A Rare Case of Amelanotic Melanoma with an Unknown Primary in the Buccinator Muscle

Malignant melanoma demonstrates strikingly varied clinical presentations, on account of which it is often challenging to diagnose. We describe herein a rare case of amelanotic malignant melanoma (AMM) in the buccinator muscle presenting as a solitary mass. A middle-aged woman presented with a non-pigmented swelling under the right buccal mucosa. Magnetic resonance imaging (MRI) revealed a well-circumscribed lesion without local invasion. A needle biopsy did not establish the diagnosis and was instead interpreted as suggestive of a salivary gland tumor, reinforcing an initial impression of myoepithelioma. Histopathological analysis of an excised specimen revealed poorly defined, pleomorphic tumor cells and a scattering of melanin-containing cells. The diagnosis of AMM was established by integrating these histopathological features with the clinico-radiological findings and the immunohistochemical results, which were positive for HMB-45, S-100, and SOX10 and negative for cytokeratin. Whole-body positron-emission tomography-computed tomography found no primary lesion. Because tumor nests suggesting possible residual disease were observed near the resection margin, a wider, secondary excision including the affected portion of the buccal mucosa and overlying skin was performed together with neck dissection. Histopathological examination of the re-resection specimen showed no residual tumor, indicating that no malignant cells were identified in the adjacent oral mucosa or facial skin. The patient has remained disease-free for three years.