A 35-year-old woman undergoes MRI brain for chronic headaches. Imaging reveals a well-demarcated, intra-axial tumor in the pons with hyperintense T2 signal and minimal enhancement. What is the most common primary CNS tumor at this location?

Explanation

## Most Common Tumor of the Pons **Key Point:** Diffuse intrinsic pontine glioma (DIPG) is the most common intra-axial brainstem tumor, accounting for 80% of all brainstem gliomas in children and young adults. ### Imaging Characteristics of DIPG | Feature | Finding | | --- | --- | | **Location** | Pons (intrinsic, diffuse infiltration) | | **T1 Signal** | Hypointense or isointense | | **T2 Signal** | Hyperintense | | **Enhancement** | Minimal to moderate, heterogeneous | | **Expansion** | Expands the pons, widens brainstem | | **Hemorrhage** | Rare | ### Clinical Context **High-Yield:** DIPG typically presents in children aged 5–9 years with: - Cranial nerve palsies (CN VI, VII) - Long tract signs (weakness, hyperreflexia) - Ataxia - Rapid progression over weeks to months **Clinical Pearl:** Unlike pilocytic astrocytomas (which are well-circumscribed and often cystic), DIPG is diffusely infiltrative with poor prognosis. Median overall survival is 9–11 months despite multimodal therapy. ### Why DIPG Is Most Common at This Site ```mermaid flowchart TD A[Brainstem Gliomas]:::outcome --> B{Location?}:::decision B -->|Pons| C[DIPG - 80%]:::action B -->|Midbrain| D[Pilocytic astrocytoma]:::action B -->|Medulla| E[Ependymoma or astrocytoma]:::action C --> F[Diffuse infiltration, poor prognosis]:::outcome D --> G[Well-circumscribed, better prognosis]:::outcome ``` **Mnemonic: DIPG Features — "PONS"** - **P**ons location (intrinsic) - **O**ften in children - **N**o good prognosis - **S**pread diffusely [cite:Osborn's Brain Imaging, Pathology, and Anatomy 2e Ch 8]