Brain metastases most commonly arise from lung and breast cancers
Scientists believe they have found a potential way to stop cancers establishing themselves in the brain, and essentially becoming terminal.
A UK team discovered cancer cells hijack the brain's blood vessels to get all the nourishment they need to seed themselves there.
Key to this is a protein on the surface of cancer cells called integrin which allows them to stick to the vessels.
Drugs that block integrin may stop cancer spread PLoS ONE journal reports.
A fifth of all cancer patients will eventually have disease that has spread to the brain.
Indeed, brain metastases are the most common malignant tumours of the central nervous system, outnumbering by 10 times those that originate in the brain.
Once a cancer has spread to the brain the outlook is not good - even with maximal treatment the median survival is nine months.
Scientists at Oxford University, with funding from Cancer Research UK, the Medical Research Council and the US National Institutes of Health, wanted to investigate exactly how cancers spread.
Previously it had generally been assumed that tumour cells grew on the cells that make up the grey and white matter of the brain - the neurons and glial cells.
But Dr Shawn Carbonell and his team found that the metastatic cancer cells start to grow on the walls of blood vessels in the brain in over 95% of cases, and not on the nerve cells.
They looked at samples of a range of cancer cell types from humans and mice.
From this they also discovered that the removal of the integrin stopped the cancer cells from attaching to the blood vessels and starting to grow.
Dr Carbonell said although this finding was still a long way from coming up with a new treatment for those with brain metastases, it was exciting.
"We have identified the protein that cancer cells use to anchor themselves to blood vessels in the brain. Now we can try to come up with drugs to target this protein and stop metastatic cancer cells from taking hold in the brain."
Dr Helen George of Cancer Research UK said the discovery was "an important part of the puzzle" and paved the way for new and much-needed treatments to tackle cancers that have spread to the brain.