Do more frequent mammograms pick up some breast cancer tumors that might have gone away without treatment? Possibly, according to a controversial study published this week in Archives of Internal Medicine. However, experts caution that the research raises an interesting question, but can’t definitively answer it.
In the study, a research team led by Per-Henrik Zahl, MD, of the Norwegian Institute of Public Health, looked at two groups of women from before and after Norway stepped up its mammogram screening program in 1996.
One group of 119,000 women ages 50 to 64 had mammograms every two years, for a total of three mammograms between 1996 and 2001. The researchers compared them to a second group of nearly 110,000 women who were ages 50 to 64 in 1992 but didn’t have routine mammograms. Those women had a single mammogram in 1997.
Not surprisingly, the women who had more mammograms had more cases of invasive breast cancer—if you look for cancer, you tend to find it. However, at the end of the six-year period, cases of invasive breast cancer were still 22% higher among regularly screened women, although the researchers expected the same number of cases in both groups.
They suggest that some of the tumors detected by mammography would have spontaneously regressed had they not been caught and treated.
It's a controversial idea, but one worth considering, says Robert M. Kaplan, PhD, the chairman at UCLA's Department of Health Services, who cowrote an editorial accompanying 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."
Little is known about how untreated breast cancer grows, because it would be considered unethical to not treat women with breast tumors. However, some types of cancer, such as metastatic melanoma, have been known to spontaneously regress or shrink on their own. And there have been about 32 reported cases of invasive breast cancer that have spontaneously regressed.
Could there be some type of breast tumor, as yet unknown, that's unlikely to grow rapidly and threaten a patient's life? Maybe, but many doctors remain skeptical.
There are reasons the study numbers don’t match, they say, including that women in the second group only had one mammogram, and there may have been more cases of cancer than were detected. Or use of hormone therapy may have differed between the two groups.
"The authors try to dismiss some of the reasons why those two numbers might not be equal and they latch on to what I think is one of the more far-fetched explanations—that a large fraction of these tumors have regressed," says Chris Kagay, MD, a clinical fellow at Harvard School of Medicine. "The authors are making a bold proposition about the natural history of breast cancer based on pretty limited evidence."
There’s little scientific evidence that breast cancers can regress, adds Robert A. Smith, PhD, director of Cancer Screening at the American Cancer Society. "The conclusion that more than 1 in 5 invasive breast cancers is destined to regress without incident if not detected by mammography is nothing more than an overreaching leap in logic."
The study is also missing some important information—mortality data. Women who have routine mammograms should be less likely to die of breast cancer than women who do not have them. A follow-up study examining death rates could either confirm or dismiss the study, experts say.
"Now we're left in this odd position of having this intriguing finding but not knowing where to go with it," says Kaplan. "It's important to understand that this is a pretty preliminary finding. It's at odds with what most people believe."
Cancer specialists worry that this type of research can be dangerous if it discourages women from having mammograms.
"It's important to remember that mammography saves lives," says Dr. Kagay. "Suggesting that screening is not valuable or that these cancers detected will spontaneously regress is quite frankly dangerous if it convinces women that they don't need to be screened."