MRI-Guided Reduced-Volume RT for Early-Stage Glottic Cancer: A Prospective Phase 1 Trial with Quantification of Laryngeal Motion.

Abstract

Five to 7 weeks of whole-larynx radiation (RT) is standard for treatment of early-stage glottic cancer. Previous studies evaluated CT-guided hypofractionated/SBRT treatment to a smaller volume. Magnetic resonance-guided radiotherapy (MRgRT) allows real-time motion tracking and gating by continuously imaging the larynx. We performed the first prospective study of MRgRT in patients with early-stage glottic cancer with quantification of laryngeal motion. This phase 1 trial evaluated MRgRT in patients with stage I/II squamous cell glottic cancer undergoing definitive RT. The RT prescription was either 42.5 Gy/5 fractions (treatment volume 10cc / smokers). Treatment was delivered using gated 0.35 T MRgRT with continuous tracking. Coprimary endpoints were feasibility and safety (grade 3+ toxicity rate). Patient-reported outcome measures (PROMs), voice quality, MRgRT treatment parameters, and local control were assessed. Intrafraction cine-MRI quantified laryngeal motion. MRgRT was feasible in all patients. Median time on study was 10.9 months. Treatment was well tolerated with no acute or late grade 3+ toxicities. There was no change in PROMs over the course of the study. Two patients who received 42.5 Gy experienced locoregional failure, one in-field only and one in-field and nodal. MR tracking resulted in 20/94 (21%) of treatment fractions requiring ≥ 1 shift necessitating intra-fraction re-imaging. In 19/94 (20%) of treatment fractions, the beam was off more than 20% of the treatment time due to stochastic beam interruption events (BIEs). During BIEs, larynx displacement exceeded 3 mm in 53% of cine-MRI frames. MRgRT targeting gross disease instead of the whole larynx is feasible and safe for patients with early-stage glottic cancer with minimal treatment-associated toxicity and no detriment in PROMs. MRI guidance provides real-time verification of larynx position. Data regarding real-time intrafraction motion has important implications for patients receiving standard laryngeal RT.