Malignant Melanoma in Paranasal Sinus Without Expression of S-100 Protein: a Case Report and Review in Literature.

Introduction: In more than 95% cases of malignant melanoma, the expression of S-100 protein is positive in immunostaining. We report a case with malignant melanoma in paranasal site without the expression of S-100 protein. Case presentation: A 52-year-old man was referred to our hospital on May 22th, 2017 because of 2-month history of nasal bleeding and diplopia. Physical examination revealed temporal hemianopsia, visual disturbance with light perception, total ophthalmoplegia, and absence of light, convergence reflex in his left eye. Laboratory data showed the count of complete blood cells was within normal range and levels of LDH or soluble IL2 receptor was 163 U/l, 443.0 U/ml. PET-CT showed hypermetabolic lesions in the left ethmoid sinus compressing the left optic nerve, liver and L5 spinous process. HE staining of biopsied specimens from left paranasal tumor showed diffuse proliferation of anaplastic large cells classified as a small round cell tumor. Immunostaining revealed the negativity of CD3, CD20, chromogranin A, synaptophysin, AE1/3, CAM5.2, CD5/6, p40, p63, NUT-CST, myogenin, desmin, MIC2, S-100 and NSE. But CD45(focal), CD56 and INI-1 were positive. We suspected this tumor as NK/T-cell lymphoma from clinical and pathological findings. Concerning the progression of his visual disorder, we immediately started SMILE regimen and radiotherapy. To confirm the diagnosis, the additional immunostaining was performed. The tumor cells were negative for CD4, CD8, CD33, CD34, CD68, c-kit, myeloperoxidase, calretinin, PAX-5, CD79a, TdT, EBER-ISH, TIA-1, and were positive for CD10(focal), vimentin, HMB45, and Melan A. Finally, he was diagnosed with malignant melanoma with liver and bone metastases. He has been treated by an immune checkpoint inhibitor of anti-PD-1 monoclonal antibody. Conclusion: The diagnosis of malignant melanoma without S-100 protein is complicated. We require careful interpretation of pathological findings without being misled by CD56 positivity.