A 62-year-old man with a 10-year history of hypertension is found incidentally to have a 4 cm left renal mass on CT imaging done for abdominal pain. The mass is hypodense on non-contrast imaging and shows heterogeneous enhancement on contrast-enhanced CT. Core needle biopsy shows cells arranged in a nested pattern with abundant eosinophilic, granular cytoplasm and prominent cell membranes. Immunohistochemistry is positive for Hale's colloidal iron stain. Which of the following is the most likely diagnosis?

Explanation

## Diagnosis: Oncocytoma ### Histopathological Features **Key Point:** Renal oncocytoma is characterized by: - Cells with **abundant eosinophilic, granular cytoplasm** (due to packed mitochondria) - **Prominent cell membranes** - Nested (organoid) or solid sheet architecture - Round, regular nuclei with prominent nucleoli ### Special Stains & Immunohistochemistry **High-Yield:** Hale's colloidal iron stain is **positive** in oncocytoma (diffuse cytoplasmic staining), which is a key distinguishing feature from other renal tumors. - **Positive:** Hale's colloidal iron, CK7 (focal), CD117 (c-kit) - **Negative:** CD10, CAIX, vimentin > **Note:** Chromophobe RCC is also Hale's colloidal iron positive, but its cytoplasm is classically **pale and flocculent ("fluffy")** — NOT eosinophilic and granular. The stem's description of "eosinophilic, granular cytoplasm" with "prominent cell membranes" in a nested pattern is the textbook description of oncocytoma (Robbins & Cotran Pathologic Basis of Disease, 10th ed.). ### Epidemiology - Accounts for ~5–7% of all renal epithelial neoplasms - Arises from the intercalated cells (Type B) of the collecting duct - Peak incidence: 60–70 years; male predominance - Associated with Birt-Hogg-Dubé syndrome ### Clinical Presentation - Typically asymptomatic and discovered incidentally (as in this case) - Can present with haematuria or flank pain - Hypertension is a common comorbidity in this age group ### Imaging Characteristics - Hypodense on non-contrast CT - Heterogeneous enhancement on contrast-enhanced CT - Classic "spoke-wheel" pattern of enhancement (central scar) — though not always present ### Prognosis **Clinical Pearl:** Oncocytoma is a **benign** neoplasm with an excellent prognosis. Surgical resection (partial or radical nephrectomy) is curative. ### Differential Diagnosis Table | Feature | Oncocytoma | Chromophobe RCC | Clear Cell RCC | Angiomyolipoma | | --- | --- | --- | --- | --- | | **Cytoplasm** | Eosinophilic, granular | Pale, flocculent | Clear | Variable (fat) | | **Cell membrane** | Prominent | Prominent | Delicate | N/A | | **Hale's stain** | **Positive** | **Positive** | Negative | Negative | | **Prognosis** | Excellent (benign) | Intermediate | Poor | Excellent (benign) | | **Architecture** | Nested/solid | Nested | Nested/acinar | Triphasic | **Mnemonic:** **Oncocytoma = O**nly eosinophilic granular cells, **N**ested pattern, **C**olloidal iron positive, **O**utcome benign.