The majority of women with a common form of breast cancer may be able to skip chemotherapy after surgery, based on their score on a genetic test, researchers said today.
As many as 65,000 women in the United States alone could be impacted by the study, described as the largest breast cancer trial to date, released at the American Society of Clinical Oncology annual meeting in Chicago.
Until now, women have faced a great deal of uncertainty about whether to add chemo to hormone therapy after a diagnosis with hormone-receptor positive, HER-2 negative breast cancer when found at an early stage before it has spread to the lymph nodes.
"With results of this ground-breaking study, we now can safely avoid chemotherapy in about 70% of patients who are diagnosed with the most common form of breast cancer," said co-author Kathy Albain, an oncologist at Loyola Medicine.
"For countless women and their doctors, the days of uncertainty are over."
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A 21-gene test called Oncotype Dx that has been around since 2004 has helped guide some decisions, post-surgery.
A high recurrence score, above 25, means chemo is advised to ward off a recurrence while a low score, below 10, means it is not.
The current study involved more than 10,000 women and focused on those whose scores were in the middle range, from 11 to 25.
Patients, aged 18 to 75, were randomly assigned to receive chemotherapy followed by hormonal therapy or hormone therapy alone.
Then, researchers studied the outcomes, including whether or not cancer recurred, and overall survival.
"For the entire study population with gene test scores between 11 and 25 - and especially among women aged 50 to 75 - there was no significant difference between the chemotherapy and no chemotherapy groups," said the findings, published in the New England Journal of Medicine.
The results show that all women over 50 with a recurrence score of 0 to 25 can be spared chemotherapy and its toxic side effects.
For women under 50 with a score of 0 to 15, chemo could be skipped.
However, among younger women with scores 16 to 25, outcomes were slightly better in the chemotherapy group, so in those cases doctors may urge patients to consider a chemo regimen.
The results "should have a huge impact on doctors and patients," Ms Albain said.
"We are de-escalating toxic therapy."
According to first author Joseph Sparano of Montefiore Medical Centre in Bronx, New York, "any woman with early stage breast cancer, 75 or younger, should have the test and discuss the results" with her doctor.
Breast cancer is the leading cause of cancer death in women worldwide, causing some 1.7 million new cases annually and over half a million deaths.
The study's primary funding came from the US National Institutes of Health.