July 9 (HealthDay News) -- One of every three breast cancers detected by a screening mammogram is unlikely to ever cause a problem, a new study predicts.
The report of a so-called overdiagnosis rate of 35 percent came from an examination of breast cancer screening programs in five countries conducted by Danish researchers and published online Thursday in BMJ.
The finding echoes those of a study published late last year that concluded that some breast cancers may naturally disappear without treatment.
That study found that more cases of breast cancer were diagnosed after a regular screening program was put in place than had been diagnosed before. The finding led specialists to suspect that some of the diagnosed tumors would have spontaneously regressed had they not been detected and treated as the result of more rigorous mammography guidelines.
However, experts say such findings do not diminish the importance of mammograms.
"We do know that breast cancer survival has increased, and we do know that screening increases survival," said Dr. Richard J. Bleicher, a breast cancer surgeon with Fox Chase Cancer Center in Philadelphia. "Some of that screening has let us detect earlier cancers."
But, he added, "in a clinical context, we can't distinguish between lethal and harmless cancers."
Overdiagnosis, described as the detection of cancers that would not cause any problems during a person's lifetime, has been reported with other cancers as well, notably prostate cancer.
Bleicher also said that he looked at the Danish study "with a bit of a skeptical eye, based on how it was performed and on the reality of how we care for patients."
The researchers, from the Nordic Cochrane Centre in Copenhagen, Denmark, combined the findings of studies that spanned 14 years -- seven years before public, or free, screening programs were started in five different regions and seven years after the programs were in place.
The regions included were the United Kingdom; Manitoba, Canada; New South Wales, Australia; Sweden; and parts of Norway.
They found an estimated overdiagnosis rate of 52 percent for all cancers, including in situ malignancies, which have not spread, and an overdiagnosis rate of 35 percent for invasive breast cancer. The study also detected a jump in incidence rates after the introduction of the screening programs.
An editorial accompanying publication of the study noted that overdiagnosis can cause women much trauma and alarm and that the choice to get a mammogram remains a personal one.
Dr. Jay Brooks, chairman of hematology/oncology at Ochsner Health System in Baton Rouge, La., observed the situation as having pros and cons.
"The good news is we have affected survival dramatically, and we have affected the ability to diagnose cancers earlier so that most women don't have to remove their breast," Brooks said. "There is a trade-off."
SOURCES: Richard J. Bleicher, M.D., breast cancer surgeon, Fox Chase Cancer Center, Philadelphia; Jay Brooks, M.D., chairman, hematology/oncology, Ochsner Health Clinic, Baton Rouge, La.; July 9, 2009, BMJ, online