In the Gleason system, which involves a pathologist’s assessment of how ominous the prostate cells look, 6 is actually pretty much the lowest score for cells that are cancer, despite the Gleason scale officially starting at 2. The highest is a 10. But many men, hearing that their cancer is a 6, assume the worst.

In the new system, which has been endorsed by the World Health Organization, instead of calling the cells Gleason 6, they will be called Group 1 in a scale that goes from 1 to 5.

One issue complicating the active surveillance questions, said Dr. Alan J. Wein, the chief of urology at the Perelman School of Medicine, is that the long-term outcomes are unclear.

“We need follow-up of at least 10 to 15 years to be sure we are not hurting these people,” he said. “The problem is we’ve been in the active surveillance business only since about 2000, and everyone started off very, very slowly. No one really has a number of patients who have gone for years and years.”

Dr. Wein advocates active surveillance for most men with low-risk cancers. But he emphasizes that they should have a second biopsy within a year, followed by regular biopsies every year or two and regular PSA tests, a blood tests that looks for a protein linked to prostate cancers.

“I tell patients, this is a bet,” he said. “You are betting that the disease is not going to progress, or if it progresses, you will be able to tell before it progresses to a situation where it is less treatable. You enhance the chances of winning the bet by doing a confirmatory biopsy.”

Mike Steskal, a commodities trader who lives and works just outside Philadelphia, decided to take the bet. He is 55 and was told last summer that he had prostate cancer after his doctor ordered a blood test for flulike symptoms that happened to include a PSA test. His PSA level was slightly elevated, which led to more tests, including a biopsy that showed a Gleason 6 cancer.