Which imaging finding is most characteristic of pilocytic astrocytoma on MRI?

Explanation

## Pilocytic Astrocytoma — Imaging Hallmark **Key Point:** Pilocytic astrocytoma (WHO Grade I) characteristically presents as a **cystic lesion with an enhancing mural nodule** — this is the pathognomonic imaging pattern. ### MRI Features | Feature | Appearance | |---------|------------| | **Cyst** | Large, fluid-filled, T2 hyperintense, minimal/no enhancement | | **Mural nodule** | Small solid component, enhances homogeneously with gadolinium | | **Edema** | Minimal perilesional edema (benign behavior) | | **Location** | Cerebellum (50%), brainstem, optic pathway, supratentorial | ### Why This Pattern? The cyst forms due to **slow growth and local ischemia**, while the mural nodule represents the **viable tumor tissue**. This cystic-nodular morphology reflects the indolent, low-grade nature of the tumor. **High-Yield:** Pilocytic astrocytomas are **WHO Grade I** (benign) with excellent prognosis; the cystic-nodular pattern is so characteristic that it is virtually diagnostic on imaging alone. **Clinical Pearl:** In children presenting with cerebellar mass, a cystic lesion with mural nodule should immediately raise suspicion for pilocytic astrocytoma — often managed by resection of the mural nodule alone, leaving the cyst. **Mnemonic:** **PILO** = **P**erfect **I**maging **L**esion **O**f cyst-nodule (cystic with enhancing nodule).