Introduction: Trismus is a treatment-related late side effect in patients treated for cancer in the head and neck region (HNC). The condition can have a considerable negative impact on nutrition, dental hygiene, ability to speak and quality of life. We have previously studied trismus within the frame of a randomized phase 3 study of HNC patients treated with mainly three-dimensional (3D) conformal radiotherapy (CRT) and found a strong association to mean radiation dose to the mastication muscles, especially the ipsilateral masseter muscle (iMAS). In the present study we have investigated trismus prevalence and risk factors in a more recent cohort of patients, treated with todays' more updated radiation techniques.
Material and methods: Maximal interincisal distance (MID) was measured on 139 consecutive patients. Trismus was defined as MID ≤35 mm. Patient-, disease- and treatment-specific data were retrospectively recorded. Differences between groups were analyzed and mean absorbed dose to mastication structures was evaluated. Dosimetric comparisons were made between this study and our previous results.
Results: The prevalence of trismus was 24% at a median of 16 months after completion of radiotherapy. In bivariate analysis treatment technique (3DCRT vs. intensity modulated radiotherapy or helical tomotherapy), tumor site (oropharynx vs. other sites) and mean radiation doses to the ipsilateral lateral pterygoid muscle, the paired masseter muscles and the iMAS were significantly associated with MID ≤35 mm. In multivariable analysis only mean radiation dose to the iMAS was significantly associated to MID ≤35 mm.
Conclusion: Mean radiation dose to the ipsilateral masseter muscle is an important risk factor for trismus development. Dose reduction to this structure during radiotherapy should have a potential to diminish the prevalence of trismus in this patient group.