Metastatic lesions are pretty common in the brain. If you look at all brain tumors in hospitalized patients, about 25% – 50% are metastatic tumors.

The top five primary sites (accounting for 80% of all intracranial metastases) are:
1. Lung
2. Breast
3. Skin (melanoma)
4. Kidney
5. Gastrointestinal tract

It’s strange that some tumors (like choriocarcinoma) are super-likely to metastasize to the brain, but others (like prostate carcinoma) almost never do. Also, the pattern of involvement can vary from tumor to tumor. Lung and breast carcinomas, for example, are likely to show up as multiple nodules studding the meninges (this is called meningeal carcinomatosis).

Metastatic tumors in the brain usually present like mass lesions (with signs of increased intracranial pressure and localized symptoms). Sometimes, it’s the brain metastasis that’s the first sign of the primary tumor.

Grossly, metastatic lesions are usually sharply-demarcated masses (as you can see in the image above). They are generally much more sharply circumscribed than gliomas (like astrocytoma) – with the exception of metastatic melanoma, which often is sneakily infiltrative. They often present at the junction between gray and white matter. Sometimes, the tumor is centrally necrotic, and surrounded by reactive gliosis.

For treatment, if the metastasis is solitary, it’s often useful to use local treatment to shrink the metastasis. If there are multiple metastases, there is less benefit to treating each metastasis – you could do more harm than good.