The answers vary greatly by age, sexual potency before treatment, PSA levels and whether a man has surgery, standard radiation or radioactive seeds, the study found.
Using the findings, men can get a rough idea of their personal odds by answering questions that also include weight and race, experts said. Unsurprisingly, older men whose sexual function is already low have the worst chances of good sexual function after treatment.
“This will make it possible for patients to have a more realistic view of what to expect for themselves, rather than trying to guess where they fit in overall compared to the average guy with prostate cancer,” said study co-author Dr. Martin Sanda of Beth Israel Deaconess Medical Center in Boston.
The federally funded study, whose findings were published in the Journal of the American Medical Association, included only men with early-stage cancers and it didn't address cure rates for different treatments. In the real world, some patients have limited treatment choices. For instance, only men with early-stage, slow-growing cancers can choose radioactive pellets.
That means the findings may be most useful in giving a realistic picture of the sexual side effects of a treatment that's already chosen, rather than helping men pick a treatment.
Researchers studied factors affecting potency in men and developed a questionnaire to help measure it. They evaluated its usefulness among 1,027 prostate cancer patients ages 38 to 84 and validated its accuracy in a separate group of more than 1,900 men. It correctly predicted the chances of post-treatment potency in 77 percent of the men who had surgery to 90 percent of the men who had radiation.
The study found great variation in men's odds of having firm erections after treatment.
For surgery, the prospects for maintaining top sexual function for a 50-year-old man could range from 21 percent to 70 percent, depending on his PSA level and whether a nerve-sparing technique was used.
For standard radiation, the study found, the odds for a man keeping top sexual function ranged from 53 percent to 92 percent, depending on PSA level and whether hormones were used along with radiation. The researchers couldn't find an age-related difference for standard radiation.
For radiation pellets, a 60-year-old man's chances of keeping top sexual function ranged from 58 percent to 98 percent, depending on race and body mass index. African-Americans and trimmer men had better chances.
The study has some deficiencies, experts said.
It didn't include men who chose what is called “active surveillance,” in which a doctor keeps track of a tumor through regular tests and treats it only if the cancer markedly worsens. Many prostate cancer patients with slow-growing tumors can live their whole lives without symptoms or treatment, said Dr. Durado Brooks, prostate cancer director for the American Cancer Society. They die of something else before the cancer kills them.
“There are a lot of prostate cancers that are not likely to benefit from treatment,” Brooks said. Including an active surveillance group would have shown how much sexual function changes because of factors other than treatment, such as age and general health, he said.
Most patients treated for early-stage prostate cancer survive, so the common side effects of treatment — incontinence and sexual problems — have become more important for patients making choices.
The researchers followed the men for two years after treatment.
In the men who were potent before treatment, 52 percent reported problems with erections two years after treatment. That included 60 percent of the men who had their prostates removed, 42 percent who had regular radiation and 37 percent of the men who had radioactive seeds, also called brachytherapy.
Brooks of the American Cancer Society cautioned that sexual problems can show up later for men who have either type of radiation therapy. A two-year follow-up may not have been long enough, he said.
The study was done from 2003 through 2006, a time when laparoscopic surgery, with small incisions and often performed robotically, was less common than it is today.