X-rays may be less effective than expert physical examination in detecting breast cancer in women, according to a new Canadian study.

Indeed, the researchers, who followed 1,000 high-risk women for six years, found with physical exams 22 cancers that had gone undetected in breast X-rays, or mammograms, as they are called. The X-rays, by contrast, turned up only four cancers that the physicals had missed.

The findings, which surprised the University of Toronto researchers, run contrary to much of the research done in the United States. They are thus certain to add fuel to the controversy over the use of mammography as a tool for early detection of breast cancer.

Based on evidence from a long-term study that mammograms can prevent breast cancer deaths in older women, the National Institutes of Health guidelines currently call for an annual X-ray for women over 50.

But because the radiation may pose a risk of cuasing cancer as well as detecting it, doctors disagree over whether there is any net benefit.

Because of the radiation risk mammograms are currently recommended for women under 50 only if they have a personal or family history that predisposes them to cancer.

Recent American research still shows mammography to be superior to the physical exam in detecting breast cancer. Of 180 cancers found so far among 10,000 women being screened yearly at the University of Michigan, 45 percent are detected only by mammograms, said Dr. Barbara Threatt, a radiologist involved in that study.

She said another 45 percent of the cancers show up on both physical exams and mammograms, but only 10 percent are found on physical exams when mammograms appear normal.

Mammograms are detecting cancer very early at the Michigan center, often when the tumore is less than one-quarter inch across and has not spread beyond the gland where it grows, Threatt said.

The 1,000 patients in the Canadian study, who were all at breast cancer - because of previous breast cancer, a family history or other factors - received yearly physicals by Dr. Leo J. Mahoney, the surgeon who headed the study, said Dr. Bruce L. Bird, the mammographer who worked with Mahoney.

The women were trained in breast self-examination, and also received a yearly thermogram, which looks for cancer by recording temperatures of breast tissues, and a mammogram every two to four years. All had normal mammograms and physicals when the study began.

Over almost six years, the doctors found 30 new breast cancers. Twenty-two showed up on physical examination while the mammogram remained normal. Four caused both a breast lump and an abnormal mammogram. Only four were found by mammogram with no lump felt by physical exam.

Bird said he could not explain why the Canadian findings are so different from American results. But based on their statistics, the Toronto researchers advocate a yearly physical, with mammograms done less frequently.

Threatt disagreed, saying the Michigan program bears out mammography's ability to find cancer earlier. a palpable two- or three-centimeter cancer. That is a late cancer," probably already spread through the patient's body, she said.