RADIATION ONCOLOGY—ORIGINAL ARTICLE Larynx motion considerations in partial larynx volumetric modulated arc therapy for early glottic cancer Houda Bahig, 1,2 Phuc F elix Nguyen-Tan, 1,2  Edith Filion, 1,2 David Roberge, 1,2 Pensavan Thanomsack, 1 Jacques de Guise, 2 Danis Blais, 1 Robert Doucet, 1 Laurent L etourneau-Guillon 2,3 and Louise Lambert 1,2 1 Radiation Oncology Department, Centre Hospitalier de l’Universit e de Montr eal, Montreal, Quebec, Canada 2 CHUM Research Center, Montreal, Quebec, Canada 3 Radiology Department, Centre Hospitalier de l’Universit e de Montr eal, Montreal, Quebec, Canada H Bahig MD; PF Nguyen-Tan MD;  E Filion MD; D Roberge MD; P Thanomsack; J de Guise PhD; D Blais MSc; R Doucet MSc; LL etourneau-Guillon MD; L Lambert MD. Correspondence Dr Louise Lambert, Department of Radiation Oncology, 1560 Sherbrooke St. E., Montreal, QC H2L 4M1, Canada. Email: louise.lambert.chum@ssss.gouv.qc.ca Conflict of interest: HB and DR declare having received honoraria from Siemens Healthineers. HB, EF and DR declare having received a research grant from Varian Medical Systems Inc. unrelated to the current project. Submitted 12 December 2016; accepted 11 March 2017. doi:10.1111/1754-9485.12612 Abstract 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 tomog- raphy (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 ante- rior 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 dur- ing swallowing. Mid-treatment 4D-CT revealed larynx shift relative to verte- brae in 30% of cases. PL-VMAT allowed for significant reduction of mean doses to ipsilateral carotid, contralateral carotid, thyroid gland, contralateral ary- tenoid 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. Key words: 4D-computed tomography; glottic cancer; larynx motion; magnetic resonance imaging; volumetric modulated arc radiotherapy. Introduction Radiation therapy is an essential component of early glottic cancer treatment. Radical radiotherapy outcomes for early glottic cancer are excellent with reported 5-year local control (LC) rates reaching 95% and 85% for T1 and T2 lesions, respectively, and 5-year overall survival (OS) exceeding 90%. 1–5 In recent years, increasing interest in reducing radiation treatment volumes has emerged, with the objective of reducing toxicity while maintaining LC. Although surgical approach involves the resection of tumour-bearing vocal cord, radiotherapy currently involves irradiation of the entire larynx. In an era of intensity-modulated radiotherapy (IMRT) and image-guided radiotherapy (IGRT), the rationale for this discrepancy becomes difficult to justify; it is therefore appealing to mirror surgical approaches and migrate towards partial larynx irradiation. 1 © 2017 The Royal Australian and New Zealand College of Radiologists Journal of Medical Imaging and Radiation Oncology  (2017) – Journal of Medical Imaging and Radiation Oncology