For many people, a breast cancer diagnosis is quickly followed by a second layer of complexity: learning that breast cancer is not a single disease, but a group of distinct subtypes, each defined by its own biology and each responding differently to treatment. For patients and families, these distinctions are far more than mere scientific details, they influence treatment decisions, prognosis, and their available options.
For example, two people may both be diagnosed with breast cancer, yet their treatment paths can look very different depending on the biology of their tumours.
Because of this, researchers continue to refine how breast cancer is classified, with the goal of developing more precise treatments and expanding options for patients who may previously have had fewer.
One area where this is becoming especially important is HER2 breast cancer. HER2 refers to a protein that can help drive cancer growth when present at high levels in tumour cells. For years, breast cancers were broadly classified as either HER2-positive or HER2-negative based on how much HER2 was detected in the tumour. Patients whose cancers fell below a certain threshold were generally not considered candidates for HER2-targeted therapies.
Over the last two decades, HER2-targeted treatments have dramatically improved outcomes for many patients living with HER2-positive breast cancer. However, researchers are now learning that HER2 may not be as simple as “positive” or “negative,” and that there may be an important category in between these two extremes.
More recently, researchers and clinicians have identified a new category known as HER2-low breast cancer. Although these tumours express lower levels of HER2, emerging evidence suggests that some patients may still benefit from HER2-targeted therapies, particularly a drug called trastuzumab deruxtecan (pronounced tras-TOO-zoo-mab deh-ROOKS-teh-kan). In a phase 3 clinical trial, patients treated with trastuzumab deruxtecan experienced longer progression-free and overall survival compared to those receiving standard chemotherapy.
This change represents more than a difference in terminology. It reflects a broader movement toward precision medicine in breast cancer care, one that aims to better match treatments to the biology of each patient’s disease. For patients, classifications once thought to close doors may now help open new ones. And for families watching a loved one navigate breast cancer, advances like these offer a powerful reminder that research continues to move cancer treatment toward more personalized, precise, and hopeful approaches to care.
