Count them on thin post-contrast MRI — the number and size decide stereotactic radiosurgery versus whole-brain radiotherapy — at the grey–white junction and in the posterior fossa, and separate a solitary metastasis from a glioma or abscess.
Orient first
- Metastases lodge at the grey–white junction and watershed zones; most are multiple.
- Lung, breast, melanoma and renal cancer are the commonest primaries; melanoma and renal metastases bleed.
- Surrounding oedema is disproportionate to size; the peritumoural region has no infiltrating tumour (unlike glioma).
Acquire the study
- 3D post-gadolinium T1 at ≤ 1 mm (spin-echo preferred for small lesions), FLAIR, DWI/ADC, SWI.
The manoeuvre
- Scroll the 3D post-contrast series in all three planes; count lesions and measure each in mm.
- Posterior fossa and the cortex near the vertex — the commonest missed sites.
- SWI: haemorrhagic metastases (melanoma, renal, thyroid, choriocarcinoma).
- DWI: a restricting centre suggests abscess rather than a necrotic metastasis.
- Leptomeningeal enhancement on post-contrast FLAIR or T1.
What confirms it
- Multiple enhancing lesions at the grey–white junction in a patient with a known primary.
What licenses you to exclude it
- A negative thin-section post-contrast MRI makes metastases very unlikely at this time.
The classic misread
- Calling vessels on 3D gradient-echo T1 small metastases — follow them across slices.