Doctors seldom tell patients about the possible harms of getting screened for cancer, a new study shows.

During any screening test, there is a chance of so-called overdiagnosis - finding something that looks like cancer but isn’t, or a cancer that’s so small and slow-growing it would never cause a problem.

In those cases, patients may get biopsies, surgeries, radiation and drugs that won’t bring them any benefit, but could come with side effects, known as overtreatment.

“Usually when you talk to your doctor, you only learn about the benefits,” said Odette Wegwarth. She led the study at the Max Planck Institute for Human Development in Berlin.

“Nobody questions whether there could potentially be any harm,” Wegwarth said.

The chance of overdiagnosis and overtreatment varies from one screening test to another and isn’t always known.

With PSA tests to detect prostate cancer and mammograms for breast cancer, there are 10 or more cases of overdiagnosis for each life saved, the researchers write.

Wegwarth and her co-author Gerd Gigerenzer surveyed 317 American adults in their 50s and 60s. All of those people had been invited to undergo cancer screening by their doctors in the past. None had been diagnosed with cancer.

Just 30 of the participants - less than one in 10 - said their doctor had brought up the chance of overdiagnosis and overtreatment when talking about screening. But 80 percent said they would want to be told about those potential harms.

It’s possible some people in the study were told about overdiagnosis, but they don’t remember it or doctors talked over their heads, said David Howard, a health policy researcher at Emory University in Atlanta, who was not involved in the new study.

But he also said many doctors probably aren’t bringing up the risks of screening.

“Many doctors themselves may be vaguely aware of the concept of overdiagnosis, but unaware of the magnitude or unable to quantify the risk,” he told Reuters Health.

“I can also imagine physicians being wary of introducing the topic because it also prolongs the visit … and out of concern that patients might not be able to understand it.”

Most people in the study said they would continue with cancer checks they were already receiving regularly if they learned 10 people would be overtreated for each life saved by the test.

But 69 percent would not start a new screening test that had those odds, according to the findings published in JAMA Internal Medicine.

Wegwarth said people seem to have “an emotional attachment” to screening tests they get regularly.

“It’s probably really difficult to make people understand that the screening they believe in can be sometimes bad too,” she told Reuters Health. “They believe in what they have been taught and told.”

But just about everything in medicine comes with a downside, Wegwarth said.

For patients, Howard recommended “going into the conversation with the physician being ready to ask questions.”

Dr. H. Gilbert Welch from The Dartmouth Institute for Health Policy and Clinical Practice in Lebanon, New Hampshire, advised seeking out information about overdiagnosis even before going to the doctor.

“This is not a simple discussion,” Welch, who wrote a commentary published with the study, told Reuters Health.

“The truth is these are complex enough issues that the patients need some time with these ideas before they come to the doctor’s office.”

SOURCE: JAMA Internal Medicine, online October 21, 2013.