Maciej Frolow—Getty Images
By Alice Park
August 27, 2015
TIME Health
For more, visit TIME Health.

Correction appended, Aug. 31, 2015

In the new era of personalized medicine, having more information on hand is considered the ideal situation for making more customized, and ideally, effective decisions about medical care.

And in a new study of breast cancer patients, researchers show that a relatively new genetic test for evaluating tumors is doing just that. It’s just that the test isn’t necessarily leading to the decisions that experts expected.

The Oncotype DX Breast Cancer Assay is a test launched in 2004 to help women decide how likely their breast cancer is to recur. The score, from zero to 100, is for women with breast tumors that have not spread to the lymph nodes. It places women on a scale of probability, based on an analysis of 21 genes in her tumor. Most doctors and patients use the score to decide, in part, whether the woman should receive chemotherapy following surgery.

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In previous studies, about 20% to 30% of doctors say they changed their recommendation about chemotherapy based on the Oncotype DX score. But none of the studies looked at how Oncotype DX affected the likelihood a woman would undergo chemotherapy rates in a real-world setting— outside of a trial. In clinics, says Michaela Dinan, assistant professor in medical oncology at the Duke Cancer Institute, many other factors contribute to treatment decisions, including fear, family history and physician advice. So she and her colleagues conducted a review of data on more than 44,044 breast cancer patients to see how the Oncotype test affected chemotherapy decisions.

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The results, published in JAMA Oncology, showed that overall, the test had no effect on their decision. Women who were tested were no more or no less likely to opt for chemotherapy than those not getting Oncotype DX. Younger age and a higher risk disease were more likely to predict chemotherapy use than the assay.

While many assumed that the test would lead to fewer treatments, Dinan’s data shows that how the testing affects chemotherapy decisions is less predictable. When Dinan delved further into the numbers, she found an interesting pattern. Those rated as having high-risk breast cancer according to the National Comprehensive Cancer Network guidelines were less likely to get chemo than women who were not tested. And among people with low-risk disease, those getting the genetic test were more likely to get chemotherapy than low-risk patients who did not.

Because the study did not take into consideration what the Oncotype DX scores were, it’s possible, for example, that women considered high risk who received intermediate or low Oncotype DX scores decided not to undergo chemotherapy since the testing showed their response might not be as positive as they might have expected. On the other hand, women who have low risk disease and receive an intermediate test score might decide to undergo chemotherapy since the intermediate risk might represent a slightly higher risk of recurrence than they were anticipating.

“It’s a more nuanced finding,” says Dinan. “The Oncotype DX test is impacting the receipt of chemotherapy, but the impact isn’t in one direction or another in terms of whether people are more or less likely to get chemotherapy.”

As more options for personalized treatments make their way into the clinic, Dinan says it’s worth remembering that they shouldn’t dictate decisions but inform them. “The nuanced finding of the difference between high risk and low risk patients says to me that whether or not a woman with early stage breast cancer undergoes chemotherapy is going to be affected by a number of different factors, not just this assay. It’s a personalized discussion about the individual patient’s case.”

Correction: The original version of this story misstated that the Oncotype DX test is FDA approved. It is a commercial test regulated by different laboratory guidelines.

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