A study published this week in the journal Archives of Internal Medicine raises an interesting question: Do breast tumors ever go away on their own? The answer is yes. Cancer researchers have known for years that it was possible in rare cases for some cancers to go away on their own. They found cases of melanomas and kidney cancers that just disappeared or cases of neuroblastoma, a very rare childhood tumor that just went away without treatment. Starting from these examples, the researchers involved in this study, try to learn if the same phenomenon occurs with invasive breast cancers found with mammograms.

For the study, statistician Per-Henrik Zahl of the Norwegian Institute of Public Health and colleagues examined breast cancer rates among 119,472 women age 50 to 64 who had three screening mammograms between 1996 and 2001. Then, they counted breast cancers among a control group of 109,784 women who were not screened. The study showed that breast cancer rates were higher among screened women than non-screened women. After 6 years, all the women were invited to undergo a mammogram. Surprisingly, the incidence of invasive breast cancer was 22 percent higher in the previously-screened group (1,909 vs. 1,564 per 100,000 women) than the control group.

The finding led the researchers to one simple explanation. “It appears that some breast cancers detected by repeated mammographic screening would not persist to be detectable by a single mammogram at the end of six years. This raises the possibility that the natural course of some screen-detected invasive breast cancers is to spontaneously regress,” Dr. Zahl said.

The researchers also found 32 documented cases of breast cancer regression. Though the number of women who experienced self-regression of their breast cancer is small, the researchers are confident that, “it may instead reflect the fact that these cancers are rarely allowed to follow their natural course.”

It’s a controversial idea, but one worth considering, says Robert M. Kaplan, PhD, the chairman at UCLA’s Department of Health Services who, together with Dr Franz Porzsolt, of Clincal Economics University of Ulm, Germany, wrote an accompanying editorial to the study. “Our tendency was to dismiss it when we first read it, but the more we looked at it, the more we thought that maybe there is something to this,” they wrote.

However controversial the findings seem to be, it should be studied be further studied, they agreed. They also said that the study reconfirms the idea that mammograms aren’t always accurate. However, the American Cancer Society recommends most women get annual mammograms after age 40 when they are more likely to develop breast cancer.

Mammograms save lives, despite having a downside: women having a mammogram might also need a biopsy to check on an abnormality that turns out not to be cancer. However, this is a small inconvenience compared to the benefits of this kind of screening.