Giving cancer patients a form of vitamin D could help radiotherapy work more effectively, researchers have suggested.
Radiation is used to kill off cancer cells
The combination could help wipe out cancer cells altogether.
Radiation therapy is commonly used prior to surgery to reduce the size of the tumour, and after surgery to eradicate any remaining cells and to reduce the chance of the tumour returning.
But there are often some cells which are resistant to the treatment and which could cause the cancer to recur.
Tests on mice have now shown combining a form of vitamin D with radiotherapy means these remaining cells can be destroyed.
In the study, researchers from Dartmouth Medical School compared tumour growth in mice given the combined therapy, using a derivative of vitamin D called EB 1089, and others given radiation therapy alone.
We're always trying to find drugs that will prevent cancer from recurring
Dr David Gewirtz, Virginia Commonwealth University Medical Center
Vitamin D is naturally manufactured by the body after exposure to sunlight, and it has been shown to help prevent and treat several forms of cancer.
But too much vitamin D leads to too much calcium being produced, which can affect bone metabolism and structure.
EB 1089 is modified so it has fewer calcium-related side-effects.
In those given the combined treatment, tumours were around 50% smaller than in those that received radiation alone.
Lead researcher Dr Sujatha Sundaram said: "The results of our latest study with EB 1089 are very encouraging.
"The vitamin D analog has proven effective in enhancing radiation treatments in our prior studies with cell cultures and now in live mice.
"We are eager to push ahead to clinical trials with breast cancer treatments in humans."
Human tests needed
Dr David Gewirtz, of the Virginia Commonwealth University Medical Center in Richmond, who also worked on the study, said: "We're always trying to find drugs that will prevent cancer from recurring, yet be less toxic to the patient than the current chemotherapy regimens.
"We're seeing very encouraging results in cell culture and animal studies when we add vitamin D analogs to radiation therapy."
The researchers hope that the treatment could help patients with radiation-resistant brain tumours and prostate cancer, as well as breast cancer.
Dr David Morgan, a member of the oncology committee of the British Institute of Radiology, told BBC News Online it was an interesting piece of research.
But he added: "Vitamin D and its analogs have been looked at a good deal in cancer generally.
"But it hasn't been established it really does have an anti-cancer effect in human trials.
"There are many new drugs which seem work in the laboratory or in mice but in real life, they don't."
The research is published in the Journal of Clinical Cancer Research.