Renal Splenosis: Renal Mass Biopsy Diagnosis of a Tumor Clinically Mimicking Renal Cell Carcinoma.

Renal mass biopsy is increasingly used to guide conservative tumor management, placing increasing importance on pathologists' ability to diagnose small tumor samples. A 66-year-old man with a history of prior splenectomy for trauma presented for urologic evaluation after identification of a left 5.8 cm renal mass. Partial envelopment of the mass by renal parenchyma and equivocal enhancement on computed tomography raised concern for renal cell carcinoma. Needle-core biopsy revealed blood, subtle vasculature, few aggregates of lymphocytes, and rare renal tubules. Immunohistochemical staining revealed CD8-positive sinusoids but negative reactivity for epithelial antigens (PAX8, keratin, epithelial membrane antigen, carbonic anhydrase IX), supporting diagnosis of renal splenosis. Renal splenosis is a rare phenomenon that can form a sizeable intrarenal mass, mimicking renal cell carcinoma. Pathologists' awareness of this uncommon occurrence may avoid unnecessary surgery. CD8-positive sinusoids and negative epithelial markers are helpful confirmatory features in the biopsy setting.