Larynx motion considerations in partial larynx volumetric modulated arc therapy for early glottic cancer.

INTRODUCTION: To assess laryngeal motion in early glottic cancer in order to determine safe margins for partial larynx volumetric modulated arc therapy (PL-VMAT), and to quantify dosimetric advantages of PL-VMAT.

METHODS: This prospective study included T1-2N0 glottic cancers treated with whole larynx VMAT (WL-VMAT). Pre- and mid-treatment 4D-computed tomography (4D-CT) and dynamic magnetic resonance imaging (MRI) allowed for assessment of larynx swallowing and respiratory motion. For 10 patients with lateralized lesions, PL-VMAT plans were calculated using margins derived from 4D-CT analysis.

RESULTS: Twenty patients were accrued from 2014 to 2016. Mean amplitude of larynx swallowing excursion was 23 mm and 6 mm in the superior and anterior directions, respectively. Mean respiratory motion reached 4 mm and 2 mm in superior-inferior and antero-posterior directions, respectively. Pre-treatment 4D-CT analysis identified one patient with planning CT acquired during swallowing. Mid-treatment 4D-CT revealed larynx shift relative to vertebrae in 30% of cases. PL-VMAT allowed for significant reduction of mean doses to ipsilateral carotid, contralateral carotid, thyroid gland, contralateral arytenoid and larynx. Using 8 mm internal margin for PL-VMAT, swallowing resulted in clinical target volume excursion beyond 95% isodose line during ≤1.5% of total treatment time in all patients.

CONCLUSION: Although swallowing motion is rare, rapid and easily suppressed by patients, there is a risk of systematic miss-targeting if planning CT is acquired during swallowing. Larynx position shift relative to vertebrae occurs in 1/3 of patients over the course of radiotherapy. With soft-tissue image guidance and margins accounting for respiratory motion, PL-VMAT allows safe reduction of dose to organs at risk.