## Pilocytic Astrocytoma (WHO Grade I) — Imaging Pearls ### Key Imaging Features **Key Point:** Pilocytic astrocytomas are low-grade, slow-growing tumors with favorable prognosis, and their imaging reflects this benign behavior. | Feature | Characteristic | |---------|----------------| | **Location** | Cerebellum (50%), brainstem, optic pathway, cerebral hemispheres | | **T1 signal** | Hypointense to isointense | | **T2/FLAIR signal** | Hyperintense, well-demarcated borders | | **Enhancement** | Homogeneous, avid gadolinium uptake | | **Cystic component** | Present in ~50% (cyst + mural nodule pattern) | | **Perilesional edema** | **Minimal to absent** — key distinguishing feature | | **Mass effect** | **Mild** — despite size, causes less mass effect than expected | ### Why Perilesional Edema is NOT a Hallmark **High-Yield:** The **absence or minimal perilesional edema** is a defining imaging characteristic that separates pilocytic astrocytoma from higher-grade gliomas and other aggressive tumors. This reflects the slow growth rate and benign nature of the tumor. **Clinical Pearl:** When you see a well-circumscribed, enhancing cerebellar tumor with a cyst-nodule appearance but **NO significant edema**, think pilocytic astrocytoma first. The lack of edema despite the tumor size is reassuring and prognostically favorable. ### Differential Imaging Patterns | Tumor | Edema | Enhancement | Cystic | |-------|-------|-------------|--------| | **Pilocytic astrocytoma** | **Minimal/absent** | Homogeneous, avid | Common | | **Diffuse astrocytoma (Grade II)** | Moderate | Minimal or heterogeneous | Rare | | **Glioblastoma (Grade IV)** | **Extensive** | Ring-like, heterogeneous | Common | | **Medulloblastoma** | Moderate | Homogeneous | Rare | **Mnemonic:** **PEACE** = Pilocytic Astrocytoma Edema is Absent, Cyst-nodule Enhancement [cite:Robbins 10e Ch 28]
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