PO-0635: Dose to the masseter muscle and risk of trismus after chemoradiation for advanced head & neck cancer

Purpose / objective: Head and neck cancer patients treated with chemoradiation are at risk for developing trismus (reduced mouth opening). Trismus is often a persisting side-effect and difficult to manage. It impairs eating, speech and oral hygiene, affecting quality of life. Although several studies identified the masseter muscle (MM) as one of the main organs at risk, currently this structure is rarely considered during treatment planning. Prospective studies for chemoradiation are lacking. The aim of our study was to quantify the relationship between radiation dose to the MM and development of radiation-induced trismus in an IMRT-VMAT population. Results: At the first evaluation, 6-12 weeks post-treatment, fourteen patients had developed radiation-induced trismus (15%). On average, mouth opening decreased with 4.1 mm, or 8.2 % relative to baseline. Mean dose to the ipsilateral MM was a stronger predictor for trismus than mean dose to the contralateral MM, as indicated by the lowest -2 log likelihood (Table 1). Figure 1A shows the correlation between the ipsilateral mean masseter dose and the relative decrease in mouth opening, with trismus cases indicated in red. No trismus cases were observed in 33 patients (35%) with a mean dose to the ipsilateral MM 50 Gy. Patients with tumors located in the oropharynx were at highest risk.