– A deadly form of prostate cancer is far more common than previously thought, leading researchers to believe that more specific diagnoses could lead to better treatment and survival rates, according to a study released Monday by researchers at the University of California at San Francisco.

The study looked at 202 men with prostate cancer that had spread beyond the prostate and was resistant to standard treatment. It found that about 17 percent of these cases of metastatic prostate cancer were a deadlier subtype with specific genetic mutations. Researchers previously believed that less than 1 percent of all prostate cancers were in that category.

The findings, published in Journal of Clinical Oncology, suggest that this kind of cancer — called treatment-emergent small cell neuroendocrine prostate cancer, or t-SCNC — could be more successfully treated with targeted drugs.

"Think of advanced, hormone-treatment-resistant prostate cancers as a pie," said Dr. Rahul Aggarwal, assistant professor of medicine in the UCSF Division of Hematology and Oncology and one of the study's authors. "Instead of treating these advanced cases homogeneously as we do with today's standard treatments, we want to split the pie according to tumor characteristics."

That way, it may be possible to develop treatments tailored to individual tumors, based on their unique genetic mutations and other characteristics, he said.

In breast cancer, he said, there is already a clear understanding of subtypes, and different treatments are available for them. "We're trying to get to that same place in prostate cancer," Aggarwal said.

He added that knowing the prevalence of this aggressive form of prostate cancer will help inspire trials to treat it.

Only men have a prostate, which is a gland related to semen production located just below the bladder.

Typically, prostate cancer is treated with hormones and chemotherapy, according to the study. In cancers resistant to conventional treatment, patients with t-SCNC died, on average, nearly eight months sooner after their treatment failed than those without that type of cancer.

Nearly 30,000 men die from prostate cancer each year, with about 1 in 10 cases spreading beyond the prostate at the time of diagnosis.