False positives on a mammogram may lead to unnecessary biopsies or treatments.
"You really decrease you chance of harm if you screen every two years," lead researcher Karla Kerlikowske, a primary care physician and professor of medicine at the University of California, San Francisco, said to Bloomberg Businessweek. "The fear was that if you do it every two years versus every year, you are going to have a really big tumor when you are diagnosed. The reality is these tumors just don't grow that rapidly."
The results, which were published on March 18 in JAMA Internal Medicine, follow the 2009 recommendation by the U.S. Preventive Services Task Force that advocated for biennial mammography for women in this age group instead of the previous suggestion of getting screened every one to two years.
However, the study authors pointed out that the original recommendations only looked at age as a factor for influencing breast cancer risk. This new study factored in age, breast density and postmenopausal use of hormone therapy (HT).
About 211,731 women received a breast cancer diagnosis in the U.S. in 2009 -- the latest year for which statistics were available -- and 40,676 will die each year from the disease, according to the Centers for Disease Control and Prevention. It is the most common cancer among women in the U.S. and one of the leading causes of cancer death among women of all races and Hispanic women.
The researchers looked at data from 11,474 women with breast cancer and 922,624 women without breast cancer who underwent screenings at U.S. facilities involved in the long-running Breast Cancer Surveillance Consortium (BCSC) from January 1994 to December 2008.
Women who went every two years for a screening were not associated with an increased risk of advance stage breast cancers or large tumors, even if the woman had dense breasts or used hormone replacement therapies, when compared with women who were screened every year.
For women who were between the ages of 40 to 49, those with extremely dense breasts were 1.89 times more likely to be found with advance-stage cancers and 2.39 times more likely to have large tumors when they were screened every other year versus annually.
"This study is not about whether you should start screening at 40 or 50, but if you are going to be screened, does it matter if you do it every year or every two years," Kerlikowske said. "If you're in that younger group, with dense breasts, it probably does matter."
Women ages 50 to 74 were less likely to receive a false-positive diagnosis if they were screened every two years. For women who had mammographies every two and three years, they had a 30.7 and 21.9 percent respective risk of false positives than women who got screened each year (about 50 to 60 percent regardless of age, according to Susan G. Komen for the Cure).
But, women in the younger age group with extremely dense breasts had a 65.5 percent chance of a false positive if they were screened biennially, and women who used hormone therapy who had dense breasts regardless of age were at a 65.8 percent chance of a false positive.
Unlike the U.S. task force, the American Cancer Society recommends annual mammogram screening starting at age forty.
"I don't think any one study ought to change everything,"said Dr. Otis Brawley, chief medical officer of the American Cancer Society, to Reuters.
Brawley said he did not expect changes in official screening policies for professional organizations, but he did think doctors were moving towards a more personalized approach when it came to screening for breast cancer.
"This is one of several studies that are all pointing in the same direction over the last several years," he said.
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