Eosinophilic solid and cystic renal cell carcinoma (ESC-RCC) is a recently recognized and rare subtype of renal cell carcinoma, formally established as a distinct entity in the 2022 WHO classification of renal tumors. Previously categorized under “unclassified RCC,” ESC-RCC has now been defined based on its unique morphological, immunohistochemical, and biological characteristics. The diagnosis of ESC-RCC remains challenging due to its rarity and the limited number of cases reported in the literature.

Case presentation: A 43-year-old male with no significant past medical history or symptoms underwent a health examination, during which an incidental renal mass was detected on ultrasonography. A contrast-enhanced abdominal CT was subsequently performed, revealing a 3.7 cm well-circumscribed, enhancing solid mass with cystic component located at the lower pole of the right kidney. The imaging findings were suggestive of renal cell carcinoma (cT1N0). The RENAL nephrometry score was 4p, indicating low complexity and an estimated 6.4% risk of operative complications. Laboratory studies showed a serum creatinine level of 0.91 mg/dL. After detailed counseling, the patient consented to undergo partial nephrectomy. A transperitoneal off-clamp robotic partial nephrectomy was performed using three robotic arms and one assistant port. Final pathological examination confirmed the diagnosis of eosinophilic solid and cystic renal cell carcinoma, staged as pT1a (Stage I).

ESC-RCC typically presents as a solitary, small-sized and low-stage renal mass. It can occur across a wide age range but shows a marked female predominance. Most patients are asymptomatic, and the tumor is often discovered incidentally during imaging studies. ESC-RCC is characterized by indolent biological behavior and a low potential for metastasis. The primary treatment for ESC-RCC is surgical resection, either partial or radical nephrectomy, depending on the tumor stage and anatomical complexity. The typical gross morphology of ESC-RCC is solitary, generally grayish-yellow, ranging from 12 to 135 mm in diameter (mean size, 34 mm). The tumor typically exhibits both solid and cystic components and is clearly demarcated from the surrounding renal parenchyma. Microscopically, distinct solid and cystic areas are observed. The solid components are arranged in diffuse, acinar, or nested patterns. The tumoral cells present eosinophilic voluminous cytoplasm with coarse basophilic granules, often described as “stippling.” The cystic areas vary in size, and their walls are typically lined by hobnail-like tumor cells. The most important immunohistochemical profile of ESC-RCC is CK20 positivity and CK7 negativity.

This case of incidentally detected ESC-RCC in an asymptomatic patient was successfully treated with robotic-assisted partial nephrectomy. This case highlights the importance of considering it in the differential diagnosis of cystic renal masses, particularly in asymptomatic individuals.