The dilemma arises from the unique nature of prostate cancer — a common diagnosis that occurs in one out of every six men, but that isn’t lethal in most cases. As many as two-thirds of men diagnosed with prostate cancer have low-risk tumors that are small and slow-growing, yet nearly 90 percent of them immediately undergo harsh treatments including surgery or radiation, which can cause permanent side effects such as urinary incontinence or impotence. PSA testing and the biopsies that follow abnormal screening results often can’t distinguish between non-life-threatening tumors and those that will spread and kill a man, which leads to overtreatment.

“It’s always hard to stop something you’ve started,” said Dr. Leigh Simmons, an internist at Massachusetts General Hospital. “Before patients even get PSA screening, we need to talk about what we’re going to do if it comes back elevated.”

The task force’s new recommendation was based on the latest research showing that PSA screening saves few, if any, lives and that it has an overall net negative impact, leading to overtreatment of slow-growing cancers and resulting side effects.

While experts debate the merits of screening all men over age 50 for prostate cancer, most agree that the PSA test has significant flaws, both detecting too many harmless cancers and missing lethal growths. For every 100 men who get a PSA test, three will have real cancers detected and two will have cancers that are missed, according to the American Society of Clinical Oncology. Another five will have false positive results that may warrant a biopsy that turns out to be negative for cancer.

Of those real cancers detected via PSA screening, most men who have their tumors surgically removed enjoy no significant life-extending benefits, according to a study published last Thursday in the New England Journal of Medicine. The research, involving 731 men who were screened and diagnosed with early-stage prostate cancer, found that those who had immediate surgery to remove their cancer didn’t live any longer than those who had their cancers regularly monitored for 12 years and excised only if the cancer grew larger or more aggressive. Those with high PSA levels — above 10 nanograms per milliliter — did have a modest reduction in their risk of dying of prostate cancer if they were treated right after diagnosis.

Simmons and her colleagues practice what they call “shared decision-making,” believing it’s their role to help men determine for themselves whether or not to have PSA screening. One of her patients, 82-year-old Peter Albrecht from Newbury, decided to stop PSA screening a few years ago after the same government task force — called the US Preventive Services Task Force — issued a recommendation in 2008 against PSA screening in men over age 75 because of its lack of life-saving benefits. (At that time, the task force also said there was insufficient evidence to recommend for or against screening in men under 75.)

“Dr. Simmons and I discussed my age and that prostate cancer is slow growing and rarely becomes invasive, so it didn’t seem to make sense to continue screening,” said Albrecht.

Simmons has incorporated DVDs into her practice to teach patients about the pros and cons of screening. The video gets into the numbers, such as the potential of regular PSA testing over several decades to lower a man’s lifetime risk of dying from prostate cancer from 3 percent to, at best, 2 percent. On the other hand, the video explains, screening raises a man’s risk of being diagnosed with prostate cancer during his lifetime from 10 percent to 20 percent because it finds tiny cancers that wouldn’t otherwise be detected and likely wouldn’t have been fatal.