Low-Grade Oncocytic Tumor of Kidney (CK7-Positive, CD117-Negative): Incidence in a single institutional experience with clinicopathological and molecular characteristics.

Low-grade oncocytic tumor of the kidney (LOT) is characterized by cytoplasmic eosinophilia and a CK7-positive/CD117-negative immunophenotype. Morphologically, they exhibit overlapping features with oncocytoma and chromophobe renal cell carcinoma. Our aim was to obtain long-term clinical follow-up data, clinicopathological and molecular characteristics, and incidence of LOT. Tissue microarrays were constructed from 574 tumors historically diagnosed as oncocytoma and surgically treated at Mayo Clinic between 1970 and 2012, and immunostained for CK7 and CD117. An extended immunophenotype was obtained on whole slide sections, along with FISH for CCND1 rearrangement status and chromosomal microarray for copy number status. In addition, two cases were retrospectively identified in a set of tuberous sclerosis complex (TSC)-associated neoplasms and three more cases diagnosed on needle core biopsies were obtained during routine clinical practice. Twenty-four cases of LOT were identified among 574 consecutive tumors diagnosed as oncocytoma and treated with partial or radical nephrectomy, corresponding to an incidence of 4.18% of tumors historically diagnosed as oncocytomas, and 0.35% of 6944 nephrectomies performed between 1970 and 2012. Overall, 29 cases of LOT were identified in three clinical settings: sporadic, TSC-associated, and end-stage renal disease (ESRD). Multifocality was seen only in the setting of TSC and ESRD. No metastases attributable to LOT were identified (median follow-up 9.6 years). There were no recurrent arm level copy number changes detected by chromosomal microarray and all tested cases were negative for CCND1 rearrangement by FISH. LOT is an uncommon eosinophilic renal neoplasm with an indolent prognosis that constitutes ∼4% of tumors historically diagnosed as oncocytoma. The morphologic, immunophenotypic, and molecular features of this neoplasm suggest it is a distinct entity of renal neoplasia.