HyperArc VMAT SRT for Locally Recurrent Previously-Irradiated Head and Neck Cancers: Plan Quality, Treatment Delivery Accuracy and Efficiency.

Purpose/Objective(s) To demonstrate the feasibility and efficacy of HyperArc VMAT treatments for locally recurrent, locally advanced or previously irradiated head and neck cancers treated with stereotactic radiotherapy (SRT). Materials/Methods First, an anthropomorphic head phantom from the MD Anderson's IROC credentialing laboratory containing a 1.9 cm diameter spherical target and in vivo-dosimetry system was imaged, planned and irradiated (25 Gy in 1 fraction) using HyperArc VMAT module with 10MV-FFF beam. Second, RANDO head phantom was imaged, planned and irradiated (35 Gy in 5 fractions) by generating eight HyperArc VMAT plans (4 right, 4 left neck tumors) at different anatomical locations (C1-C4). Average tumor volume was 21.7 cc (13.0–32.3 cc). Distance to isocenter from the central marker of the Encompass device down to neck was 25.8 cm (23.8–28.0 cm) and 24.3 cm (22.0–27.1 cm) for the left and right neck tumors and 9 cm (in) from the both lateral markers that defined the patient protection zone. Third, two head and neck cancer patients with recurrent large mass of 56 cc (4.8 cm, diameter) on right maxillary sinus and 159 cc (6.7 cm, diameter) on right ethmoid sinus were imaged, planned and treated (35 Gy in 5 fractions) with HyperArc SRT. Plan quality, treatment delivery accuracy and efficiency were reported. Results Phantom irradiation results met all the compliance criteria set forth by the IROC for HyperArc SRS. For end-to-end tests, the average conformity index, Paddick conformation number, gradient and heterogeneity indices, and dose to 2 cm away from the target were 1.08 ± 0.05 (1.02–1.14), 0.85 ± 0.02 (0.83–0.89), 2.71 ± 0.27 (2.28–2.92), 0.14 ± 0.02 (0.11–0.17) and 23.5 ± 1.5% (21.4–25.0%). Average maximal dose to skin and spinal cord was 12.8 ± 3.5 Gy (9.2–16.7 Gy) and 4.6 ± 0.8 Gy (4.0–5.6 Gy). Mean values of modulation factor, beam on time and treatment time was 2.96 ± 0.5 (2.39–3.78), 3.1 ± 0.2 min (2.67–3.78 min), and 13.96 ± 0.50 min (13.39–14.78 min) with no collision issue. Pretreatment portal dosimetry quality assurance results demonstrated a high pass rate of 99.1 ± 1.6% for clinical gamma passing criteria of 2%/2 mm. Similar results were found on the patient's plans including highly conformal target coverage, sharp dose fall-off (50% dose fall-off within 5 mm from the target) and sparing adjacent critical organs including spinal cord ( Conclusion For recurrent head and neck SRT treatments, HyperArc VMAT provided highly conformal dose distribution, rapid dose fall-off, excellent sparing of adjacent critical organs, highly precise and accurate treatment that could be delivered to as low as C4 level. It could potentially allow HyperArc SRT to glomus tumors to those patients who may not tolerate frame-based SRS. Clinical follow up of these patients is ongoing to confirm the therapeutic benefits of this novel and attractive head and neck SRT. Author Disclosure D. Pokhrel: None. M.E. Bernard: None. D. Lockhart: None. J. Jeremiah: None. D. Fabian: None. M.R. Kudrimoti: None.