Perigastric Follicular Dendritic Cell Sarcoma Mimicking Metastatic GIST: Insights from Imaging and Immunohistochemistry

Follicular dendritic cell sarcoma (FDCS) is an exceptionally rare malignant neoplasm, accounting for less than 0.4% of all soft tissue sarcomas. Intra-abdominal cases are even rarer and are frequently misdiagnosed because of morphologic and immunophenotypic overlap with gastrointestinal stromal tumor (GIST) in the gastrointestinal tract. We report a 53-year-old woman with intra-abdominal FDCS presenting with a 6.5 × 5.2 cm perigastric mass, hepatic metastases, and retroperitoneal lymphadenopathy, initially suspected as metastatic GIST. Immunohistochemistry resolved the diagnosis: CD21 +, CD23 +, D2-40 +, with negative C-KIT, DOG1, and cytokeratin, confirming FDCS. Baseline 18 F-fluorodeoxyglucose (FDG) positron emission tomography/computed tomography (PET/CT) demonstrated marked metabolic activity (primary maximum standardized uptake value [SUVmax] 10.9, hepatic metastases 10.6, lymph nodes 5.2). Following gemcitabine-docetaxel chemotherapy, sequential PET/CT at 12 weeks showed significant metabolic response per PERCIST 1.0 criteria (primary SUVmax 4.0, 61% reduction; lymph nodes SUVmax 1.4, 73% reduction). This case highlights diagnostic pitfalls of FDCS mimicking GIST and illustrates the emerging utility of serial PET/CT in treatment response evaluation.