Multifocal Low-Grade Oncocytic Tumor of the Kidney Associated With Germline TSC1 Alteration: Case Report and Literature Review.

Low-grade oncocytic tumor (LOT) of the kidney is a rare, indolent neoplasm within the spectrum of eosinophilic renal tumors. It typically presents as a small, solitary mass and is associated with molecular alterations in TSC1, TSC2, or MTOR. We describe a 72-year-old male with a history of pancreatic neuroendocrine tumor, gastrointestinal stromal tumor, and prostate cancer who presented with multiple bilateral renal masses. Partial nephrectomy revealed two distinct tumors: one consistent with LOT and the other with a sclerosing angiomyolipoma (AML). The LOT consisted of oncocytic cells with round to oval nuclei and delicate perinuclear halos, arranged predominantly in solid architecture, and immunoreactive for PAX8, EMA, and CK7, whereas negative for CD117 and AMACR. The AML was composed of spindle and epithelioid cells embedded in sclerotic stroma, positive for MiTF and SMA, and negative for conventional melanocytic markers, including HMB45 and Melan-A. Germline testing identified a TSC1 intron 6, c.509-15G > A variant of uncertain significance. Somatic analysis showed increased allelic imbalance at the TSC1 locus, suggesting loss of heterozygosity and a potential pathogenic role. This case adds to the limited reports of multifocal LOT and demonstrates the consistent association of germline TSC1 alterations with this rare tumor presentation.