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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\u201350%, 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\n                    <jats:italic>BCL6<\/jats:italic>\n                    rearrangements,\n                    <jats:italic>IGH<\/jats:italic>\n                    and\n                    <jats:italic>MYC<\/jats:italic>\n                    copy-number gains in both components,\n                    <jats:italic>IGH<\/jats:italic>\n                    rearrangement testing was negative, and next-generation sequencing identified alterations in\n                    <jats:italic>JAK1<\/jats:italic>\n                    ,\n                    <jats:italic>APC<\/jats:italic>\n                    ,\n                    <jats:italic>FGFR4<\/jats:italic>\n                    ,\n                    <jats:italic>NOTCH3<\/jats:italic>\n                    , and\n                    <jats:italic>TSC1<\/jats:italic>\n                    . This case highlights biologic challenges in distinguishing transdifferentiation from synchronous neoplasms.\n                  <\/jats:p>","DOI":"10.1007\/s12308-026-00736-z","type":"journal-article","created":{"date-parts":[[2026,9,26]],"date-time":"2026-09-26T13:56:45Z","timestamp":1790431005000},"update-policy":"https:\/\/doi.org\/10.1007\/springer_crossmark_policy","source":"Crossref","is-referenced-by-count":0,"title":["Concurrent follicular lymphoma and follicular dendritic cell sarcoma: composite neoplasm or transdifferentiation?"],"prefix":"10.1007","volume":"19","author":[{"given":"Mahsa","family":"Hajizadeh","sequence":"first","affiliation":[],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Jinbo","family":"Fan","sequence":"additional","affiliation":[],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Jeffrey W.","family":"Craig","sequence":"additional","affiliation":[],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Ifeyinwa E.","family":"Obiorah","sequence":"additional","affiliation":[],"role":[{"vocabulary":"crossref","role":"author"}]}],"member":"297","published-online":{"date-parts":[[2026,9,26]]},"reference":[{"issue":"2","key":"736_CR1","doi-asserted-by":"publisher","first-page":"161","DOI":"10.1111\/cup.13571","volume":"47","author":"A Papoudou-Bai","year":"2020","unstructured":"Papoudou-Bai A, Vassou A, Marinos L, Papathanasiou K, Kapsali E, Kanavaros P (2020) Concurrent cutaneous localization of Langerhans cell sarcoma and chronic lymphocytic leukemia\/small lymphocytic lymphoma in a patient with a history of chronic lymphocytic leukemia\/small lymphocytic lymphoma. 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