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Case Reports

Abstract

Primary cutaneous follicle center lymphoma (PCFCL) is an indolent B-cell lymphoma composed of malignant lymphocytes derived from germinal center B-cells, typically presenting as lymphoid follicles with atypical germinal centers with non-polar proliferation rates and minimal mantle zones. A prominent plasma cell component is not characteristic. In contrast, primary cutaneous marginal zone lymphoma (PCMZL) often has numerous clonal plasma cells, with background benign germinal centers. On rare occasion, PCFCL may have numerous monotypic plasma cells, potentially mimicking PCMZL. This uncommon variant of PCFCL with plamacytic differentiation poses a diagnostic challenge, as plasma cell–rich infiltrates and germinal centers can be seen in both reactive processes and PCMZL. We encountered this challenge in a 25-year-old male presenting with a slowly enlarging subcutaneous nodule on the right posterior shoulder. A prior superficial biopsy was initially interpreted as granulomatous. Complete excision revealed dermal and subcutaneous nodules composed of atypical lymphoid follicles with expanded germinal centers, atypical centrocytes and centroblasts, thin mantle zones, and interspersed monotypic plasma cells. Immunohistochemistry demonstrated blc6 and CD10-positive follicle center cells, IgD-positive mantle zones, and cyclin D1 negativity, supporting a follicle center origin. Plasma cells were monotypic by light chain assessment. FISH analysis did not reveal a BCL2 rearrangement [t(14;18)], consistent with PCFCL with plasmacytic differentiation. PCFCL with plasmacytic differentiation is rare and can easily be misclassified as PCMZL or a reactive plasma cell–rich process. Awareness of this uncommon presentation and recognizing features to differentiate benign from malignant germinal centers are crucial to avoid missing the diagnosis.