HER2-low breast cancer: a new treatment category

HER2-low describes breast cancers that have a small amount of the HER2 protein, but not enough to count as HER2-positive. For years these cancers were simply grouped as HER2-negative. That changed when an antibody-drug treatment, trastuzumab deruxtecan, was shown to help women with metastatic HER2-low breast cancer live longer. HER2-low has since become an important category when treatment for metastatic disease is being planned.

Why a small amount of HER2 now matters

Trastuzumab deruxtecan carries a powerful chemotherapy drug directly to cancer cells that carry HER2. It can work even when only a little HER2 is present, which older HER2 drugs such as trastuzumab could not.

Where HER2-low status is used today

Mainly in metastatic breast cancer, usually after other treatments, including hormone therapy for hormone-sensitive cancers. In early breast cancer, HER2-low status does not currently change the standard plan outside clinical trials.

Who this may not apply to

Cancers with no detectable HER2 are called HER2-zero, and newer research is exploring whether some of these may also benefit. People with significant lung problems need particular caution with this drug.

The recognition of HER2-low has added a meaningful treatment option for many women with metastatic breast cancer. It does not make metastatic disease go away for good, but trastuzumab deruxtecan has helped women live longer and control their cancer for longer than standard chemotherapy in studies.

Test results can vary

The line between HER2-zero and HER2-low is fine, and results can differ between laboratories or between the original tumour and a metastasis. If your result was zero, it can be reasonable to ask whether another sample or a review of the slides is worthwhile.

Side effects need attention

Feeling sick, tiredness and hair loss are common. The most serious concern is lung inflammation, which is uncommon but must be reported promptly. New cough, breathlessness or fever should never be ignored.

Access and cost

This is an expensive treatment and access varies. Ask your team what is realistic for you, whether support programmes exist, and whether a clinical trial might be an option.

What this page cannot tell you

It cannot tell you where this treatment would fit in your plan. Ask your oncologist about your HER2 result and its meaning for you.

Treatment is given as an infusion every three weeks and continues for as long as it controls the cancer and side effects are manageable.

Preventing sickness

Feeling sick is common, so anti-sickness medicines are usually given before each infusion and to take at home. Tell your team if they are not working well enough, as other options can be tried.

Watching the lungs

Regular scans and questions about breathing help catch lung inflammation early. Report any new cough, breathlessness or fever straight away, even between appointments.

Blood tests and reviews

Blood counts and other tests are checked before each dose, and scans every few months show how the cancer is responding.