The technique, tested in mice, uses a protein called TNF that can track down sites in the brain where cancer has spread.
Homing in on a marker found only on tumour blood vessels, the protein temporarily opens the "blood brain barrier" to allow cancer drugs in.
The blood brain barrier (BBB) is a biological shield that keeps out toxic chemicals and other harmful agents such as bacteria.
But it also acts as an obstacle to many drug treatments, including some targeting cancer.
TNF broke down the barrier only in blood vessels passing through a tumour, the research showed. Healthy parts of the brain remained shielded and protected.
The research, funded by the charity Cancer Research UK, is published in the Journal Of The National Cancer Institute.
Lead scientist Nicola Sibson, from Oxford University, said: "Treatments that work very well against the original site of the cancer lose their effectiveness when the cancer spreads to the brain. These drugs are prevented from getting to the tumour because of the blood brain barrier.
"A number of attempts have been made to open up the BBB but they've all struggled because they're either not specific enough to open the BBB only at the site of the tumour or not effective enough to allow the drug across to kill the cancer."
Kat Arney, science information manager at Cancer Research UK, said: "Getting treatments through the blood brain barrier remains one of the greatest challenges for cancer researchers.
"This exciting result points the way to a potentially game-changing moment in finding ways to treat cancers that have spread to the brain. We now need to test this approach in cancer patients to see if it will have the same effect."