Intramasseteric Schwannoma Derived from the Masseteric Nerve

This report documents the intramasseteric onset of a schwannoma of the masseteric nerve, which is extremely rare. The patient was a 24-year-old woman who noticed an asymptomatic mass in the parotidomasseteric region 1 year before; a soft painless mass located near the angle of the left mandible was observed during the initial examination. No neuropathy, including sensory deficit, facial nerve paralysis, or trismus, was observed. Contrast-enhanced magnetic resonance imaging revealed an isolated neoplastic lesion within the body of the masseter muscle; the lesion exhibited low-signal intensity on coronal T1-weighted imaging and high-signal intensity on T2-weighted imaging with somewhat heterogeneous early uptake of contrast medium. The tumor was resected under general anesthesia and was found to be solid with distinct margins, enclosed within the masseter muscle, and continuous with a branch of the masseter nerve identified using intraoperative electrical stimulation. The lesion was a pale yellow solid mass encapsulated in a smooth membrane measuring 3.2 × 1.7 × 1.6 cm. Histopathological examination revealed a mixture comprising Antoni A pattern with Verocay bodies surrounded by oval nuclei exhibiting nuclear palisading, and an Antoni B pattern with loss of the characteristic cellular arrangement and separation of the cells, creating a more distinct individual tumor cell morphology. The final histopathological diagnosis was schwannoma. No complications, such as trismus or facial nerve paralysis, and no tumor recurrence were observed in the 1 year that has elapsed postoperatively.