Women 40 and over, not just 50 and over, should have their breasts X-rayed regularly to detect cancer, the American Cancer Society said yesterday.

The recommendation adds 12 million American women to the 34 million age 50 and over for whom the cancer society previously urged regular mammograms, or breast X-rays.

A woman over 40 should have a mammogram every year or two, depending on her or her doctor's wishes, Dr. Arthur Holleb, medical director of the society, said. Women 50 and over should have a mammogram every year, according to the society.

Until now, the group has been governed by 1977 evidence that the risk of developing cancer from excess radiation was greater than the risk of breast cancer for women in their 40s.

But Holleb and other doctors said the amount of radiation per mammogram has been cut by as much as two-thirds in recent years, and thus the risk of breast cancer now surpasses any risk of radiation-induced cancer. That risk is now "minimal . . . if it exists at all," the society said.

The action puts the cancer society far ahead of the government's National Cancer Institute in recommending routine mammograms.

After a 1977 "consensus conference" on breast-cancer screening--one where the risks of X-raying received heavy emphasis--NCI merely said that mammography "may be considered" for high-risk groups, including women 50 and older, and that physicians "may" want to recommend routine mammography for younger women.

NCI is sticking with its recommendation for now, said Dr. Richard Costlow, chief of preventive medicine in the institute's community activities division. He said there is "suggestive evidence" but "no solid proof" that mammography benefits now outweigh any risks, and "I'd rather await completion" of a study under way in Canada that should provide firmer evidence.

Still, he said, "I don't have any real objection" to the cancer society's making its own recommendation.

Holleb said two advisory groups--a national cancer-detection advisory committee and a breast-cancer task force--agreed that "mammography now offers a superb opportunity of finding asymptomatic breast cancer in very early stages with good survival rates."

Much of the evidence comes from 29 breast cancer detection demonstration projects sponsored by the cancer institute and the cancer society in the 1970s.

Mammography detected many more cancers than did doctors' physicals or self-examination by women in their late 30s and 40s in these projects. Cancers found this early, Holleb added, sometimes may allow for removal of the cancerous area or a segment of the breast, rather than the entire breast.

The society still recommends monthly self-examination for all women over 20; physical examination of the breasts by a doctor every three years for women 20 to 40, and every year after 40; and one baseline mammogram, for further comparisons, between 35 and 40.

"Our recommendations," Holleb said, "are only for asymptomatic women," women without symptoms, "and where there is any suspicion of cancer, there must be a biopsy--surgical removal of suspicious tissue for study--as well as mammograms of both breasts to find any other, still silent cancers."

The cancer society and the government cancer institute agree that certain adult women in high-risk categories also may benefit from regular mammograms. The institute specifies high-risk women as those 35 and older who have had a breast cancer, and those 40 and older with a family history of breast cancer.