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Some 70 per cent of women with early-stage breast cancer and an intermediate risk of cancer recurrence can safely skip chemotherapy after their tumors have been removed, U.S. researchers said on Sunday.

“This is a major finding,” said Larry Norton, a breast cancer expert at Memorial Sloan Kettering Cancer Center in New York, who helped organize the government-funded study more than a decade ago.

“It means that maybe 100,000 women in the United States alone do not require chemotherapy,” Dr. Norton said.

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The research, presented at the American Society of Clinical Oncology (ASCO) meeting in Chicago, studied how to treat women with early-stage breast cancer that responds to hormone therapy.

Women were deemed to have a medium-level risk of the cancer coming back based on a 21-gene panel known as Oncotype DX from Genomic Health. The test predicts the likelihood of cancer recurrence within 10 years.

Those who score low on the test – from zero to 10 – are already told to skip chemotherapy after their tumours are removed and they receive hormone therapy. Those who score highly – 26 to 100 – receive both hormone therapy and chemotherapy.

The study, dubbed TAILORx, was also published in the New England Journal of Medicine. It involved more than 10,000 women with breast cancer that had not spread to nearby lymph nodes and whose tumours respond to hormone therapy and test negative for the HER2 gene.

Of those, 6,711 scored in the intermediate range of 11-25, and were randomly assigned hormone therapy alone or hormone therapy as well as chemotherapy.

The study found that all women over the age of 50 with this type of breast cancer could skip chemotherapy, a group that represented 85 per cent of the study’s population. In addition, women 50 and younger who scored between zero and 15 could be spared chemotherapy and its toxic side effects.

However, chemotherapy did offer some benefit to women 50 and younger who had a cancer recurrence score of 16-25, researchers found.

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Steven Shak, chief scientific officer at Genomic Health, said about four in 10 women in the United States with early-stage breast cancers are not tested for recurrence risk. He expects the study’s results will change that practice.

“This is going to provide the highest level of evidence now for our test being indispensable in clinical practice,” Dr. Shak said.

The company currently provides tests to more than 900,000 patients in more than 90 countries, Dr. Shak said. In the United States, the test costs US$4,000 and is covered by Medicare and all major private insurers.

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