| Summary: | Background: Follicular dendritic cell sarcoma (FDCS) is an extremely rare malignant tumor caused by proliferation of antigen-presenting follicular dendritic cells. The tumor most commonly arises in lymph nodes, with fewer than 30% of cases originating in extranodal sites. Case Presentation: We report the case of a 39-year-old previously healthy male patient who presented with paraneoplastic pemphigus in the setting of a large intraperitoneal mass with no associated lymphadenopathy. The biopsy results showed a cellular proliferation of mildly to moderately atypical oval-to-spindle-shaped cells admixed with a variably dense lymphoplasmacytic infiltrate suggestive of FDCS versus inflammatory myofibroblastic tumor. A pathology review following total resection of the abdominal mass confirmed the diagnosis of FDCS. The tumor cells showed positivity for CD35, CD21, CD45, and CD68, negative ALK staining, and limited smooth muscle actin immunoreactivity. The surgery was complicated by severe pneumonia and acidosis, resulting in a prolonged ICU stay and death. Conclusion: It is essential to maintain a high index of suspicion for FDCS and include it in the differential diagnosis of a spindle cell neoplasm, especially in the setting of paraneoplastic pemphigus. The histological and immunohistochemical features should be sufficient to confirm the diagnosis of FDCS.
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