Session Item

Doses to mastication structures critical for radiation-induced trismus in head and neck cancer (HNC) depend on radiation therapy (RT) technique. Doses by older, less conformal, RT techniques often exceeded currently proposed tolerance doses, but it is unknown how newer techniques fare in this context. The purpose of this work was to investigate how doses to the temporomandibular (TM) joint, masseter muscles, and pterygoid muscles in VMAT for HNC compared to earlier RT techniques.

We investigated two HNC patient cohorts treated with external beam RT at the Sahlgrenska University Hospital, Sweden, in 2007-2012 (3D-CRT and/or IMRT; n=195 [study 1]) or 2017-2020 (VMAT; n=117 [study 2]). All patients had undergone RT without mastication-structure-sparing intent. All included patients had normal mouth-opening ability before RT and were followed up with respect to trismus status at 3, 6 and 12 months post RT. Trismus was defined as maximal interincisal opening [MIO] ≤35 mm at any post-RT follow up. The TM joint, masseter and medial/lateral pterygoid muscles were consistently delineated on the planning CT:s. Mean dose comparisons for structures were made using Student’s unpaired t-test. P-values≤0.05 indicated statistical significance.

Within 12 months post RT, 109/195 (56%) of patients in study 1 had experienced trismus compared to 15/117 (13%) in study 2. Averaged mean doses (±1 SD) for the masseter muscles were 17.6±4.5 Gy in study 1 and 10.0±4.6 Gy in study 2 (p<0.001). Corresponding numbers were 8.5±7.6 Gy and 2.3±2.5 Gy for the TM joints (p<0.001), 25.9±5.1 Gy and 19.1±8.4 Gy for the medial pterygoid muscles (p<0.001), and 13.0±9.0 Gy and 4.8±4.8 Gy for the lateral pterygoid muscles (p<0.001). Masseter muscle doses were above the proposed tolerance in 73% of patients in study 1 compared to 12% in study 2 (Figure 1). The corresponding numbers were 44% and 6% for the TM joints, 88% and 58% for the medial pterygoid muscles and 22% and 1% for the lateral pterygoid muscles.

Figure 1. Mean mastication structure doses for patients with trismus (red) and without trismus (blue) given older (leftmost/structure) and newer (rightmost/structure) RT techniques. Tolerance doses for 10% risk of trismus: 30 Gy for masseter muscles; 15 Gy for TM joint, and 40 Gy for medial/lateral pterygoid muscles (Pauli N et al. Acta Oncol 2016: 55:788; Rao SD et al. Acta Oncol 2016:55:99; Kamal M et al. RO 2020:149:142; Hauge C et al. Acta Oncol 2018:57:1038).

Modern RT without mastication-structure-sparing intent resulted in 77% trismus reduction in HNC patients compared to patients treated with older techniques. Mastication structures mean doses by VMAT were generally below currently proposed tolerance doses, with dose reductions around 6-8 Gy for all structures compared to older techniques. The margin to “safe” dose levels were smaller for the medial pterygoid muscles compared to the other structures suggesting that focusing on this structure may lower trismus rates further.