality assurance results, treatment delivery accuracy and efficiency.
Results: Mean EF size in CC direction was 32.1±2.3 cm (29.2–34.3 cm). Mean target conformity and heterogeneity indexes were 1.05±0.08 (1.01–1.09) and 0.15±0.09 (0.06–0.25) respectively. 95% of the PTV45Gy and PTV55Gy received 46.8±2.2 Gy (45.2–51.3 Gy) and 56.8±2.3 Gy (55.6–60.2 Gy), respectively. Plans were evaluated per RTOG-0418 and all OAR were within protocol compliance. Pre-treatment portal dosimetry QA result was 99.7%, on average for 3%/3mm gamma passing criteria. The 3D radial couch correction and repositioning from daily CBCT imaging was 0.84±0.57 cm (0.41–1.93 cm). Mean beam-on time was 2.6±0.2 min (2.4–2.9 min). Mean patient couch time including CBCT was 9.5±0.3 min (9.2–9.9 min).
Conclusion: Treating extended fields in complex gynecologic cancers patients on Halcyon utilizing dual-isocenter VMAT auto-feathering is safe, feasible and efficient. Rapid treatment delivery on Halcyon may increase patient comfort, reduce intrafraction motion errors, and improve clinic efficiency and patient throughput. Follow up for disease-control and toxicity in a larger patient cohort is warranted.