Chemotherapy can save lives, but has serious risks that make it important to avoid treatment if it is not needed. In addition to the hair loss and nausea that patients dread, chemo can cause heart and nerve damage, leave patients vulnerable to infection and increase the risk of leukemia later in life. TAILORx is part of a wider effort to fine-tune treatments and spare patients from harsh side effects whenever possible.

Endocrine therapy also has side effects, which can include hot flashes and other symptoms of menopause, weight gain and pain in joints and muscles. Tamoxifen can increase the risk of cancer of the uterus.

Patients affected by the new findings include women who, like most in the study, have early-stage breast tumors measuring one to five centimeters that have not spread to lymph nodes; are sensitive to estrogen; test negative for a protein called HER2; and have a score of 11 to 25 on a widely used test that gauges the activity of a panel of genes involved in cancer recurrence.

The gene test, called Oncotype DX Breast Cancer Assay, is the focus of the study. Other gene assays exist, but this one is the most widely used in the United States. It is performed on tumor samples after surgery, to help determine whether chemo would help. The test is generally done for early-stage disease, not more advanced tumors that clearly need chemo because they have spread to lymph nodes or beyond.

The test, available since 2004, gives scores from 0 to 100. It costs about $3,000, and insurance usually covers it. Previous research has shown that scores 10 and under do not call for chemotherapy, and scores over 25 do.

But most women who are eligible for the test have scores from 11 to 25, which are considered intermediate.