A 35-year-old woman undergoes MRI for headaches and is found to have a midline posterior fossa mass. On imaging, the lesion shows T1 hypointensity, T2 hyperintensity, and marked homogeneous enhancement with gadolinium. There is no restricted diffusion. Which imaging feature best distinguishes medulloblastoma from ependymoma in this location?

Explanation

## Medulloblastoma vs Ependymoma: Key Imaging Discriminators ### Clinical Context Both medulloblastoma and ependymoma are common posterior fossa tumors in children and young adults, but their imaging signatures differ in important ways. ### Comparative Imaging Features | Feature | Medulloblastoma | Ependymoma | |---------|-----------------|------------| | **DWI/ADC** | Restricted diffusion (high cellularity) | No restriction (lower cellularity) | | **Enhancement** | Homogeneous, intense | Heterogeneous, variable | | **Cystic change** | Rare | Common (50–80%) | | **Hemorrhage** | Uncommon | Rare | | **Location** | Midline, 4th ventricle floor | Intraventricular, lateral recesses | | **Calcification** | Uncommon | Common (40–50%) | ### Best Discriminator: Restricted Diffusion **Key Point:** Restricted diffusion on DWI (low ADC) is the single most reliable imaging feature distinguishing medulloblastoma from ependymoma. This reflects the extremely high cellularity and densely packed cells of medulloblastoma, which restrict water diffusion. **High-Yield:** Medulloblastoma is one of the most cellular CNS tumors (high nuclear-to-cytoplasmic ratio), making DWI restriction a hallmark finding. Ependymomas, by contrast, have lower cellularity and typically show no diffusion restriction. **Clinical Pearl:** In a young patient with a midline posterior fossa mass, restricted diffusion on DWI strongly favors medulloblastoma and warrants urgent staging (spine MRI, CSF analysis) and neurosurgical consultation, as medulloblastoma is highly aggressive and prone to leptomeningeal spread. ### Why Other Features Are Less Discriminatory - **Homogeneous enhancement:** While medulloblastoma tends to enhance homogeneously, ependymomas can also show intense enhancement; this is not a reliable discriminator. - **Cystic components:** Although ependymomas more commonly have cysts, medulloblastomas can also present with cystic/necrotic areas, especially larger tumors. - **Location:** Both can occur in the midline 4th ventricle; location alone does not reliably distinguish them. [cite:Robbins 10e Ch 28; Osborn Neuroradiology 3e]