A 58-year-old Indian man with a 10-year history of chronic kidney disease presents with a left renal mass. Imaging shows a solitary 4 cm mass with minimal fat stranding. On immunohistochemistry, the tumor cells are positive for cytokeratin 7 (CK7) and negative for CD10. Which feature best distinguishes this tumor from clear cell RCC?

Explanation

## Chromophobe RCC vs. Clear Cell RCC: Immunohistochemical and Morphological Distinction ### Clinical Context The patient has a solitary renal mass in the setting of chronic kidney disease (CKD). The immunohistochemical profile (CK7+, CD10−) suggests a **chromophobe RCC** rather than clear cell RCC. **Key Point:** While CK7 positivity and CD10 negativity are helpful, the **morphological hallmark of chromophobe RCC is the presence of binucleate cells with wrinkled, irregular nuclei resembling "raisin-like" or "vegetable-like" appearance**—this is the most reliable discriminator. ### Chromophobe RCC Characteristics | Feature | Chromophobe RCC | Clear Cell RCC | |---------|---|---| | **Origin** | Intercalated cells of collecting duct | Proximal tubular epithelium | | **Cytoplasm** | Pale, granular (flocculent) | Clear, lipid/glycogen-rich | | **Nuclear morphology** | Wrinkled, irregular, "raisin-like" | Round to oval, variable grade | | **Binucleate cells** | Characteristic, frequent | Rare | | **CK7** | Positive | Negative | | **CD10** | Negative | Positive | | **Hale's colloidal iron** | Positive (cell membrane) | Negative | | **Frequency** | 5% of RCC | 70–80% of RCC | | **Prognosis** | Intermediate (better than ccRCC) | Worst | | **Association with CKD** | Yes (acquired cystic kidney disease) | No | ### Morphological Discriminator **High-Yield:** The **wrinkled, raisin-like nuclear membrane** is the single most distinctive morphological feature of chromophobe RCC on H&E staining. This is pathognomonic and immediately separates it from ccRCC. **Mnemonic:** **CHROME** — **C**lear cytoplasm (flocculent, not lipid-rich), **H**igh binucleate cells, **R**aisin-like nuclei, **O**rigin from collecting duct intercalated cells, **M**ore indolent, **E**xcellent prognosis relative to ccRCC. ### Why CK7+/CD10− Is Not the Best Discriminator **Warning:** While the immunohistochemical profile (CK7+, CD10−) is useful and supports chromophobe RCC, it is NOT the single best discriminator because: - Some oncocytomas (benign) also show CK7+/CD10− - Immunohistochemistry requires special staining and is not always performed - Morphology on routine H&E is the gold standard and is always available **Clinical Pearl:** Chromophobe RCC is associated with Birt-Hogg-Dubé (BHD) syndrome and acquired cystic kidney disease in CKD patients. The patient's 10-year CKD history makes chromophobe RCC more likely than ccRCC.