Researchers who tracked breast cancer rates in Norwegian women proposed the controversial notion on Monday that some tumors found with mammograms might otherwise naturally disappear on their own if left undetected.

But leading cancer experts expressed doubt about the findings and urged women to continue to get regular mammograms, saying this screening technique unquestionably saves lives by finding breast cancer early on when it is most treatable.

Dr. Per-Henrik Zahl of the Norwegian Institute of Public Health in Oslo and Norwegian and U.S. colleagues examined invasive breast cancer rates among nearly 120,000 women age 50 to 64 who had a mammogram — an X-ray of the breast used to find evidence of cancer — every two years over a six-year period.

They compared the number of breast cancers detected with another group of about 110,000 Norwegian women of the same age and similar backgrounds who were screened just once at the end of the six-year period.

The researchers said they expected to find no differences in breast cancer rates but instead found 22 percent more invasive breast tumors in the group who had mammograms every two years.

Mammography and breast self-examination for tumors are standard methods used for early detection of breast cancer, the leading cause of cancer deaths among women worldwide.

The American Cancer Society estimated that about 465,000 women die of breast cancer globally each year, and 1.3 million new cases are diagnosed.

The researchers acknowledged many doctors might be skeptical of the idea but they cited 32 reported cases of a breast cancer regressing, a small number for such a common disease.