SO POWERFUL A SEX SYMBOL is the female breast that the American Society of Plastic and Reconstructive Surgeons has termed small breasts "underdeveloped" and called them "deformities" that are "really a disease."

Many in the medical profession would dispute these contentions, which the society of plastic surgeons made in a statement to the Food and Drug Administration. But there is apparently a very receptive audience for this document's further assertion that treatment for small breasts is "often very necessary to ensure an improved quality of life."

By the society's probably conservative estimates, over a million American women -- or close to 1 in every 100 over 18 -- have already had their breasts surgically enlarged with plastic implants, and more than 70,000 are rushing to join them each year. Thus, the plastic surgeons are hardly pleased that an FDA scientific advisory panel has urged the agency to immediately investigate evidence that the silicone gel-filled devices used to enlarge breasts may not be safe.

"We told the commissioner of the FDA (Dr. Arthur Hull Hayes Jr.) that this review should take precedence over all the others under consideration by its Bureau of Medical Devices," said Dr. Norman Anderson of the Johns Hopkins University Medical School, who chairs the advisory panel. Its recommendation for an urgent inquiry followed a public hearing in January at which witnesses for the plastic surgeons and Dow-Corning, the largest of the seven implant makers, testified.

"We feel so strongly about it that we have urged him (Hayes) to have the FDA put up some of the money for the studies, instead of just leaving it up to the manufacturers," Anderson added. "That's something the government normally doesn't do."

But the breast-implant industry argues that there is no serious danger, and that implants need not be considered one of the medical devices that must be evaluated for long-term safety under federal regulations.

Though the advisory panel made its urgent recommendation four months ago, the FDA has yet to take any action. Women continue to get breast implants at a rate of more than 1,000 a week, with no assurance that they are being told in advance of the possible risks involved.

Perhaps a third of augmentation mammoplasties -- as breast enlargements are called -- are repair procedures for patients who have lost a breast to cancer or had precautionary mastectomies because their risk of breast cancer is thought to be great, the plastic surgeons reported at the hearing. And a very few of these operations are undertaken to give breasts to the rare woman who does not develop them naturally. But the greatest number are performed for purely cosmetic reasons on women with healthy, normal breasts.

It is these patients who particularly concern Anderson, an associate professor of medicine and surgery, and his panel colleagues, Dr. Howard Igle, chief pathologist at the Children's Hospital Medical Center of Akron and Dr. Tapas K. Das Gupta, professor of cancer surgery at the Abraham Lincoln Medical School in Chicago. Breast reconstruction for cancer patients, they say, serves therapeutic as well as psychological purposes because it restores body symmetry and balance. But for the rest, the benefit is only cosmetic and may be outweighed by long-term dangers most women know nothing about when they decide to have the surgery.

According to the plastic surgeons' testimony, most patients realize that, as in any operation, hemorrhage and infection can occur. And most are told, too, that the capsule of scar tissue that forms around the implants -- a natural response to the presence of foreign matter -- will probably shrink.

This shrinkage, the plastic surgeons acknowledge, sometimes leads to breasts so hard, painful and misshapen that the devices either have to be removed and replaced or manually squeezed open; a tactic that, if unsuccessful, may also lead to additional surgery. According to Dr. Norman Hugo of the society of plastic surgeons, complications can be expected with 25 to 33 percent of implants.

But it is other, more subtle risks that have particularly aroused the FDA advisory panel's anxiety.

For example, the society of plastic surgeons maintains that it is easier to detect early breast cancer in patients who have had implants because the devices push the breasts forward and so make them easier to examine. But the advisory panel is not alone in believing just the reverse.

"The trouble," Anderson explained, "is that you can't tell clinically (by touch) whether a lump is an extension of old scar tissue from the capsule around the implant or a possible malignancy. And if you turn to X-ray mammography for the answer, the image is usually poor because the device has gotten in the way. That leaves you with the unhappy choice of hoping for the best and letting things ride or repeatedly subjecting patients who have already had breast surgery to exploratory biopsies."

Breast implants pose another possible danger. Small amounts of silicone that "bleed" or "weep" from the envelopes containing the silicone gel -- even if the envelopes themselves do not rupture or leak, as has been known to happen with serious consequences, from punches or accidental blows.

There is no hard and fast proof that the slow seepage of plastic into nearby and distant tissues is harmful. In fact, the society of plastic surgeons has said that "breast augmentation has an excellent record of achieving important health-related objectives for the vast majority of patients" and that concern about the escape of silicone gel is "based on only old, outdated information." On the other hand, there is no conclusive evidence that the seepage is innocuous.

The FDA's advisory panel raised a number of questions: If a woman wearing implants becomes pregnant, can the silicone cause the embryo to abort or cause malformation in the fetus? Can a woman safely nurse a child after an implant? Do these devices increase her cancer risk, in the breast or elsewhere? When the plastic gets into the bloodstream, is blood circulation impaired?

Although the implants have been fitted since 1963, no studies of the kind that would even begin to answer such questions with certainty have been carried out.

Under a 1976 law, the Food and Drug Administration is supposed to require manufacturers of permanent body implants to fund studies of their long-term safety and effectiveness as a condition of putting new products or keeping old ones on the market. Until now, however, the FDA has done nothing to enforce this law in regard to breast implants. Now the industry wants to be excused from this requirement altogether.

Seven firms that make breast implants and the society of plastic surgeons have argued to the FDA that 20 years' experience using the implants is sufficient evidence of their safety. Besides, they say, it would cost the firms so much to collect the missing information that the price of the silicone breast cushions would have to be raised accordingly. Since cosmetic surgery is not usually covered by health insurance plans, this is a matter of understandable concern to both the plastic surgeons and the makers of the device.

But the FDA's advisory panel expressed concern about the results of the one study of breast implant patients which has followed their fates for as long as ten years.

Starting with 50 patients, the study -- conducted by Dr. Ben P. Gregory, a plastic surgeon in Pensacola, Fla. -- could keep track of only 34 for as long as a decade. Sixteen of these 34 women have had their implants removed or replaced.

A 17th patient in the sample has had to have her spleen removed. Although the plastic surgeons testified that this was unrelated to her earlier breast enlargement, the panel said it suspected that silicone carried through her bloodstream into her spleen may have injured the organ by depriving it of circulation. In any case, the panel was concerned that 16 of the 34 women studied had undergone further surgery that surely would have been unnecessary had they not been fitted with implants to begin with.

According to FDA sources, the agency is likely to decide to require the missing safety studies. But millions of dollars are at stake and there is nothing to prevent the plastic surgeons, the implant manufacturers -- or both -- from seeking in court a reversal of any decision to require a safety evaluation. And even if the courts were to rule for the FDA, study results suggesting that regulatory action was needed could be legally challenged, too.

In other words, it could take at best many years to resolve the uncertainties surrounding silicone gel breast implants.