## Imaging Characteristics of Simple Renal Cyst **Key Point:** A simple renal cyst is a benign lesion that requires NO follow-up and is classified as Bosniak category I. ### CT Criteria for Simple Cyst | Feature | Simple Cyst | Complex Cyst | Cystic Neoplasm | |---------|-------------|--------------|------------------| | **Attenuation (HU)** | <20 (water density) | 20–90 | Variable | | **Enhancement** | None | Minimal to moderate | Significant | | **Wall** | Thin, imperceptible | Thin to thick | Thick, irregular | | **Septations** | None | Thin or thick | Present | | **Bosniak Category** | I | II–IV | III–IV | **High-Yield:** The hallmark of a simple cyst is **homogeneous low attenuation (water density, <20 HU) with NO enhancement** on contrast-enhanced imaging. This is the most reliable criterion for benignity. **Clinical Pearl:** Simple renal cysts are extremely common (prevalence increases with age) and are incidental findings in up to 50% of patients >50 years old. They require no intervention or surveillance. **Tip:** On unenhanced CT, a simple cyst appears as a round or oval lesion with attenuation similar to water (0–10 HU). On contrast-enhanced scans, there is NO increase in attenuation — this lack of enhancement is the key differentiator from solid masses and complex cysts. ## Why Other Findings Are NOT Simple Cysts - **Homogeneous enhancement >20 HU** → suggests a solid lesion or hyperdense cyst (may contain protein/hemorrhage); requires follow-up. - **Heterogeneous enhancement with septations** → complex cyst or cystic neoplasm; Bosniak II–IV; needs surveillance or intervention. - **Peripheral rim enhancement with central necrosis** → suggests malignancy (renal cell carcinoma with central necrosis) or infected cyst; requires urgent evaluation. 
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