By Andrew Seaman

An influential panel of U.S. experts issued final recommendations on Monday reaffirming their controversial position that mammogram screening should start at age 50, but also said some women may benefit from screening starting at age 40.

Under the U.S. Preventive Services Task Force guidelines, mammogram screening every two years for women 50 to 74 got a grade of "B", meaning doctors should offer the service. Screening for women in their 40s got a "C" grade, meaning doctors should offer the service for select patients, depending on individual circumstances.

Critics have stressed that keeping 50 as the starting age for screening could threaten insurance coverage for millions of women aged 40 to 49. That is because insurers are not required to cover screening for women in their 40s, according to provisions in the Affordable Care Act.

Lawmakers, however, have already weighed in, adding an amendment to the 2016 U.S. spending bill that guarantees coverage for mammograms for women starting at age 40 through 2018.

Debate over the proper age at which doctors should start offering screening mammograms has raged since the task force first issued its recommendation in 2009. At the time, the panel cited evidence showing that the harm from over screening outweighed the benefits in cancer prevention.

Since that time, the task force has maintained that 50 is the best age to start routine screening. But it has left the door open for individuals who might benefit from screening starting at age 40.

"The task force believes that the science supports a range of individual choices for women to make for when to start screening, all the way from starting at age 40 or waiting until age 50, or anywhere in-between," said Dr. Michael LeFevre of the University of Missouri School of Medicine, Columbia, former chair of the government-backed panel.

The new, more inclusive wording of the guidelines is more in line with new recommendations from other cancer groups, such as the American Cancer Society. The society released new breast cancer screening guidelines in October pushing back the starting age for screening mammograms to 45 from 40, and recommending that younger women should have the choice to start screening as early as 40.

Dr. Otis Brawley, chief medical officer for the American Cancer Society, said he is happy with the Task Force's conclusions.

"We all recognize that the patient should be empowered and the patient should control her own body," he said.

According to the task force, screening 10,000 women in their 50s will result in eight fewer deaths, versus four fewer deaths for women who start screening at age 40.

Possible harms of breast cancer screening include unnecessary treatment for potentially harmless forms of breast cancer, incorrect results known as false positives and unnecessary additional testing.

    As long as women understand the balance between benefits and harms, they can make a reasonable decision to start screening anytime in their 40s, LeFevre said.

Women with mothers or sisters with a history of breast cancer may benefit more from screening in their 40s, according to the task force.