Low-grade Oncocytic Fumarate Hydratase-deficient Renal Cell Carcinoma: An Update on Biologic Potential, Morphologic Spectrum, and Differential Diagnosis With Other Low-grade Oncocytic Tumors.

Fumarate hydratase-deficient renal cell carcinoma (FH-deficient RCC) is typically considered a high-grade, aggressive subtype of RCC that frequently arises in the setting of hereditary leiomyomatosis-renal cell carcinoma (HLRCC) syndrome. Increasing experience with HLRCC-associated RCC and FH-deficient RCC has resulted in recognition of tumors with lower grade morphologic features, overlapping with those of succinate dehydrogenase-deficient RCC and other low-grade oncocytic tumors. In this review article, we report a previously unpublished case that was recently encountered in our practice and review cases in the current literature with an aim of getting a better understanding of these oncocytic tumors and their morphologic spectrum. The 13 cases reviewed were approximately equally distributed across males and females, occurred at a younger age, and were more frequently seen in the right kidney, with both unifocal and multifocal presentations. While most presented an exclusive, low-grade oncocytic morphology, in 4 cases they were associated with either separate high-grade tumors, or as a secondary pattern in an otherwise conventional high-grade FH-deficient RCC. Loss of FH and 2 succinyl cysteine (2SC) positivity by immunohistochemistry supported their diagnosis, and are recommended to be performed alongside CD117, CK7, and CK20 in to aid classification in challenging oncocytic tumors. When occurring in isolation, these tumors are distinctive from their high-grade counterparts, with no reported adverse outcomes in cases reported thus far. As such, accurate diagnosis of this low-grade pattern among FH-deficient RCCs is worthwhile not only due to its association with HLRCC and need of genetic counseling and surveillance, but also due to more favorable prognosis. Finally, increasing experience with the low-grade end of the morphologic spectrum of FH deficient RCC reiterates that not all tumors of this subtype of RCC have a uniformly aggressive outcome.