A new test can predict the risk of breast cancer recurrence and may identify women who will benefit most from chemotherapy, according to research supported by the National Cancer Institute (NCI). These results suggest that almost half of over 50,000 U.S. women diagnosed with estrogen-dependent, lymph-node negative breast cancer every year are at low risk for recurrence and may not need to go through the discomfort and side effects of chemotherapy. The researchers used tissue samples and medical records from women enrolled in clinical trials of the cancer drug tamoxifen, which blocks the effect of estrogen on breast cancer cells. These women had a kind of breast cancer defined as estrogen receptor-positive, lymph node-negative. This kind of breast cancer needs estrogen to grow but has not spread to the lymph nodes. Currently, many American women with this type of breast cancer do receive chemotherapy in addition to hormonal therapy. Using samples from 447 patients and a collection of 250 genes, 16 cancer-related genes were found that worked best. The scientists created a formula that generates a “recurrence score” based on the expression patterns of these genes in a tumor sample. Ranging from 1 to 100, the recurrence score is a measure of the risk that a given cancer will recur. The results validate the ability of the recurrence score to predict risk of recurrence. Using biopsy tissue and medical records from another tamoxifen trial, researchers divided 668 women into low, intermediate, and high risk of recurrence groups. Fifty-one percent were in the low risk group (with a score of less than 18); 22 percent were at intermediate risk (recurrence score 18 or higher but less than 31); 27 percent were at high risk (a score of 31 or higher). There was a significant difference in recurrence rates between women in the low and high risk groups. In the low risk group, there was a 6.8 percent rate of recurrence at 10 years; in the intermediate and high risk categories these rates were 14.3 and 30.5 percent, respectively. Up to a recurrence score of 50, rates of recurrence increased continuously as the recurrence score increased. These trends held across age groups and tumor size. The same test has also been used to predict how beneficial chemotherapy will be for women with estrogen receptor-positive, lymph node-negative breast cancer that are on tamoxifen. In the treatment study, women with high recurrence scores, who are representative of about 25 percent of patients with this kind of breast cancer, had a large benefit from chemotherapy in terms of 10 year recurrence-free rates. Women with low recurrence scores, who represent about 50 percent of these patients, derived minimal benefits from chemotherapy. The group under study was not large enough to determine whether chemotherapy is detrimental to the low risk group. However, the test has the potential to change medical practice by sparing thousands of women each year from the harmful short-and long-term side effects associated with chemotherapy.