LOS ANGELES, Nov. 25 (Xinhua) -- Some invasive cancers like breast cancer may sometimes go away without treatment and in larger numbers than anyone ever believed, a new study showed.

    In the study, a group of U.S. and Norwegian researchers compared two groups of women ages 50 to 64 in two consecutive six-year periods.

    A group of 109,784 women in Norway was followed from 1992 to 1997. Mammography screening was initiated in 1996. In 1996 and 1997, all were offered mammograms, and nearly every woman accepted.

    The second group of 119,472 women was followed from 1996 to 2001. All were offered regular mammograms, and nearly all accepted.

    It might be expected that the two groups would have roughly the same number of breast cancers, either detected at the end or found along the way.

    Instead, the researchers report, the women who had regular routine screenings had 22 percent more cancers. For every 100,000 women who were screened regularly, 1,909 were diagnosed with invasive breast cancer over six years, compared with 1,564 women who did not have regular routine screening.

    There are other explanations, but researchers say that they are less likely than the conclusion that the tumors disappeared.

    The most likely explanation is that "there are some women who had cancer at one point and who later don't have that cancer," said Dr. H. Gilbert Welch, a researcher at Dartmouth Medical School in New Hampshire.

    The finding does not mean that mammograms caused breast cancer, nor does it bear on the question of whether women should continue to have mammograms, because so little is known about the progress of most cancers, said the study.

    The study, published on Tuesday in the Archives of Internal Medicine, was conducted by Welch and researchers from the Norwegian Institute of Public Health and the Ulleval University Hospital in Oslo.