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Abstract

Homozygous deletion of CDKN2A (9p21) predicts aggressive behavior in primary cutaneous diffuse large B-cell lymphoma, leg type; it has only been reported in one prior case of primary cutaneous follicle center lymphoma (PCFCL) that we previously reported; the patient had an aggressive clinical course. We describe the cytogenetic evolution of a case of PCFCL into one with a 9p21 loss. The 2024 biopsy showed a T-cell-rich follicle center B-cell lymphoma composed of CD20+/CD79a+/Bcl-6+ CD10- centrocytic and centroblastic cells and variably Bcl-2+; cytogenetics were normal. In May 2025, a clinically enlarging nodule on the thigh showed interstitial and nodular dermal and subcutaneous infiltrate represented by a mixture of centrocytic and centroblastic cells mirroring the earlier biopsy, although without features of large cell transformation. FISH identified a homozygous 9p21 deletion in 64% of neoplastic B cells without MYC or Bcl2 rearrangements. Multiple synchronous lesions subsequently arose, signaling biological acceleration. Acquisition of 9p21 loss in nodal follicular lymphoma is associated with a worse prognosis. In the setting of primary cutaneous B cell lymphoma, the 9p21 deletion is almost exclusively reported in the leg type lymphoma, but clearly it can occur in follicle center lymphoma, where the clinical course could potentially be more aggressive, at least based on the two cases we have encountered and the literature precedent as it applies to nodal lymphoma. Routine FISH testing for CDKN2A in recurrent and progressive PCFCL, including cases that show morphologic evolution, should be performed.