Abstract

Metastatic melanoma with unusual histology can be diagnostically challenging. Malpractice lawsuits in dermatopathology represent about 10-15% of all pathology claims, with the diagnosis of melanoma representing the majority of the cases. One
exceptionally rare, cutaneous manifestation of metastases is a blue nevus-like metastatic melanoma (BNLMM). Gene expression profiling, preoperative high-resolution ultrasonography and fluorescence in situ hybridization probes have been used to
differentiate BNLMM from other lesions. BNLMM can be extremely difficult to distinguish from benign lesions, such as blue nevi and peripheral nerve sheath tumors, and other malignant entities such as desmoplastic melanoma. A 74-year-old male presented with a dark brown-black 1 x 0.5 cm lesion on his left superior helix in the same anatomical region of a previously resected lentigo maligna. Histopathologic sections demonstrated dermal atypical melanocytic proliferation with predominant dendritic morphology and pigment incontinence with epidermal sparing.  Perineural and angiotropic involvement by melanocytes was identified. Immunohistochemical (IHC) stains for SOX-10, HMB45 and PRAME highlighted melanocytic proliferation. P16 IHC stain showed complete loss of p16, which also favored a diagnosis of melanoma. The histologic and immunohistochemical presentation rendered a diagnosis of BNLMM. BNLMM can mimic desmoplastic melanoma and benign tumors of the skin. It is essential that pathologists be provided with adequate clinical history and a history of melanoma, in such cases to avoid a potentially serious diagnostic pitfall. We report this case to highlight the importance of this rare entity and to avoid misdiagnosis of BNLMM, which would significantly impact patient treatment, follow-up and outcomes.