Concurrent follicular lymphoma and follicular dendritic cell sarcoma: composite neoplasm or transdifferentiation?

Follicular dendritic cell sarcoma (FDCS) is a rare neoplasm, and its synchronous occurrence with follicular lymphoma (FL) has not been reported. Whether composite tumors arise independently or through clonal evolution remains unclear. A 52-year-old woman presented with night sweats, weight loss, and an obstructing hepatic flexure colonic mass with mesenteric disease. Biopsy showed two tumor populations: an atypical B cell infiltrate positive for CD20, CD10, focal BCL6, and weak/focal MUM1, with kappa predominance and Ki-67 of 20-50%, consistent with follicular large B cell lymphoma/FL grade 3B, and epithelioid-to-spindled cells expressing CD21, CD23, CD35, CXCL13, and D2-40 and lacking B cell and epithelial markers, supporting FDCS. Fluorescence in situ hybridization demonstrated shared BCL6 rearrangements, IGH and MYC copy-number gains in both components, IGH rearrangement testing was negative, and next-generation sequencing identified alterations in JAK1, APC, FGFR4, NOTCH3, and TSC1. This case highlights biologic challenges in distinguishing transdifferentiation from synchronous neoplasms.