Mammograms every two years offer the same benefit as annual screening for women aged 65 and older, regardless of comorbidity, and also significantly reduce the number of false positive results, according to a study published online Feb. 5 in the Journal of the National Cancer Institute. 

“Our analyses indicated no association between comorbidity, screening interval, and tumor stage at diagnosis,” wrote Dejana Braithwaite, PhD, of the University of California, San Francisco (UCSF), and colleagues.

The authors explained that uncertainty about the appropriate use of screening mammography among older women exists, primarily because comorbid illnesses may diminish the benefit of screening. “Current guidelines reflect the uncertainty surrounding screening mammography among older women: the US Preventive Services Task Force breast cancer screening guidelines recommend universal screening for women aged 50 to 74 years, whereas the American Cancer Society proposes annual screening for all women aged older than 40 years with no upper age limit,” they wrote.

Findings in the current study were based on prospectively collected national data from January 1999 to December 2006 that included 2,993 older women with breast cancer and 137,949 without breast cancer. The women underwent mammography at facilities that participated in a data linkage between the Breast Cancer Surveillance Consortium and Medicare claims.

Results showed no statistical difference in adverse tumor characteristics by comorbidity, age or screening interval. Braithwaite and colleagues noted this result is in contrast with previous population-based analysis of Surveillance, Epidemiology and End Results-Medicare data which found different comorbidities were associated with differing disease characteristics. One difference between the current study and previous efforts was that the UCSF study used the Charlson index, a measurement of comorbidity used in cancer research, as opposed to examining the effect of individual comorbidities, explained the authors.

“Efforts to investigate this question further will require a better understanding of the extent to which comorbidity serves as a marker for healthcare contact, health or lifestyle behaviors, or care-seeking style and the extent to which specific comorbid conditions increase risk of more (or less) aggressive disease via biological pathways,” wrote Braithwaite et al.

While the rates of late-stage breast cancer were unchanged by screening interval, the authors found that 48 percent of women between the ages of 66 and 74 who were screened every year had false positive results, while 29 percent of women in the same age range who were screened every two years had false positives. “As is the case in younger women, most older women who undergo annual mammography are at high risk of false-positive mammography results and biopsy recommendations without added benefit from more frequent screening.”

The authors estimated that there are 4.9 million women aged 66 to 89 years in the U.S. with comorbidities and 14.3 million without comorbidities. If they all underwent annual instead of biennial mammography, that would translate to approximately one million additional false-positive exams and 290,000 additional biopsy recommendations among women with comorbidities, and 2.86 million false-positives and 860,000 additional biopsies among women without comorbidities.

“Because a randomized controlled trial of mammography in older women is unlikely, more high-quality observational research examining additional measures of comorbidity and breast cancer mortality may facilitate improved understanding of the benefits and harms of different screening mammography frequencies among older women and, ultimately, inform clinical and policy decisions about the appropriate use of screening in this growing population,” concluded Braithwaite and colleagues.