The Canadian-led research shows that men who were given intermittent courses of drugs that suppress the production of male hormones lived as long as men who received continuous therapy.
But the men on the intermittent course had fewer of the unpleasant side-effects that go along with this type of prostate cancer treatment.
Androgen-suppression therapy, as it's called, can induce hot flashes, impotence, growth of breast tissue, insomnia, weight gain, worsening of diabetes, loss of muscle mass and osteoporosis.
The study looked only at men who did not have metastatic prostate cancer, meaning cancer that had moved to other parts of the body.
It is published in this week's New England Journal of Medicine.
The work was led by the NCIC Clinical Trials Group, the research arm of the Canadian Cancer Society. The Canadian Cancer Society provided much of the funding for the trial.
About two-thirds of the patients in the trial were Canadians, though trial sites were also located in the United States and Britain.
Dr. Laurence Klotz, one of the principle investigators of the trial, said since the trial was started a number of years ago, many doctors in Canada have adopted intermittent androgen-suppression therapy for their prostate cancer patients.
But Klotz, a prostate cancer specialist at Toronto's Sunnybrook Health Sciences Centre, says these findings provide support for the move. Klotz said he expects intermittent therapy to become the standard of care for these patients.
The findings don't relate to men whose prostate cancers have spread. A companion study — which was presented recently to a major cancer research conference — found for men with metastatic cancer, intermittent therapy did not have the same safety profile. But those findings haven't yet been published in a scientific journal.
In this study, nearly 1,400 men who had undergone treatment for prostate cancer — radiation or surgery and radiation — and whose PSA levels had begun to rise again were randomly selected to either receive continuous treatment or intermittent treatment.
PSA stands for prostate specific antigen, a protein found in the blood that can be a signal of prostate cancer.
All the men were given eight months of hormone therapy; it suppresses production of testosterone, which fuels prostate cancer. Then the men in the intermittent arm were taken off the drug until their PSA levels reached a set threshold.
For some, the drug holiday lasted six months. Others were off the drugs for as long as five years. "Some guys were big winners," Klotz said in an interview.
The men were followed over time. There was no statistically significant difference in the number of deaths in the two groups; nor was there a significant difference in the median survival time.
The median age of men in the trial was 74 years and the median survival was roughly nine years.
The study doesn't provide much information on the improvement in quality of life for the intermittent group. That will be detailed in another scientific paper, Klotz said.
But these drugs are pricey, and given the number of men using them, a major cost to health-care systems. Men in the intermittent arm ended up getting about one-third less drug, without risk to themselves, the study suggests.