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Case Reports

Abstract

Introduction: Blue nevi (BN) are benign dermal melanocytic tumors named for their grossly distinctive blue-black appearance. BN are neurocristic lesions that arise independent of UV irradiation, deviating from the etiology of common acquired nevi and cutaneous melanomas. While primarily cutaneous, rare cases of nodal BN have been reported, posing a potential diagnostic pitfall in their distinction from metastatic melanoma. Herein, we report a nodal blue nevus identified incidentally in a sentinel lymph node (SLN) of a patient with multifocal carcinoma of the breast, emphasizing histopathological characteristics. Clinical Presentation: A 71-year-old female presented with multifocal invasive ductal and lobular carcinomas of the left breast. Histopathological examination of a SLN revealed an incidental, heavily pigmented melanocytic proliferation. A thorough history and physical examination revealed no history of previous melanoma (ocular, mucosal, or cutaneous) and no evidence of an undetected primary melanoma elsewhere. Results: The characteristics of the proliferation were consistent with a nodal blue nevus: confinement primarily to the nodal capsule and trabeculae, dendritic cytomorphology, lack of significant cytological atypia/pleomorphism, low proliferative rate (per mitotic count and Ki67 immunohistochemistry), retention of BAP1 expression, absence of PRAME expression, and absence of BRAF V600E mutant protein expression. Conclusion: The incidental discovery of nodal blue nevi during SLN biopsies can lead to diagnostic dilemmas. This case reinforces the importance of including benign nevi in the differential diagnoses of a nodal melanocytic neoplasm. Histopathological evaluation, supported by immunohistochemical and/or molecular analyses, can prevent misdiagnosis and unnecessary aggressive investigations or treatments.