Today's biopsy of an abnormality in Nancy Reagan's left breast is likely to show cancer, but because the suspicious area was found by X-ray when it was too small to feel, it will probably be curable with surgery, breast cancer experts said yesterday.

Cancer experts praised some aspects of the First Lady's treatment, saying that the fact that the abnormality showed up on an annual mammogram will help persuade women of the value of regular X-rays of the breasts. Breast cancer strikes 1 out of 10 American women, and mammograms are widely recommended after age 40 to increase the chance of finding tumors at an early, curable stage.

But they said that Nancy Reagan's advance decision to have her breast removed immediately if a malignancy is found is at odds with many doctors' preference for a two-step procedure, in which a biopsy is done first and further treatment is delayed to allow the patient time to choose among options.

The appearance of a abnormal area on the First Lady's mammogram -- one that was not seen on last year's test -- is very likely, in a woman her age, to mean cancer, experts said. "Finding it by mammogram is the most favorable possible situation," said Dr. Samuel Hellman, medical director of Memorial-Sloan Kettering Cancer Center in New York. "The most important variable in the prognosis is whether it has spread to lymph nodes under the arm. The smaller the {tumor}, the less likely the spread."

Experts said early breast cancer can show up on a mammogram either as a shadow or a pattern of white flecks called calcifications. Nancy Reagan's doctors will use a technique called needle localization to pinpoint the suspicious area of the breast so that surgeons can remove it for examination. In this technique, the skin of the breast will first be numbed with a local anesthetic. Then, a radiologist will place a fine needle into the area as a marker and will take an X-ray to make certain that the tip of the needle is at the exact site of the abnormality.

With the needle still in place, Nancy Reagan will be moved to the operating room. Using a local anesthetic, her surgeons will remove a tissue sample from the area around the needle's tip. The sample will be X-rayed to make sure it contains the entire abnormality. Then pathologists will freeze part of it, cut it into thin slices and examine it under a microscope, looking for cancer.

If cancer is found, Nancy Reagan will be put under general anesthesia and the surgical team, led by Dr. Donald McIlrath of the Mayo Clinic, will perform a modified radical mastectomy. In this operation, the entire breast is removed, as well as small, bean-shaped structures called lymph nodes found underneath the breast and in the armpit. Some surgeons also remove a chest muscle underlying the breast called the pectoralis minor.

Removal of the lymph nodes is critical because they must be examined microscopically to determine whether breast cancer has spread. If the tumor is smaller than four-fifths of an inch in diameter and no cancer cells are found in the lymph nodes, the tumor is considered Stage 1. The chance that such a cancer will be cured by surgery is 90 to 95 percent, several experts said yesterday.

If cancer cells are found in lymph nodes, it often means that the cancer has spread to other regions of the body. The tumor is then considered Stage 2, and the chance that the woman will be alive five years later falls to between 65 and 75 percent, experts said.

The smaller a tumor, the smaller the likelihood that it will have spread to the lymph nodes. But even when tumors are so small that they can only be detected by mammogram, they still are found to involve the lymph nodes in 20 percent of cases, according to Dr. Rudolph Almaraz, an assistant professor of surgery and oncology at Johns Hopkins University Medical School.

If cancer is found, pathologists will examine the cells microscopically to determine which of the approximately 15 types of breast cancer is present. They will also test the tumor to see if its cells are sensitive to the female hormones estrogen and progesterone.

Whether any further treatment such as chemotherapy or other medications would be needed depends upon the size and type of tumor, the result of lymph node examination, and on whether the tumor is hormone-sensitive, experts said.

Doctors interviewed yesterday said that if Nancy Reagan undergoes a mastectomy, she will have chosen the more conservative of two equally effective treatments available to most women with early breast cancer.

Between one-quarter and one-third of women with breast cancer treated at Memorial Sloan-Kettering, Georgetown University Medical Center and hospitals affiliated with Harvard Medical School, are choosing as an alternative treatment lumpectomy and lymph node removal followed by radiation therapy, according to Dr. William F. Feller, an associate professor of surgery at Georgetown University Medical Center.

Whether radiation therapy can be used as an alternative to mastectomy depends upon the size and location of the tumor, the size of the breast and other factors, according to Dr. Christine Berg, a radiation oncologist at Georgetown. It requires six to seven weeks of daily treatments, and in an older woman, can sometimes weaken the ribs and cause painful fractures.

Its advantage, Berg said, is that "you do have a breast that looks very much like the original breast after radiation therapy."

Surgeons interviewed said that a modified radical mastectomy is a low-risk operation, seldom causing excessive blood loss or other complications. Unlike the outmoded radical mastectomy, in which removal of several muscles left a patient with a weakened, swollen arm, a modified radical mastectomy causes no disability, Almaraz said. He added that most patients leave the hospital within three to five days after surgery and resume their normal activities after about three weeks.