In a study conducted to inform American Cancer Society (ACS) breast cancer screening guidelines, a screening mammography once every 2 years was found to be safe for postmenopausal women at average risk of breast cancer. These findings were published in JAMA Oncology (2015; doi:10.1001/jamaoncol.2015.3084).

The research also suggests that premenopausal women older than 40 years deciding whether to undergo mammography may want annual mammography to increase their chances of finding cancers at an earlier stage; however, they should weigh this potential benefit against the increased potential for false alarms, which occur more often with more frequent screening.

The research was conducted with data from the Breast Cancer Surveillance Consortium (BCSC), the nation’s largest collection of information on mammography, which consists of 6 breast imaging registries across the United States.

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The American Cancer Society, which sponsored the BCSC study, published its new breast screening guidelines in JAMA (2015; doi:10.1001/jama.2015.12783). ACS used the consortium findings, as well as a review of existing evidence on screening mammography outcomes, to develop its new recommendations.

For the study, researchers looked at 15 440 women age 40 to 85 years with a breast cancer diagnosis within 1 year of an annual screening mammography or within 2 years of a biennial screening mammography. They defined annual mammography as mammographies performed 11 to 14 months apart, and biennial mammographies as mammographies performed 23 to 26 months apart. (The new study refines earlier research by the BCSC, which used wider screening intervals to classify women as annual or biennial screeners).

The current research goal was to compare the proportion of breast cancers with less favorable prognostic characteristics (stage IIB or higher, tumor size greater than 15 mm, positive lymph nodes and any 1 or more of these characteristics) with the proportion of those with more favorable prognostic characteristics in women screened annually vs. every 2 years. They separated the women by age and by menopausal status, and among postmenopausal women, by whether they were using postmenopausal hormone therapy.

What they found was that premenopausal women with a diagnosis of invasive breast cancer after a biennial screening mammography were more likely to have tumors with less favorable prognostic characteristics than women whose diagnosis was made after an annual screening mammography.

“What was surprising is that based on previous studies we expected to see differences in breast cancer severity by screening interval among women 40 to 49 years of age and no differences by screening interval among women 50 years or older,” said lead author Diana L. Miglioretti, PhD, of the University of California Davis School of Medicine.

Instead, she said, the differences found were based on the women’s menopausal status, not their age.

“For example, younger women were 17% more likely to have later-stage tumors if they were screened every 2 years vs. annually, but that was not statistically significant. For premenopausal women, however, biennial screeners were 28% more likely to have later-stage tumors than annual screeners, which was statistically significant.”

Conversely, they found that postmenopausal women who were not using hormone therapy and whose breast cancer diagnosis followed a biennial or annual screen had similar proportions of tumors with less favorable prognostic characteristics. This was also true for women age 50 years and older.

“Our results suggest that menopausal status may be more important than age when determining screening intervals,” Miglioretti said. “They suggest that postmenopausal women may be safely screened every 2 years. In contrast, premenopausal women who are undergoing screening may want to be screened annually to increase their chances of diagnosing cancer at an early stage.”