Among men whose low-risk prostate cancer was managed with so-called active surveillance for up to 15 years, just 1.5 percent died of the cancer, according to new data from a Canadian study. 

That result is similar to outcomes in men whose cancers are treated immediately, the authors write.

Prostate cancer often grows very slowly. In some men, such as the elderly or those with serious health problems, it may never need to be treated, says the American Cancer Society.

In the Canadian trial, 993 men with low or intermediate risk cancers were enrolled in active surveillance between 1995 and 2013. By now, more than 200 of them have been observed for more than 10 years and 50 for more than 15 years.

“This is the third time we’ve published the key results of our long term surveillance cohort,” said lead author Dr. Laurence Klotz of Sunnybrook Health Sciences Center in Toronto.

The men were monitored with regular testing. Treatment was started if the cancer progressed.

As of now, only 27 percent of the men have been treated for their cancers with radiation therapy, radical prostatectomy or androgen-deprivation therapy.

Of the 933 patients, 149 have died, but only 15 died from prostate cancer, the researchers reported in the Journal of Clinical Oncology.

All the men who died from the cancer had metastases by the end. Another 13 patients had metastases but died from causes other than prostate cancer. In all, less than three percent of the men developed metastatic cancer.

That’s similar to the rate of metastases in another study of men with low-risk disease who were treated immediately, according to Dr. Matthew R. Cooperberg of the University of California, San Francisco.

“In recent years, active surveillance has evolved from an experimental protocol to a broadly accepted - in fact, preferred - management strategy for men diagnosed with low-risk prostate cancer,” he wrote in an editorial in the journal.

Twenty years ago, treating every prostate cancer patient was the norm, Klotz told Reuters Health by phone.

“Over the years this has evolved,” he said.

“This whole approach is one of evolution and we can do better with that one or 1.5 percent,” who end up dying from the disease, Klotz said.

In this group of low-risk cancers, about 25 percent turned out to be “wolves in sheep’s clothing,” he said. Those that metastasized weren’t low-grade disease that spread, rather they were hidden higher-grade disease that doctors missed, he said.

But doctors are getting better at identifying those cases. Now, magnetic resonance imaging can detect many of the more dangerous cancers that may have missed with a biopsy 20 years ago, Klotz said.

Men in the study who died from prostate cancer succumbed about 15 years after diagnosis, usually in their 80s, he noted.

“It really looks like (active surveillance) is a safe strategy for the management of probably 40 to 50 percent of newly diagnosed prostate cancer patients,” he said.

Overtreating prostate cancers that would not ultimately be fatal can lead to incontinence, erectile dysfunction and other problems, he said.

“That’s why I think this approach is so important, if you can significantly reduce overtreatment but you still have the benefit of screening,” Klotz said.

Active surveillance has been widely embraced in Canada and has been somewhat slower to catch on in the U.S., but is becoming more common, he said.

“The bottom line is, it’s catching on and I also think the role of MRI will provide further reassurance,” for doctors and patients, Klotz said.