Blue Nevus-Like Metastatic Melanoma in a Patient With Primary Lentigo Maligna Melanoma.

Metastatic melanoma with unusual histopathology can be diagnostically challenging. One exceptionally rare cutaneous manifestation of metastases is blue-nevus-like metastatic melanoma (BNLMM). A 74-year-old male presented with a blue-gray lesion on his left helix in the same anatomical region of a previously resected lentigo maligna. Histopathological sections demonstrated an atypical biphasic proliferation of dendritic melanocytes with pigment incontinence and epidermal sparing, measuring > 0.05 mm in diameter. Although the majority of the cell population exhibited cytologic features consistent with a blue nevus, there was a subset of dendritic cells with irregular epithelioid contours and rare, typical dermal mitotic figures. Sections showed an increased Mart-1/Ki67 rate. Perineural and angiotropic involvement by the atypical melanocytes was identified. Immunohistochemical (IHC) stains for SOX-10, HMB45, and PRAME highlighted melanocytic proliferation. BAP-1 IHC was retained, and p16 IHC showed complete loss. No previous procedure site changes were present, indicating that the dendritic cell proliferation was separated from the primary invasive melanoma by normal dermis. The histopathological and immunohistochemical findings led to a diagnosis of microsatellite BNLMM. We report this case to highlight the importance of this rare entity and to avoid misdiagnosis of BNLMM, which would significantly impact patient prognosis, treatment, follow-up, and outcomes.