A lump is discovered on a mammogram or through self-examination.

The lump is biopsied to see if it is cancerous. In many cases, this can be done with a needle in a doctor's office. In other cases, it must be surgically removed for biopsy. Sometimes, as was the case with Nancy Reagan, other factors, such as the shape or location of the lesion, indicate before the biopsy that cancer is present.

THE LUMP IS BENIGN Nine times out of 10, the lump is found to be a cyst or other growth that is not a threat to health or a predictor of future cancer. No further treatment is needed.

THE LUMP IS MALIGNANT

It contains cancer cells. Today, a two-stage procedure is usually used -- biopsy first, surgery later. Decisions about how to further treat the cancer may be made over the next few days. No breast cancer patient need wake up from a biopsy with a surprise mastectomy.

TYPE OF SURGERY

SMALL TUMOR

If the tumor is very small, or small relative to the size of the breast, then breast-preservation surgery, such as a lumpectomy accompanied by radiation, is an option. Sometimes, if the tumor was removed for biopsy, additional tissue around it is taken in a second procedure.

LARGE TUMOR, OR TUMOR NEAR NIPPLE

If the tumor is large, or close to the nipple, a modified mastectomy -- removal of the breast -- is often indicated. (The modern mastectomy is less disfiguring than the old "Halsted radical" procedure and may be accompanied by immediate reconstruction.)

REMOVAL OF AXILLARY (UNDERARM) LYMPH NODES FOR BIOPSY This is the only way to determine if the cancer has already spread. A "positive" finding indicates the presence of cancer cells beyond the breast.

'NODE NEGATIVE'

Women who are node-negative show no evidence of cancer's spread. According to official statements, the first lady was in this category. Probably, no further treatment is necessary. But one in four node-negative women will die of a recurrence, so more tests should be performed to help predict which women are especially at risk. None of these tests is fool-proof, but better ones are under development.

'NODE-POSITIVE'

The cancer has spread. Depending on age, other risks and results of more tests, some type of additional therapy -- cancer-fighting chemicals or hormones -- is chosen.

HORMONE-RECEPTOR TEST

This test helps doctor and patient choose the right treatment. It is also a predictor of the course of the disease.

ESTROGEN OR PROGESTERONE 'RICH'

About one third of breast cancers fall into this relatively optimistic category -- more in post-menopausal women. The cancer cells still resemble normal cells in their response to the body's hormones, and cancer growth in women with node involvement might be blocked by anti-hormone therapy like tamoxifen.

ESTROGEN OR PROGESTERONE 'POOR' The cancer cells have "forgotten" their hormonal response. Other cell-killing, or cytotoxic, drugs may be needed.

THE OUTLOOK

Half the women diagnosed with breast cancer eventually die of it, but individual survival chances vary greatly. A post-menopausal woman whose tumor is discovered early -- before it can even be felt -- and who has no family history of breast cancer and no evidence of spread, has the best outlook -- perhaps more than a 90 percent chance of surviving five years. The outlook is not as bright in the case, for example, of a young woman whose cancer has spread -- but many of these patients also do well.