Metastatic Undifferentiated Melanoma Mimicking a Primary Bone Tumor: A Potential Diagnostic Pitfall.

ABSTRACT: Undifferentiated melanoma (UM) is defined by the loss of classic morphologic and immunohistochemical melanocytic markers. Reports in the literature are rare and show that UM usually occurs as a metastasis in the setting of a known primary cutaneous melanoma. The most common mutations in UM include those involving BRAF , NRAS , and KIT , which are almost invariably present in the parent melanoma. In this study, we report a case of a primary sinonasal melanoma with metastatic UM presenting with osteoclast-like giant cells and resembling a primary bone tumor. The retention of an unusual KRAS mutation in UM that was also present in the primary lesion provided critical information for the diagnosis. Our report highlights the importance of considering mutational analysis to identify undifferentiated melanomas in patients with metastatic tumors which do not have the typical histopathologic and immunohistochemical features of melanoma.

Primary cutaneous Epstein-Barr virus-positive diffuse large B-cell lymphoma (DLBCL) in a patient taking fingolimod.

A 55-year-old man with relapsing-remitting multiple sclerosis on fingolimod presented to the dermatology clinic with skin lesions on the left temple and cheek. Histopathology showed a diffuse infiltrate of enlarged, atypical lymphocytes throughout the dermis with an overlying grenz zone and a subpopulation of scattered smaller lymphocytes and plasma cells. Epstein-Barr virus-encoded RNA in situ hybridization stain was positive. Based on the morphologic and immunophenotypic findings, a diagnosis of EBV-positive diffuse large B-cell lymphoma was made. This case aims to raise awareness for the dermatologist that patients on fingolimod may be at increased risk of lymphoproliferative disorders.