Atypical Presentation of IgG4-Related Disease: A Case Report of Obstructive Seminal Vesiculitis.

Immunoglobulin G4-related disease (IgG4-RD) is a systemic fibroinflammatory condition known to form "pseudotumors" that mimic malignancy. We previously reported the case of a 59-year-old male who presented with acute kidney injury from a unilateral ureteric obstruction, diagnosed as right-sided seminal vesiculitis. We now report the crucial clinical follow-up. During surveillance, the patient developed a new, contralateral pelvic mass and presacral thickening. A PET-CT scan confirmed high metabolic activity, raising suspicion for malignancy. Serological testing was definitive, revealing a markedly elevated serum IgG4 level of 7.76 g/L (normal <1.3 g/L). A CT-guided biopsy of the new lesion excluded malignancy and showed a dense lymphoplasmacytic infiltrate with an IgG4/IgG ratio >40%. The diagnosis was revised to multifocal IgG4-RD. The patient was commenced on Rituximab, which led to a marked improvement in both his serological markers and associated joint symptoms. This case demonstrates that a focal urological finding like seminal vesiculitis can be the novel debut presentation of a systemic IgG4-related disease, highlighting the need for a high index of suspicion for systemic inflammatory conditions in cases of atypical or evolving urological masses.