TUESDAY, Nov. 25 (HealthDay News) - Some breast cancers may naturally disappear without treatment, a study of women undergoing mammography suggests.

The Norwegian study found that more cases of breast cancer were diagnosed after a regular screening program was put in place than before. That has 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.

But since doctors can't yet determine which tumors might regress and which might go on to be dangerous, the finding isn't likely to change recommendations for mammography, experts said.

"The problem is, we don't know the natural history [of breast cancer]," said Dr. Jay Brooks, chairman of hematology/oncology at Ochsner Health System in Baton Rouge, La. "I am sure there are some that do regress. The problem is, we can't pick up those that are going to regress. It's one of the unanswerable questions."

The study was published in the Nov. 24 issue of the Archives of Internal Medicine.

Cancer experts have long suspected that some cancers may grow and then, for reasons that are unclear, simply shrink again and disappear. The new breast cancer trial appears to support that notion.

The study took advantage of the fact that new biennial (once every two years) screening mammography programs were instituted throughout Europe in the 1990s. Such a program began in Norway in 1996.

Researchers from 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.

They then compared that data to the rates of breast cancer for almost 110,000 women of similar age between 1992 and 1997, largely before the new guidelines came into effect. When Norway began offering mammograms in 1996, almost all these women went and got a one-time screen.

Although it might be expected that the two groups of women would have breast cancer at similar rates, they did not. In fact, the women in the later cohort -- who got mammograms once every 2 years -- had a 22 percent higher rate of tumors than the women in the earlier, less heavily screened group.

According to the authors, changes in hormone therapy use over the study period did not explain the difference. So why would more frequent screening mean more cancers?

What probably happened, the authors theorized, is that some of the tumors detected during more frequent screening would not otherwise have caused women trouble if they had remained undetected. These tumors might have either remained stable or, more likely, spontaneously regressed.

In fact, there have been 32 cases of spontaneous regression of breast cancer reported in one recent review of the medical literature. And not all actual cases of spontaneous regression end up being documented.

In addition, autopsy studies have revealed that many women die without ever knowing they had a breast cancer.

One expert was intrigued by the findings.

"The study is very provocative and it generates an interesting hypothesis: that it's possible some screen-detected breast cancers would not ever lead to death from breast cancer and are unnecessarily diagnosed and treated," said Dr. Jeanne Mandelblatt, associate director for population sciences at Georgetown University's Lombardi Comprehensive Cancer Center in Washington, D.C.

"But this study design can't prove or disprove that hypothesis," she added. "This study is inconclusive because these women were not randomized; there's no data about the tumor sizes or tumor characteristics in the two groups, and no data about the breast cancer death rate in the two groups."

Still, the finds should impact breast cancer research, experts say. "If the spontaneous remission hypothesis is credible, it should cause a major re-evaluation in the approach to breast cancer research and treatment," wrote Robert Kaplan, of the University of California, Los Angeles, and Dr. Franz Porzsolt of the Clinical Economics University of Ulm, Germany, in an editorial that accompanied the journal article. "Certainly, it is worthy of further evaluation."

Right now, however, doctors cannot tell a "bad" breast tumor from a potentially harmless one, so regular mammography screening is still valuable.

"For women, the take-home message remains that mammography done as recommended does decrease the chance of dying from breast cancer across the general population," Mandelblatt said. "Women need to know that there are risks and benefits to all medical interventions, including screening mammography, and the policy recommending mammography is based on the fact that the benefits outweigh some of these harms."

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