The largest breast cancer trial ever conducted has yielded surprising results: Thousands of women can potentially skip grueling chemotherapy when it comes to treating early-stage breast cancer.

The 10-year study, discussed Sunday at the American Society of Clinical Oncology conference in Chicago, revealed that most patients with an intermediate risk of cancer recurrence won’t hurt their chances of beating the disease by forgoing chemo.

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The study cast doubt on chemo’s necessity in treating women in the early stages of the disease if: 1) there is no spread to lymph nodes; 2) the cancer is hormone-positive; and 3) the cancer is not the type targeted by the drug Herceptin.

This could affect up to 70,000 women a year in the U.S. and many more worldwide, the study noted.

Researchers gave 10,273 patients a test called Oncotype DX, which measures the activity of genes involved in cell growth and the response to hormone therapy, to estimate the risk a cancer would recur, multiple outlets reported.

For the study, the 67 percent of women showing an intermediate risk of recurrence had surgery combined with hormone therapy, and half of that group also received chemo. After nine years, 94 percent of both groups were still alive — and about 84 percent were alive without signs of cancer, indicating that the chemo made no difference.

“The impact is tremendous,” study leader Joseph Sparano of Montefiore Medical Center in New York told USA Today. “Most women in this situation don’t need treatment beyond surgery and hormone therapy,” he said, and “the rest of them are receiving chemotherapy unnecessarily.”

The study is an important tool in an oncologist’s toolbox.

“Moving forward, when women are making this decision, this study will help us put it into perspective and give them better advice next week than we were able to give them last week,” Jennifer Litton, an associate professor and oncologist at MD Anderson Cancer Center in Houston, Texas, explained to USA Today.

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“This is a hard call when you are actually the one with the breast cancer,” an early-stage breast cancer survivor from Middleton, Massachusetts, told LifeZette. “I am a wife and a mom, and my attitude about treatment was ‘Throw the kitchen sink at it. I want to live — period.’ I can’t say how I would have responded to this study, but when it’s your life, you’d rather deal with the horrible effects of chemo for a relatively short amount of time than die.”

For the past several years, cancer care has moved away from chemotherapy, with its harsh side effects — including debilitating nausea, hair loss, anemia, fatigue and infection — to gene-targeting therapies, hormone blockers, and immune system treatments.

Another study presented at Sunday’s conference found that Merck’s immunotherapy drug Keytruda worked better than chemo as initial treatment for most people with the most common type of lung cancer, and with far fewer side effects, noted USA Today.

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Women with early-stage breast cancer tend to have high survival rates, but their outlook worsens tremendously if the cancer returns to other parts of the body. That has prompted many people to turn to chemo, according to multiple reports.

“Oncologists have been getting much smarter about dialing back treatment so that it doesn’t do more harm than good,” Steven Katz, a University of Michigan researcher who examines medical decisionmaking, told The Washington Post. “That’s important because chemo is toxic; it whacks patients out and can result in long-term job loss.”

“This [study] doesn’t tell us chemotherapy is not effective in some cases.”

Litton, the MD Anderson oncologist, said doctors need to consider each case on its own merits; she cautioned against ruling out chemo too quickly.

“This study gives us information on a very specific group of women with a very specific type of cancer,” she said, “but it doesn’t tell us chemotherapy is not effective in some cases.”

Deirdre Reilly is a senior editor for LifeZette. Follow her on Twitter.