A Case of Skeletal Muscle Metastasis Secondary to Carcinoma of the Tongue Base

Distant metastases in cases of head and neck cancer are commonly detected in the lungs, bones and liver. Metastasis in skeletal muscle is rare, because the tumor cells cannot survive the mechanical stimuli associated with muscle contractions. We report a case of skeletal muscle metastasis secondary to an oropharyngeal carcinoma. An 80-year-old man presented to our hospital with a hard mass lesion in his left neck. An ulcerative tumor of the tongue base and left cervical lymphadenopathy were found, and biopsy revealed oropharyngeal squamous cell carcinoma (cT2N2bM0). Upfront neck dissection on the left side was performed and radiotherapy (2 Gy/fraction, total 70 Gy) was administered, followed by palliative therapy for the residual oropharyngeal carcinoma. Three months later, the patient presented with walking difficulty and a 4-cm mass lesion in the left thigh. Contrast-enhanced magnetic resonance imaging (MRI) revealed an enhancing mass lesion with a low apparent diffusion coefficient in the left thigh. A needle biopsy revealed skeletal muscle metastasis from the oropharyngeal squamous cell carcinoma. At 7 months after the diagnosis of the skeletal muscle metastasis, the patient died from progression of the oropharyngeal carcinoma. Skeletal muscle metastasis is rare, and sometimes, it is difficult to differentiate this condition from soft tissue sarcomas. Needle biopsy and contrast enhanced MRI are useful for the diagnosis of skeletal muscle metastasis.