Follicular dendritic cell sarcoma associated with IgG4-related disease: a case report and literature review.

Follicular dendritic cell sarcoma (FDCS) is a rare malignant tumor involving lymph nodes and extranodal sites, classified into conventional and inflammatory pseudotumor (IPT)-like variants. The IPT-like variant predominantly involves the liver and spleen. Immunoglobulin G4-related disease (IgG4-RD) is a systemic immune-mediated disorder characterized by multiorgan involvement, whereas isolated esophageal and gastric involvement is exceedingly rare. Although FDCS may exhibit infiltration of IgG4-positive plasma cells, the coexistence of FDCS and IgG4-RD has not been previously reported. Herein, we describe a unique case of FDCS associated with IgG4-RD involving the liver, esophagus, and stomach. The patient initially presented with hepatic lesions and gastrointestinal ulcers, accompanied by elevated serum IgG4 levels and histopathological features suggestive of IgG4-RD. Following glucocorticoid and immunosuppressive therapy, gastrointestinal lesions improved; however, the hepatic lesion progressed and was subsequently confirmed as FDCS by histopathology and immunohistochemistry. This case highlights the diagnostic challenges posed by overlapping clinicopathological features between FDCS and IgG4-RD.