Primary Malignant Melanoma of the Maxillary Sinus Initially Misdiagnosed as Large Cell Lymphoma: A Cytological Pitfall.

We report a case of malignant melanoma (MM) originating in the maxillary sinus in an 84-year-old man, who was initially misdiagnosed with large-cell lymphoma based on fine-needle aspiration cytology (FNAC). The cytological specimens showed a clear background with loosely cohesive, monotonous, and large atypical cells exhibiting high nuclear-to-cytoplasmic ratios and irregular nuclear contours without recognizable melanin pigment at the time of evaluation. These features led to the preliminary interpretation of malignant lymphoma. A biopsy specimen obtained from the nasal cavity revealed nests of atypical cells with enlarged, irregular nuclei, and prominent nucleoli. A few tumor cells contained brown granules, suggesting melanin pigmentation. Immunohistochemistry revealed tumor cells positive for HMB-45, melan-A, and SOX10, confirming the diagnosis of MM. A retrospective review of the cytological specimens revealed scattered pigmented tumor cells and intranuclear cytoplasmic pseudo-inclusions (Apitz bodies). This case illustrates a diagnostic pitfall in which amelanotic MM cytologically mimics large-cell lymphoma, particularly in the sinonasal region. Even in the absence of overt pigmentation, careful attention to subtle cytological features, such as Apitz bodies and scattered pigmented tumor cells, is essential, and MM should be considered in the differential cytological diagnosis of large-cell neoplasms.