Primary Breast Langerhans Cell Histiocytosis in an Adolescent Girl: A Rare Diagnostic Case with Clinicopathologic and Immunohistochemical Insights

Background: Langerhans Cell Histiocytosis (LCH) is a rare clonal myeloid neoplasm characterized by the proliferation of CD1a-positive dendritic cells driven by activation of the MAPK pathway. Breast involvement is exceptionally uncommon, particularly in adolescents, where it may mimic more prevalent benign and malignant breast lesions, posing a significant diagnostic challenge. Case Presentation: A 16-year-old female presented with a progressively enlarging left breast mass. Radiologic assessment revealed a large heterogeneous lesion extending from the axilla into the breast, with cystic components, skin thickening, and vascular encasement, categorized as BI-RADS 4C. The patient underwent surgical excision. Histopathologic examination demonstrated sheets of large histiocytic cells with oval to grooved nuclei and abundant eosinophilic cytoplasm, accompanied by a prominent inflammatory infiltrate rich in eosinophils, lymphocytes, plasma cells, and multinucleated giant cells. Immunohistochemical analysis showed diffuse positivity for CD1a, S100, and CD68, confirming the diagnosis of LCH. Comprehensive staging revealed no evidence of multisystem involvement, consistent with single-system disease. The patient was treated with complete surgical excision without systemic therapy and remains clinically stable, 3 months postoperatively. Conclusion: This case highlights an exceedingly rare presentation of LCH as a primary breast mass in an adolescent. It underscores the importance of considering LCH in the differential diagnosis of atypical breast lesions in young patients. Definitive diagnosis relies on histopathologic and immunophenotypic evaluation, which is critical for appropriate management and avoidance of misdiagnosis.