Breast cancers are often classified as either HER2-negative or HER2-positive, which helps doctors know what the best options are for the treatment of that specific type of cancer. However, new research suggests that nearly 40 percent of patients with a recurrent breast cancer case will have a different HER2 status than they had with their primary cancer diagnosis.

The human epidermal growth factor receptor 2 gene (HER2) normally helps control how a healthy breast cell grows, divides, and repairs itself. In about 10 to 20 percent of breast cancers, however, HER2 makes too many copies of itself (known as gene amplification). All those extra HER2 genes then cause breast cells to make too many HER2 receptors (known as protein overexpression), forcing breast cells to grow and divide uncontrollably.

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Breast cancers that have HER2 gene amplification or HER2 protein overexpression are known as HER2-positive and tend to be more aggressive than other types of breast cancer. They are often treated using drugs that specifically target the HER2 protein.

Because a cancer patient’s HER2 status determines what treatments will be most effective for them, it’s vital for most patients with invasive breast cancer to find out what their HER2 status is. And new research is showing that it’s also important to get your HER2 status retested if you get a recurrent cancer case after your primary cancer has already been treated.

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For the study, the researchers analyzed both the primary and secondary tumors of 575 patients who had recurrent breast cancer. Overall, they discovered that 38 percent of recurrent breast cancers had a different HER2 status than their primary cancers.

15 percent of HER2-negative primary breast cancers switched to HER2-low recurrent breast cancers (meaning the HER2 proteins present were elevated but not high enough to warrant a “positive” HER2 diagnosis). 14 percent of HER2-low primary breast cancers switched to HER2-negative recurrent breast cancers. And nine percent of the recurrent cancers either became HER2-positive or were not HER2-positive anymore.

The rate of HER2 status switching was highest among those whose original diagnosis was hormone-receptor-positive and HER2-negative.

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“The results provide a whole new insight on how HER2-low tumors might evolve as a subgroup, possibly challenging the current dichotomy between HER2-positive and HER2-negative breast cancer,” says Federica Miglietta, M.D., of the University of Padova, who presented the research, in a statement. “Our findings stress the importance of re-testing HER2 expression on tumor relapse since it might provide the option of new therapeutic opportunities, currently in a trial, and hopefully in the near future, in the clinic.”

So if you’ve had breast cancer before and you happen to develop it again, please don’t automatically assume that your new tumor will be the same HER2 status as your prior one was. If you doctor doesn’t order a new test for HER2 status and other biomarkers, request one so that you know you’ll be getting the best care possible for your individual case.

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