After a breast cancer diagnosis, one of the first words you hear from the doctor is "HER2." At first everyone wonders what that is. Put simply, it is a kind of switch-like protein attached to the surface of cancer cells. Normal cells have a little of it too, but when an abnormally large amount of it forms, the cancer cells keep receiving the signal to grow. Think of it as being similar to a car stuck with the accelerator pressed down. When a biopsy shows there is an excessive amount of this HER2, it is called "HER2-positive," and roughly one in five of all breast cancer patients falls into this category.
In the past, being HER2-positive was regarded as a rather bad sign, because it tends to progress quickly. But now the mood has changed a lot, thanks precisely to targeted therapy. So what makes targeted therapy different from the chemotherapy we commonly know? Ordinary chemotherapy, that is, cytotoxic chemotherapy, attacks rapidly growing cells indiscriminately. It catches cancer cells, but healthy hair cells and stomach lining cells take a hit too. That is why hair falls out and you feel nauseous. Targeted therapy, on the other hand, pinpoints and blocks only that HER2 switch I mentioned. By cutting off the signal, the cancer cells can no longer receive the command to grow.
A representative drug is trastuzumab (Herceptin). The name is hard, but think of it as an antibody drug that sticks tightly to HER2 and keeps it from working. Pertuzumab may be used alongside it, and recently there are even methods that attach a chemotherapy component to the antibody to deliver the drug only to the cancer cells (this is called an antibody-drug conjugate). The drugs have gradually become smarter. To clear up one misunderstanding: doing targeted therapy does not mean skipping ordinary chemotherapy entirely. Usually the two go together. Shrinking the large mass with chemotherapy and catching the HER2 signal with targeted therapy, coordinating the two in this way.
Of course, targeted therapy is not entirely without side effects. However, things like hair loss or severe nausea are definitely milder than with ordinary chemotherapy. Instead, there is something else to watch for, and the part of greatest concern is the heart. Because HER2 targeted drugs can affect the heart muscle, during treatment you periodically undergo heart function tests (such as echocardiography). Rather than something to be afraid of, it is easier on the mind to accept it as simply a process of going along while checking in regularly. For most people it recovers once treatment ends.
And one more thing. Because the treatment direction splits completely depending on whether you are HER2-positive or negative, it is good to be sure to check this item on the biopsy report and ask your doctor, "What is my HER2?" Even with the same breast cancer, the type differs from person to person, and there are drugs suited to each, which is the heart of treatment these days. Once you actually understand it, knowing the exact character of your cancer is half of treatment.
What is written here is a general explanation to aid understanding. Your treatment plan is most accurately decided in consultation with the medical team who has directly seen your test results.