Sinonasal mucosal melanoma of the maxillary sinus: Diagnostic challenges and therapeutic insights from a case report.

Primary sinonasal mucosal melanoma of the maxillary sinus is an exceptionally rare and highly aggressive malignancy that frequently presents with vague, nonspecific symptoms, resulting in substantial diagnostic delay. This case involves a 50-year-old male with an 18-month history of progressive left-sided diplopia, nasal obstruction, recurrent epistaxis, and hemifacial pain. CT demonstrated a large expansile maxillary sinus mass with bone erosion and superior extension toward the orbit, while MRI revealed a multiloculated T2-hyperintense lesion compressing adjacent structures. Histopathological evaluation confirmed mucosal melanoma with immunohistochemical positivity for HMB-45, Melan-A, and focal S-100 expression. Because of the locally advanced disease and the tumor's proximity to critical structures, only a biopsy was performed. Adjuvant intensity-modulated radiotherapy was subsequently delivered. This case highlights the pivotal role of multimodality imaging in characterizing sinonasal tumors, guiding staging and therapeutic decision-making, and emphasizes the importance of maintaining a high index of suspicion when encountering atypical or persistent maxillary sinus lesions.