Some older patients may receive screening for breast or prostate cancer even though the tests aren't recommended because they aren't expected to live long enough to benefit from treatment, a U.S. study suggests.

Screening isn't advised for patients with a life expectancy of less than 10 years because even if tests find cancer, it probably won't grow fast enough to kill them before something else like heart disease or diabetes does.

But about 16 percent of people aged 65 years and older who shouldn't get breast or prostate cancer screenings based on their limited life expectancies are still getting tests for these tumors, the study found.

"Evidence suggests that detection and treatment of early-stage tumors may reduce cancer mortality, but despite this potential benefit, screening programs also cause harm," said lead study author Dr. Firas Abdollah of the Henry Ford Hospital in Detroit.

For older people without long to live, over-diagnosis and overtreatment of cancer can lead to significant physical and mental health problems, Abdollah and colleagues report in JAMA Oncology.

In the U.S. alone, the cost of over-diagnosis may also run as high as $1.2 billion a year, the researchers also note.

To assess how often older patients get unnecessary screenings, researchers reviewed survey data collected in 2012 for almost 150,000 people 65 and older.

About 51 percent of these individuals reported having either a blood test to detect prostate cancer or a mammogram to look for breast tumors in the past year, the study found.

Roughly 31 percent of those screened had a life expectancy of less than 10 years, which corresponds to an estimated overall rate of non-recommended screening of 15.7 percent.

Rates of non-recommended screenings varied across the country, from a low of 11.6 percent in Colorado to a high of 20.2 percent in Georgia.

One limitation of the study is that it's possible some people over-estimated life expectancy, the authors note. Another shortcoming is that some people may have been previously diagnosed, treated or observed for prostate or breast cancers.

The recommendations against screening people with a limited life expectancy apply to individuals without a prior cancer diagnosis or current symptoms, noted Dr. Therese Bevers, medical director of the cancer prevention center at MD Anderson Cancer Center in Houston.

"There is screening that is appropriate to do, but I don't think screening should be done without a reason," said Bevers, who wasn't involved in the study.

A woman who feels a lump in her breast, for example, or a man who has difficulty with urination or ejaculation, might benefit from screening because these symptoms can point to cancer, Bevers added. These cases are considered necessary diagnostic tests, as opposed to preventive screenings.

Often, though, the challenge with patients who don't have symptoms is determining life expectancy, said Dr. Eric Klein, chairman of the Glickman Urological and Kidney Institute at the Cleveland Clinic in Ohio.

"It is simply an educated guess and nobody wants to be treated as a statistic," Klein, who wasn't involved in the study, said by email. "Some docs may not be aware of the guidelines, but most patients think they are going to live forever and want to do everything they can to make that happen."

The study findings underscore the importance of doctors and patients taking the time to discuss both the risks and harms of screening, including a frank discussion about what might happen next, said Dr. Ronan Factora, of the Center for Geriatric Medicine at Cleveland Clinic.

"The person should ask themselves if they would go through treatment if something was found - if the person would not go through treatment, they should reconsider looking for a disease that they are not going to treat," Factora, who wasn't involved in the study, said by email.

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