Real-time quantification of laryngeal motion in early glottic cancer using MR-Linac cine imaging: Implications for adaptive PTV margins.

Abstract

e18109 Background: To quantify intra-fraction laryngeal motion in patients with early-stage glottic cancer treated on a 1.5T MR-Linac and evaluate the potential for planning target volume (PTV) margin reduction. MR-guided radiotherapy (MRgRT) offers real-time visualization that could in turn facilitate personalized adaptive treatment strategies, while standard protocols used in conventional treatment options often have to rely on generous margins to account for uncertainty that in turn increases toxicity. Methods: A prospective observational study was conducted involving 6 patients diagnosed with T1–T2 early glottic carcinoma undergoing MRgRT on the Elekta Unity system. Patients were immobilized using an MR-safe head and neck base plate with a headrest and standard 5-point thermoplastic mask. Intra-fraction motion of the thyroid cartilage (surrogate for the glottis) was continuously monitored using the Comprehensive Motion Management (CMM) system via real-time cine MRI. The target volume with OARs and the tracking structures were contoured by an expert head and neck radiation oncologist. Motion data was analysed across Superior-Inferior (SI), Left-Right (LR), and Anterior-Posterior (AP) axes. Non-physiological outliers (>2 cm) representing swallowing or coughing events were excluded. Descriptive statistics (mean, SD, 95th percentile) were calculated to inform margin efficacy. Results: Analysis of continuous motion revealed that laryngeal stability is predominantly maintained during normal respiration, with transient excursions due to swallowing. After exclusion of outliers, the mean displacement was 0.81 ± 1.05 mm in the LR direction, 1.13 ± 1.14 mm in the AP direction, and 2.37 ± 2.06 mm in the SI direction. The 95th percentile of motion — representing the boundary required to cover the target 95% of the time — was 2.5 mm (LR), 3.3 mm (AP), and 6.5 mm (SI). Conclusions: Laryngeal motion during MRgRT is anisotropic, with the greatest displacement observed in the SI direction. The data suggests that conventional PTV margins (often uniform 3-5mm) may be safely reduced in the axial plane (LR/AP) to 3 mm, while the SI margin requires careful management or gating to account for swallowing. CMM allows for safe margin personalization, potentially reducing toxicity to the pharyngeal constrictors and larynx minus gross tumor volume (GTV). The result of our study provides novel insight into the dynamics of laryngeal motion using MR-Linac cine imaging and underscores the clinical potential for motion-based margin reduction in adaptive radiotherapy for early-stage glottis carcinoma.