Dual Malignancies in IgG4-Related Disease: A Diagnostic and Therapeutic Challenge.

We report a rare case of IgG4-related disease (IgG4-RD) presenting with retroperitoneal fibrosis and autoimmune pancreatitis, followed by sequential development of papillary thyroid carcinoma and diffuse large B cell lymphoma. Histopathologic and positron emission tomography-computed tomography findings were essential for diagnosis and guided timely treatment. This case highlights the potential oncogenic link between chronic IgG4-driven inflammation and B cell malignancy, emphasizing the need for vigilant cancer surveillance in patients with IgG4-RD.