Atlanta ? Mammograms remain the most important tool in detecting breast cancer and women need not worry about performing breast self-exams, the American Cancer Society said Thursday.

The Atlanta-based society updated its breast cancer guidelines for the first time since 1997. More research has confirmed the society’s 1997 recommendation for women to receive mammograms annually from age 40.

“A lot of women were reading a year or so ago that some people were not sure whether mammography had any benefit,” said Debbie Saslow, the society’s director of breast and gynecologic cancers. “The level of confidence in the benefit is higher than ever. Mammograms find 80 percent to 85 percent of cancers — we know they increase survival dramatically.”

The largest change in the guidelines involves the breast self-exam, which previously was recommended once a month. But research has found the exams did not contribute to breast cancer survival rates.

Where mammograms typically find cancers that have grown for two years, self-exams typically detect cancer that has been growing for six years, Saslow said.

“We don’t have evidence that doing it every month is having any survival benefit,” she said. “For us it’s not a huge change as a lot of people weren’t doing breast self-exams anyway. To the public it probably is a big change.”

The recommendations say women in their 20s should be told about the benefits and limitations of the self-exam and that it is OK for women to choose to perform it occasionally or not to perform it at all.

“Unfortunately by the time you can feel something, it’s big enough where it’s either had a chance to spread and grow or it’s pretty benign and it wouldn’t hurt if you didn’t find it,” Saslow said.

The society also said women ages 20 to 39 should receive a clinical breast exam every three years and annually for women age 40 or older.

Women at increased risk may wish to have mammographies at age 30 as well as breast ultrasound or MRI.