Separate the big three by location, diffusion and behaviour: medulloblastoma (midline vermis, restricts), pilocytic astrocytoma (cyst with an enhancing nodule, no restriction), ependymoma (fourth ventricle, squeezes through the foramina) — then image the whole neuraxis for drop metastases.
Orient first
- Posterior fossa tumours are the commonest brain tumours in children.
- ADC is the best single discriminator: low in medulloblastoma, high in pilocytic astrocytoma (verify local values).
- Diffuse midline glioma of the pons (H3 K27-altered) expands the pons and is not biopsied in typical cases.
Acquire the study
- DWI/ADC, T2, FLAIR, 3D T1 pre- and post-gadolinium; whole spine post-contrast sagittal T1.
The manoeuvre
- Location on sagittal T1: vermis (medulloblastoma), hemisphere cyst with nodule (pilocytic), fourth ventricle floor (ependymoma), pons (diffuse midline glioma).
- ADC map: solid tumour ADC — low vs high.
- Extension through the foramina of Luschka and Magendie (ependymoma).
- Hydrocephalus; tonsillar herniation.
- Whole spine post-contrast: leptomeningeal drop metastases.
What confirms it
- Histology and molecular classification.
What licenses you to exclude it
- A normal MRI excludes a posterior fossa tumour.
The classic misread
- Imaging the spine only after surgery — post-operative blood mimics metastases.