Immunoglobulin G4-related Disease Mimicking Portal Vein Tumor Thrombus

We report the case of a 72-year-old Japanese man with an incidental portal vein mass that was surgically resected and diagnosed as immunoglobulin G4 (IgG4)-related disease. The mass was discovered during an atrial fibrillation examination. The patient had a history of gastric cancer and was also diagnosed with rectal cancer, raising concerns about metastasis. Due to technical challenges, a biopsy was not feasible. Imaging findings suggested portal vein tumor thrombosis, complicating the diagnosis. This case highlights a rare presentation of IgG4-related disease mimicking portal vein tumor thrombus.