Cancer drugs can be "posted" direct to tumours, researchers have said.
Radiotherapy can damage healthy cells
An anti-cancer drug was delivered directly to cancer cells inside hollow balls of molecules called liposomes.
The Institute of Cancer Research team said this molecular "envelopes" should mean powerful drugs can be sent directly to tumours.
Writing in the British Journal of Cancer, they said tests on mice showed giving drugs in this way reduced the risk of damage to healthy cells.
It is hoped that the finding will enable scientists to improve the effectiveness of radiotherapy for cancer patients.
Radiotherapy is used to kill cells in tumours, but nearby healthy cells are often damaged during the treatment.
The researchers were looking at ways of delivering the drug IUdR direct to cells.
IUdR is a type of drug known as a radiosensitiser - which makes cancer cells more sensitive to radiotherapy.
It can allow lower doses of radiation to be used in treatment, reducing the amount of damage to healthy tissue.
But the drug also has its own severe side effects, such as bone marrow suppression, which means it is not yet used on humans.
However, when the scientists used the molecular "envelopes", they found the drug did far less harm to healthy cells - because it came into less contact with them - but still made cancer cells more sensitive to radiation.
If similar results are seen in cancer patients, it would mean doctors could use larger doses to boost the effectiveness of radiotherapy treatment.
Dr Kevin Harrington, of Cancer Research UK's Targeted Therapy Laboratory, who led the research, said: "The benefits of using liposomes to deliver a drug stem from
their ability to deliver relatively more drug to where it's wanted.
"The immune system doesn't reject the liposome, and the 'enveloped' drug does far less harm to healthy cells than the drug on its own.
He added: "This is the first time that liposomes have been used with this particular drug.
"Further work is needed, but our results suggest that in a matter of years patients could safely receive active doses of IUdR.
"That would enable them to receive lower levels of radiation and still kill the cancer cells in their bodies."
Professor John Toy, medical director at Cancer Research UK, added: "As well as developing new drugs to fight cancer, we need new and better ways to deliver
"Reducing the side effects of this particular drug could, in turn, reduce the side effects associated with radiotherapy."
"Novel technologies such as liposomes offer the hope of more targeted, and therefore safer, treatments for cancer in the future."