Review of renal oncocytosis (multiple oncocytic lesions) with focus on clinical and pathobiological aspects

Renal oncocytosis is a recently established disease entity characterized by numerous oncocytic tumors and diffuse involvement of oncocytic changes in renal parenchymal epithelia. In this article, we review this disease with a focus on its clinical and pathobiological aspects. Clinically, renal oncocytosis may occur in a sporadic form without any underlying disease or may be associated with chronic renal failure/long-term hemodialysis. However, Birt-Hogg- Dubé syndrome, characterized by skin tumors such as fibrofolliculoma or trichodiscoma, pulmonary lesions including bullae and spontaneous pneumothorax, and renal tumors should be evaluated in the differential diagnosis. The disease can develop either unilaterally or bilaterally. The involved renal parenchyma contains several to multiple brownish-colored nodules of varying size and is entirely replaced by lesions at the overt stage. Histologically, oncocytic tumors in both the dominant mass and smaller lesions encompass so-called hybrid tumor, chromophobe renal cell carcinoma (RCC), and renal oncocytoma (RO). Regarding renal parenchymal abnormalities, infiltrative growth of oncocytic cells, cortical cysts with oncocytic features, or extensive oncocytic change in non-neoplastic tubules can also be observed. Histochemical, immunohistochemical, and molecular genetic features of chromophobe RCC and RO arising in the setting of renal oncocytosis are generally identical to those in the sporadic type. However, hybrid tumors seem to be histologically distinct from chromophobe RCC and RO. In FISH analyses of some hybrid tumors, a gain of chromosomes 1, 2, 6, 10, and 17 was identified. In one tumor, no germ line mutation of folliculin gene was identified. Published data show that tumors follow a benign course. Further studies will be necessary to clarify the pathogenesis of renal oncocytosis.