When is it safe for a woman with breast cancer to skip chemotherapy?

A new study helps answer that question, based on a test of gene activity in tumors. It found that nearly half of women with early-stage breast cancer who would traditionally receive chemo can avoid it, with little risk of the cancer coming back or spreading in the next five years.

The so-called genomic test measures the activity of genes that control the growth and spread of cancer, and can identify women with a low risk of recurrence and therefore little to gain from chemo.

“More and more evidence is mounting that there is a substantial number of women with breast cancer who will not need chemotherapy to do well,” said Dr. Rachel A. Freedman, a breast cancer oncologist at the Dana-Farber Cancer Institute in Boston. She was not involved in the study.

The researchers estimated that their findings, published Wednesday in The New England Journal of Medicine, would apply to 35,000 to 40,000 women a year in the United States, and 60,000 to 70,000 in Europe. They are patients with early disease who because of tumor size, cancerous lymph nodes and other factors would normally be prescribed chemo.