A major national study of breast cancer treatment suggests that surgical removal of just the malignant lump followed by radiation may be as effective as removal of the entire breast for tens of thousands of women each year.

The study, supported by the National Cancer Institute at 89 institutions around the country, found that the less-disfiguring surgery is a "perfectly acceptable alternative to breast removal" for most patients with small tumors that are detected early, Dr. Bernard Fisher reported yesterday.

"It is a very tangible reward for early detection," he said.

The new study, the largest of its kind, provides the most definitive evidence to date that the least disfiguring type of breast surgery, known as "lumpectomy" or "segmental mastectomy," may be as effective as more extensive surgery in terms of survival five years after the disease is detected.

It continues a trend over the past 20 years away from what Fisher called the "mutilating, debilitating Halsted radical mastectomy" -- a widely used procedure for nearly a century in which a woman's breast and surrounding muscle and tissues were removed -- to less radical surgery such as total mastectomy, in which the breast but not the surrounding muscle is removed.

However, the new study's support for even lesser surgery is likely to be controversial among surgeons and critics who say more followup of patients is needed in the period beyond five years.

NCI director Dr. Vincent T. DeVita, Jr. said candidates for lumpectomy, followed by radiation, might include about half of the 119,000 women in the United States each year who discover they have breast cancer. It is most appropriate, he said, for those in whom the tumor is smaller than l 1/2 inches in diameter.

DeVita estimated that about 15 percent of American breast cancer patients today were getting lumpectomies or other breast-sparing surgery. He predicted that the new study would increase pressure for the surgery's wider use, but said that its acceptance might be limited by the number of trained radiation therapists in some areas and by the emotional concerns of women who might feel more comfortable with traditional therapy.

"What we've done here for the first time is provide firm data, so that a woman now has information at her fingertips to make a decision," said Fisher, a University of Pittsburgh cancer expert who led the nationwide lumpectomy study.

Its findings and that of a l0-year followup of an earlier study in which total mastectomy was found as effective as the radical mastectomy are reported in this week's New England Journal of Medicine.

An editorial in the journal noted that the 10-year study continued to show that the less radical surgery was as effective as the more radical Halsted. It called the new conclusions about the lumpectomy "encouraging" but cautioned that "it remains to be seen whether the same will be true" after 10 years.

"An historic surgical debate may be nearing resolution, but conclusions for the present must remain tentative," said Dr. C. Barber Mueller, of the Canadian McMaster University Medical Centre.

There has been criticism that publication of the new lumpectomy study underwent more than a year of scientific review before publication. The editor of the journal, Dr. Arnold Relman, yesterday defended the delay as necessary for revisions and accumulation of more data.

He said the study published was the "most exhaustive and definitive to date . . . . But I think before one can say with complete confidence there really isn't any difference in quality or quantity of life, we'll have to wait a bit longer."

The new lumpectomy study involved 1,843 patients in the United States and Canada with the early stages of breast cancers who were assigned randomly to one of three treatment groups and followed for an average of 39 months. In the lumpectomies, doctors removed the cancerous lump and a rim of normal tissue; if the surrounding tissue contained cancer cells, the woman received a mastectomy. Women with evidence of cancer in lymph nodes were also given chemotherapy.

From this data, the researchers concluded that at the five-year point, treatment by lumpectomy, with or without radiation, resulted in recurrence and survival rates that were "no worse" than total breast removal, but that the addition of radiation treatment showed added benefits.

Overall projected 5-year survival was statistically about the same for the three groups, ranging from 76 percent to 85 percent.

The researchers also found that lump removal followed by radiation helped protect against the recurrence of lumps in the same breast, showing that among survivors at five years, 28 percent of those not given radiation had recurrences compared with 8 percent of those with radiation.

Medical experts noted that the study provided no new evidence as to whether overall breast cancer survival is improving. Some critics note that breast cancer mortality rates generally have remained steady for 25 years, but government cancer experts say that a slight improvement in survival has been offset by increasing breast cancer cases and that it is too early to see dramatic changes from recent improvements in care.