A new study of women undergoing mammography has suggested that some breast cancers may spontaneously regress without treatment.

The Norwegian study, published in the November 24 issue of the Archives of Internal Medicine found that the rate of breast cancer among women who received biannual mammograms over six years in four Norwegian countries was 22% higher than in a control group of women who received only 1 mammogram at the end of a 6-year period. That may mean that tumours in those who weren’t tested regressed without being treated, researchers said.

“Our findings simply provide new insight on what is arguably the major harm associated with mammographic screening, namely, the detection and treatment of cancers that would otherwise regress,” said the researchers at the Norwegian Institute of Public Health’s epidemiology department in Oslo.

Furthermore, the researchers write that the “natural course for some screen-detected breast cancers may be to spontaneously regress.” They believe there are ‘pseudo’ cancers in this population of mammography-screened women and most of these types of cancer may naturally regress without treatment.

Researchers led by Per-Henrik Zahl at the Norwegian Institute of Public Health tracked the incidence of breast cancer among more than 119,000 women aged 50 to 64 who participated in three rounds of biennial mammography screening between 1996 and 2001. The first group, comprising 109,784 women, underwent monitoring between the years 1992 and 1997, while the second one, consisting of 119,472 women, was followed from 1996 to 2001. The second group received 3 biennial screening mammograms between 1996 and 2001 as part of the Norwegian Breast Cancer Screening Program. When Norway began offering mammograms in 1996, almost all these women of similar age went and got a one-time screen.

The researchers compared the incidence of breast cancer among the “screened” group with rates among the control group. At the end of 6 years, all participants received a 1-time screening to asses the breast cancer prevalence.

Surprisingly, the researchers found that the women who got mammograms at once every 2 years had a 22 percent higher rate of tumours that the women in the other group. Scientists found that for every 100,000 women who had undergone regular screenings, 1,909 were diagnosed with breast cancer during the six year monitoring period, compared to 1,564 women who had not been routinely screened on a regular basis.

The authors noted that some of the tumours detected during more frequent screening would not have caused women any trouble if they had remained undetected. They might have spontaneously regressed without them knowing.

Many doctors, however, remain skeptical when it comes to the idea that there might be some types of breast tumours that can spontaneously regress, as there is little scientific evidence to support the conclusion that some invasive breast cancers are designed to regress without treatment. Cancer specialists worry that this type of research can be dangerous if it convinces women that they don’t need to be regularly screened. The idea one should keep in mind is that early detection and prevention are essential elements in the battle with breast cancer.