Most women diagnosed with early stages of common forms of breast cancer may be able to avoid having to undergo chemotherapy, according to a new study.

The study, led by Dr. Joseph Sparano and published Sunday in the New England Journal of Medicine, found that women who had small tumors that had not spread to the lymph nodes did almost as well with medications as those who received chemotherapy.

“The impact is tremendous,” said Sparano, of Montefiore Medical Center in New York. The study, the largest ever done on breast cancer treatment, could spare about 70,000 American women from having to undergo the toxic treatment every year.

The study found that a genetic test of the tumors could determine which women would benefit from taking only a drug that blocks the hormone estrogen or stops the body from making it.

Breast cancer is the most common cancer in women worldwide. More than half the women diagnosed worldwide have cancer that is hormone-positive, where the growth of cancer cells is stimulated by estrogen or progesterone.

After following 10,273 women for an average of nine years. researches determined that for 83.3 percent of those on hormone therapy alone the original cancer did not return and they did not develop cancer elsewhere in their bodies. For the group that had both hormone and chemotherapy, the rate was minutely better at 84.3 percent.

The overall survival rate for both groups was 93.9 percent and 93.8 percent, respectively.