Cutaneous Acral Presentation of Indolent Mantle Cell Lymphoma With Prominent Nail Involvement and Atypical Immunophenotype.

Mantle cell lymphoma (MCL) is a rare, aggressive subtype of B-cell non-Hodgkin lymphoma with rare cutaneous involvement that typically indicates advanced systemic disease. We report a case of indolent MCL in a 56-year-old previously healthy male who presented with a 5-year history of edematous, painful plaques and associated onychodystrophy in the bilateral toes. Histopathology demonstrated a multinodular dermal and subcutaneous infiltrate of atypical lymphocytes. Immunohistochemistry revealed a primary B-cell population diffusely positive for CD20 and BCL2 and focally positive for BCL1. These cells were negative for CD5, SOX-11, BCL6, and LEF1. In situ hybridization demonstrated admixed lambda-restricted plasma cells, indicating plasmacytic differentiation. Fluorescence in situ hybridization testing for an IGH/CCND1 translocation was positive, confirming a diagnosis of MCL. While bone marrow biopsy demonstrated 10%-20% involvement by MCL, staging revealed no lymphadenopathy or visceral disease. No significant peripheral blood laboratory abnormality was noted. To our knowledge, this is the first reported case of cutaneous involvement by MCL confined to the bilateral toes. This case is also notable for its absence of typical markers, namely CD5 and SOX-11, plasmacytic differentiation, and indolent clinical behavior. We discuss the diagnostic and clinical significance of these atypical clinicopathologic features and diagnostic overlap with other cutaneous B-cell lymphomas, emphasizing the importance of comprehensive immunophenotypic and molecular evaluation in atypical presentations.