Researchers compared annual with every other year screenings and the effect as to whether the different schedules affected late stage diagnoses. They also compared how the different intervals affected the number of false-positive results, where initial indications that the patient had breast cancer are later found to be wrong after further, and sometimes invasive, testing.

Dr. Braithwaite an assistant professor of cancer epidemiology at the Helen Diller Family Comprehensive Cancer Center at the University of California, San Francisco said that "Your risk of having breast cancer detected at a later stage is no greater if you screen every two years compared to every year."

Dr. Braithwaite and colleagues found that women who were screened every other year were less likely to have false positive test results than those who were having an annual screening.  In this study researchers collected data on more than 140,000 women ages 66 to 89 complied from results spanning 1999 to 2006.  Of those woman, close to 3,000 women were diagnosed with cancer during the study.

False positives are usually found only after extensive and invasive testing after an initial diagnosis. This causes the patient unnecessary anxiety goring through expensive procedures and lost time from work.  If the frequency of mammograms can be reduced and the accuracy of diagnosis increased it would benefit not only the patient, but healthcare providers and insurance companies as well.  

In this study during 10 years of screening 48% of women who had annual screenings had received a false positive diagnosis for breast cancer.  For those women who were screened every two years, 20% had at least one false positive.

The American Cancer Society suggests woman receive annual screenings after age 40, especially if there is a family history of the illness.  

Research was published in the Journal of the National Cancer Institute