Background: We review the pathological, cytopathological, and molecular features centered on renal oncocytoma and its differential diagnosis. The recent expansion of entities under the category of renal tumors with oncocytic or eosinophilic cytoplasm has important implications on how cytologic diagnosis is clinically considered. Summary: In this first of two parts, we discussed the pathological spectrum of oncocytic or eosinophilic tumors of the kidney that includes oncocytoma; chromophobe renal cell carcinoma (ChRCC) - including its eosinophilic variant (eosinophilic ChRCC); hybrid oncocytic/chromophobe tumors, either sporadic or syndromic; oncocytic papillary RCC, acquired cystic disease-associated RCC; succinate dehydrogenase (SDH)-deficient RCC; and eosinophilic solid and cystic (ESC) RCC. We describe the histomorphological and immunohistochemical features of these tumors, including the newly accepted entities, and focus on the molecular alterations reported. A practical approach for differential diagnosis and broader correlation to available cytologic findings are provided, with more in-depth cytologic descriptions for oncocytoma and eosinophilic ChRCC included in part 2 of this review. Most of the oncocytic tumors have an indolent behavior, although few aggressive cases have been reported in patients with ESC RCC, eosinophilic vacuolated tumor, and SDHdeficient RCC. Key Messages: In this era where surveillance management for low-grade oncocytic renal tumors is considered, precise diagnosis is important as it will have an impact on their subsequent management. Further, accurate diagnosis is important especially in renal tumors associated with hereditary neoplasms for monitoring and genetic counseling for their family members.
