About 5,550 American women a year with small, new-found breast cancers lose the breast to a surgeon's knife.

They no longer need do so, according to a major study published in today's New England Journal of Medicine.

Doctors at Italy's highly regarded National Cancer Institute report that in such cases, which must be carefully defined, removing only a fourth of the breast and giving the women postoperative radiation produces an equal number of cures.

In the same journal, doctors at 68 American medical centers say a new three-drug combination is producing highly improved results in treating many breast cancers after all kinds of surgery. This is particularly true in the case of women over 50 whose cancers are hormone-dependent, which includes nearly 70 percent of women of that age.

One woman in 11 gets breast cancer sometime, and there are 110,000 new cases in this country every year.

"We are seeing many changes in treatment now" after decades without change and the next 5 to 10 years should see a marked improvement in survival," Dr. Bernard Fisher of the University of Pittsburgh, chairman of the nationwide group that has been comparing several treatments, said yesterday.

Breast removal remains the standard first treatment for most breast cancers although some doctors are experimenting with removing only the cancerous lump, or only one part of the breast, or not operating and using X-rays to kill the tumor. But none of these lesser treatments had been backed by large enough studies to win universal acceptance.

The Milan, Italy, group, however, headed by Dr. Umberto Veronesi operated on 701 women over seven years, patients whose cancers measured no more than eight-tenths of an inch across and who had no cancer spread to nearby lymph nodes. About one breast cancer in 20 falls in this class.

In 349, they did a Halsted radical mastectomy (removing the breast and underlying structures and lymph nodes). In the other 352, they removed just the affected quarter of the breast plus some underlying muscle and lymph nodes. tThen this group got six weeks of X-rays to attack any remaining cancer cells. t

Five years after surgery, 89.1 percent of the first group and 86.5 percent of the second group were still free of disease. The difference is statistically insignificant.

"On the basis of this study," the Italian doctors conclude, radical surgery apparently causes "unnecessary mutilation" in women with small tumors caught early.

Both Italian and American doctors have shown in the past decade that many women, especially premenopausal women, with breast cancer can benefit from post-surgical chemotherapy to try to kill any remaining cancer. Some authorities think this includes older, post-menopause women.

Fisher and colleagues across the country tell today of adding a drug called tamoxifen to an established two-drug (L-phenylalanine mustard and 5-fluorouracil) combination. This new combination has proved effective in treating many estrogen-dependent cancers.

In women over 50 whose cancers had spread beyond the breast and whose cancers were estrogen- or female sex hormone-dependent, treatment failures were reduced by 51 to 64 percent two years after surgery. Patients 49 and under with strongly hormone-dependent cancers also showed some benefit.

There are apparently two kinds of breast cancer biologiclly, those that are somehow associated with the body's estrogen production and those that are not. A lesson now, said Fisher, is that "every woman with breast cancer should make sure her doctor gets an estrogen-receptor analysis, if one is possible," before designing her treatment.