The research team found that precisely targeted radiation therapy could completely control the signs of cancer in 21 percent of patients who had five or fewer sites of metastatic disease.
"This was proof of principle in patients who had failed the standard therapies and had few, if any, remaining options," said the study's senior author, Ralph Weichselbaum, MD, professor and chairman of radiation and cellular oncology at the University of Chicago Medical Centre.
"We had encouraging results, including several long-term survivors, in patients with stage-IV cancers that had spread to distant sites," he added.
In 2004, Weichselbaum organized a clinical trial to test the ability of local radiation therapy to control a limited number of related tumours along with colleague Joseph Salama, MD, assistant professor of radiation oncology at the University of Chicago.
The trails included patients with stage-IV cancer with one to five distant metastases.
Each patient was given three doses of targeted radiation therapy separated by at least two days.
Treatment was usually completed within one week. The first patients in the study received lower doses. As few side effects were seen, radiation doses were gradually increased in subsequent groups of patients.
The study enrolled 29 patients, with a total of 56 cancerous lesions, enrolled in the trial from November 2004 through February 2008.
Of the 29 patients, 24 had progressed after at least one round of systemic chemotherapy. For the other five, there was no promising choice of therapy.
Six of the 29 initial patients had lasting tumour control, with no evidence of disease 15 months after treatment. Many patients had a complete response in at least one tumour.
Thirty-one of the 56 treated tumours completely disappeared. Two tumours had a partial response, defined as reduction in tumour volume of more than 30 percent.
Only three of the 56 tumours progressed growing in size by 20 percent or more during the treatment phase.
Tumour control improved as the radiation dose increased.
"We have seen improved response rates with higher radiation doses without an increase in side effects yet," said Salama.
The authors wrote that patients tolerated the treatment with "limited difficulty." All had some fatigue but few had serious side effects.
The technique could also be applied after chemotherapy, the authors suggest, in cases where the drugs had eliminated most the smaller cancer, leaving only a few larger tumours behind.
The study is published in the Aug 15 Clinical Cancer Research.