CHICAGO (Reuters) — A physician's group issued new breast cancer screening guidelines on Monday for women in their 40s that leave the decision about whether to get a mammogram up to the women and their doctors.While it is clear that mammogram screenings can prevent breast cancer deaths in women aged 50 and over, the benefits of screening women in their 40s are not uniform, according to new guidelines issued by the American College of Physicians.

As a result, the group urges younger women to weigh their personal cancer risk and consult their doctor about whether regular mammogram screening makes sense.

"The evidence is not strong enough to say that all women should be screened and it should be based on individual reasons," said Dr. William Golden, who is chair of the board of regents for the American College of Physicians.

The group urged younger women to consider the harmful effects of mammograms, which include false-positive results, radiation exposure, false reassurance and pain during the mammogram procedure.

Younger women who decide not to screen for breast cancer should review the decision every one to two years until age 50, the group suggests.

The guidelines have renewed the debate about when women should start regular screening for breast cancer, which is the second-leading cause of cancer deaths among women in the United States after lung cancer.

Various groups recommend that women over the age of 40 get regular mammograms at least every three years, with some, recommending annual exams.

Twenty-five percent of all diagnosed cases of breast cancer are among women younger than 50.

Mammography screening can reduce breast cancer deaths by about 20 to 35 percent in women aged 50 to 69 and by about 20 percent in women aged 40 to 49, the US Centers for Disease Control and Prevention reported earlier this year.

The American Cancer Society called the new guidelines a "step backward" because they depart from years of consensus based on scientific evidence.

"It would be a major public health setback if these new guidelines caused some women and their doctors to conclude that screening can safely be postponed," said Robert Smith, director of cancer screening for the American Cancer Society, in a statement e-mailed to Reuters.

The new guidelines are based on a review of several studies showing differing conclusions on the benefits and harm of mammography among women in their 40s.

The study, to be published in the April 3 issue of the Annals of Internal Medicine, found that among younger women, the risk of breast cancer varies greatly.

A 40-year-old woman with no risk factors has about a 1 percent chance of getting breast cancer, while a 49-year-old woman with multiple risk factors — such as family history and early onset of menstruation — has about a 6 percent risk.