Which histological feature is pathognomonic for clear cell renal cell carcinoma?

Explanation

## Histological Features of Clear Cell RCC **Key Point:** The hallmark histological feature of clear cell RCC is the presence of cells with abundant clear cytoplasm due to high intracellular lipid and glycogen content. This is the defining morphological feature that gives the tumor its name. ### Cytoplasmic Composition and Staining 1. **Lipid content:** Abundant neutral lipids (triglycerides, cholesterol esters) 2. **Glycogen content:** High glycogen accumulation 3. **Appearance:** Cells appear "clear" or "vacuolated" on H&E stain 4. **Special stains:** Sudan Black or Oil Red O stains highlight lipid (positive in frozen sections) 5. **PAS stain:** Highlights glycogen (diastase-resistant) ### Architectural Patterns in Clear Cell RCC | Pattern | Description | Frequency | |---|---|---| | Solid/nested | Sheets of clear cells with delicate vasculature | Most common | | Acinar/tubular | Cuboidal clear cells in small tubules | Common | | Cystic | Cystic spaces lined by clear cells | Variable | | Papillary | Papillary architecture (less common) | ~10% | **High-Yield:** The "clear cell" appearance is due to lipid extraction during routine tissue processing (formalin fixation dissolves lipids), leaving behind a vacuolated appearance. In frozen sections, the lipid is retained and cells appear more granular and eosinophilic. **Mnemonic:** **CLEAR** = **C**ytoplasmic lipid and glycogen, **L**ipid-rich, **E**xtracted during processing, **A**bundant vacuoles, **R**efined vasculature ### Differential Histology - **Chromophobe RCC:** Cells with "vegetable-like" cytoplasm (binucleate, wrinkled nuclei), not truly clear - **Papillary RCC:** Papillary architecture with foamy macrophages in cores, not predominantly clear cytoplasm - **Oncocytoma:** Granular eosinophilic cytoplasm (mitochondria-rich), not clear **Clinical Pearl:** The clear cell appearance can be mimicked by lipid-rich tumors, but the combination of clear cytoplasm + abundant vasculature + VHL loss is diagnostic of ccRCC.