Focal IgG4-related periprostatic "PI-RADS 5" pseudotumor mimicking prostatic adenocarcinoma

Immunoglobulin G4-related disease (IgG4-RD) is a systemic fibroinflammatory disease characterized by raised serum IgG4 levels and tumefactive inflammation affecting multiple organ systems, typically involving the pancreas and biliary tree. Though rare, prostatic involvement has been reported in a few cases and is suspected to be an underreported entity. Our patient is a 63-year-old gentleman who has presented with an incidental "PI-RADS 5" (Prostate Imaging Reporting & Data System) prostate lesion and perivascular soft tissue cuffing of the superior rectal vessels on MRI rectum performed for surveillance of rectal neuroendocrine tumor. He had a history of lacrimal gland IgG4-RD. The lentiform prostate lesion subtly indents the prostate capsule, reminiscent of a periprostatic rather than an intraprostatic lesion. Perivascular cuffing of superior rectal vessels suggest inflammatory vasculitis of IgG4-RD. Differential diagnosis of periprostatic inflammatory IgG4-RD was considered, subsequently proven on MRI-ultrasound fusion targeted biopsy. Reported radiological findings of prostate IgG4-RD typically show diffuse chronic inflammation of the prostate, with a minority of the reports describing focal involvement, often mimicking focal prostate adenocarcinoma. Focal periprostatic involvement of IgG4-RD is an unusual manifestation which should be considered in patients with IgG4-RD who present with a periprostatic pseudotumor. IgG4-RD of the prostate usually responds well to steroid treatment without the need for surgery.