An Unusual Journey From Lymphoma to Sarcoma: Presumed Transdifferentiation of Diffuse Large B‐Cell Lymphoma to Follicular Dendritic Cell Sarcoma—A Case Study

Diffuse large B‐cell lymphoma (DLBCL) is the most prevalent form of non‐Hodgkin lymphoma, capable of occurring in lymph nodes as well as various extranodal locations. In contrast, follicular dendritic cell sarcoma (FDCS) is a rare malignancy of low‐to‐intermediate grade that originates from stroma‐derived cells typically located in germinal centers. While histiocytic and dendritic cell neoplasms have been reported following B‐cell lymphomas, cases consistent with presumed transdifferentiation from DLBCL to FDCS remain exceedingly rare. We report a 42‐year‐old woman with a history of refractory germinal center subtype DLBCL who underwent multiple lines of chemoimmunotherapy and achieved a documented complete metabolic response (CMR). However, detailed follow‐up records during the subsequent interval were not fully available. Approximately two years later, she developed a rapidly enlarging cervical mass. Histopathological and immunohistochemical evaluation confirmed FDCS. Although the clinicopathologic findings were consistent with presumed lineage switching, molecular confirmation of clonality was not available. Despite surgical resection and initiation of gemcitabine‐based chemotherapy, the patient succumbed to a pulmonary embolism. This case highlights the diagnostic challenges of FDCS in heavily treated lymphoma patients and underscores the importance of molecular diagnostic testing to confirm clonal relationships. Given the limited evidence regarding optimal management of FDCS, a multidisciplinary approach remains essential.