The report in the Journal of the National Cancer Institute said the rate of false positives could be reduced significantly if radiologists always could compare films from previous mammogram screenings.
Dr. Joann Elmore of the University of Washington School of Medicine in Seattle said the rate of false positives — breast cancer mammogram screenings that require follow-up tests — are becoming increasingly common in the United States, but it should not discourage women from having annual screenings.
"Mammography is not a perfect test (for breast cancer), but it is the best test we have," said Elmore, first author of the study. "Women should realize that they have a 10 percent chance of being called back for additional tests."
The results of the study, she said, carry this message: Women should continue to have mammograms, but they should try to return to the same testing facility each time so radiologists can compare past test films.
"If the woman's been going to the same facility for five or eight years, they can simply pull up the old films and get some reassurance that a little abnormality has been stable over five or six years," she told CBS Radio News.
False positives are reduced by about 70 percent when radiologists compare current films with images from past tests, Elmore said.
The study involved an evaluation of mammogram readings from 2,169 women by 24 radiologists in a community clinic practice from the years 1985 to 1993, which gave time for follow-up studies of the patients. The study analyzed the rate of false-positive interpretations by the doctors, then related that to the experience and training of the radiologists and to the age and other characteristics of the patients.
"If we send 100 women to get a screening mammogram, ten of them may get a letter saying that they need more testing and the majority of women don't have breast cancer, among all these women that are called back," Elmore said.
The study found that the false-positive rate ranged from 2.6 percent to 15.9 percent. But when this rate was adjusted for the effect of patient characteristics, such as age, the false-positive rate dropped to 3.5 percent to 7.9 percent. Age affects the false-positive rate because breast tissue is denser in younger women, which makes their mammograms more difficult to interpret.
The study found that doctors who graduated from medical school in the past 15 years had false-positive rates two to four times higher than more experienced doctors.
Elmore suggested the young doctors may have had training that concentrated on finding cancer without emphasizing that false positives "can cause a lot of women who don't have breast cancer to be called back" for additional tests.
Older doctors also have more experience, she said, and "experience matters."
On the other hand, "it might be possible that these younger radiologists are actually calling a lot more of the screening films positive, and maybe they're catching more cancers. We don't know," Elmore admitted.
The threat of malpractice lawsuits may affect mammogram interpretations by causing doctors to err on the side of caution, said Elmore. Mammography "is one of the top causes of medical malpractice allegations," Elmore said.
Although the study determined which of the doctors had the most false positives, it could not determine which of the doctors was most accurate in detecting cancer. Out of the 2,169 mammogram readings, 45 cases of breast cancer were verified.
False positives usually result in women being called back for added tests, which can be limited to examination by a specialist or to additional X-ray exams. For some women, the callback results in a biopsy, removal of a small bit of breast tissue for microscopic examination. This is the "gold standard" for evaluating suspicious lesions detected with mammograms, Elmore said.
In any case, false positive carry a heavy psychological and financial burden for many women, experts say.
Elmore said a women has only a 50 percent chance of having a single false positive in 10 annual mammography tests. In the United States, the average woman has a one-chance-in-eight lifetime risk of cancer, she said.