KATHERINE: What the Breast Cancer Trial Found

What happens when chemotherapy does not clear the cancer

Some breast cancers are treated with drugs before surgery rather than after. That is called neoadjuvant therapy.

It has a useful side effect beyond shrinking the tumor. When the surgeon operates, the pathologist can see how much cancer the drugs actually killed. If nothing invasive is left, that is a pathologic complete response, and the outlook is good.

If invasive cancer is still there, the outlook is worse. KATHERINE was built for exactly that group.

The HER2 story in one paragraph

HER2 is a protein on the surface of breast cells that receives a grow signal. In some breast cancers the HER2 gene is amplified, and the cell covers itself in far too many copies of the protein. That drives fast growth. A tumor with this feature is called HER2-positive.

Trastuzumab is an antibody that latches onto HER2 and blocks it. Its arrival changed HER2-positive breast cancer from one of the worst prognoses to one of the more treatable. Standard care before surgery pairs it with chemotherapy, usually a taxane.

What T-DM1 is

Trastuzumab emtansine, or T-DM1, is an antibody-drug conjugate. That means it is two things bolted together.

The antibody half is trastuzumab, which finds HER2 on the cancer cell. The other half is emtansine, a chemotherapy drug too toxic to give on its own. It works by wrecking the internal scaffolding a cell needs to divide.

The antibody carries the poison to the cell and delivers it inside. The idea is to hit HER2-positive cells hard while sparing more of the rest of the body. It is one of the clearest examples of what targeted therapy tries to do.

What the trial did

KATHERINE was a phase 3, open-label, randomized trial. A total of 1,486 people took part.

Everyone had HER2-positive early breast cancer. Everyone had received a taxane plus trastuzumab before surgery, with or without an anthracycline. And everyone still had invasive cancer in the breast or the armpit lymph nodes when they were operated on.

They were split evenly. Half received T-DM1 for 14 cycles. Half received trastuzumab for 14 cycles.

The main measure was invasive disease-free survival. That counts several things as an event: the cancer returning in the same breast, in the surrounding region, or at a distant site; a new invasive cancer in the other breast; or death from any cause.

What it found

At the planned interim analysis, an event had occurred in 91 people on T-DM1, or 12.2%, and in 165 people on trastuzumab, or 22.2%.

At three years, 88.3% of the T-DM1 group were free of invasive disease, against 77.0% of the trastuzumab group. That is a gap of 11.3 percentage points.

The hazard ratio was 0.50, with a 95% confidence interval of 0.39 to 0.64. In plain terms, the risk of recurrence or death at any given moment was halved.

Distant spread as the first event happened in 10.5% of the T-DM1 group and 15.9% of the trastuzumab group.

T-DM1 caused more side effects than trastuzumab alone. The pattern matched what was already known about the drug.

Why this changed practice

Before KATHERINE, someone with residual disease after neoadjuvant therapy simply finished the year of trastuzumab. There was no established way to act on the bad news the pathology report had delivered.

The trial turned that report into a decision point. Residual invasive cancer is now a reason to change drug, not just a prognostic marker.

That is also why breast cancer is increasingly treated with drugs first and surgery second in HER2-positive disease. The response to neoadjuvant treatment carries information you cannot get any other way.

When to get checked

Screening and symptoms are two different tracks.

For routine screening, the U.S. Preventive Services Task Force recommends mammography every two years for women aged 40 to 74. For women 75 and over it finds the evidence insufficient.

Symptoms need attention at any age, whatever your screening schedule says. See a clinician about:

• A new lump or firm area in the breast or armpit that does not come and go with your cycle.
• One breast changing shape or size.
• Skin that dimples, puckers, or looks like orange peel.
• A nipple that has newly turned inward.
• Discharge from one nipple, particularly if clear or bloody.
• A rash or scaling on the nipple that will not settle.

If you are already diagnosed, ask directly whether your tumor is HER2-positive and whether treatment before surgery is an option for you. Our page on breast cancer sets out what the receptor results mean.

What this trial cannot tell you

• It applies only to HER2-positive early breast cancer with residual invasive disease after neoadjuvant treatment. It says nothing about other groups.
• The main measure combined several different events. A trial can improve that combined figure without every component moving equally.
• More people had side effects on T-DM1. Better disease control was not free.
• Three-year figures are not lifetime figures. Breast cancer can return much later.
• Treatment for HER2-positive disease has kept moving since 2019, so this is one step rather than the current standard in full. Clinical trial phases explains what a phase 3 result settles and what it leaves open.

The wider picture

For 2026 the American Cancer Society projects about 321,910 new female breast cancer diagnoses in the United States, and about 42,140 deaths; SEER publishes that forecast. Five-year relative survival across all stages was 91.9% for 2016 to 2022, and 64% of cases are found while still confined to the breast.

Those are population figures from past years of care. They summarize a very large and varied group, and they cannot describe what will happen to any one person.

Sources

• von Minckwitz G, Huang CS, Mano MS, et al. Trastuzumab Emtansine for Residual Invasive HER2-Positive Breast Cancer. N Engl J Med 2019;380:617-628
• NCI PDQ: Breast Cancer Treatment (Health Professional Version)
• SEER Cancer Stat Facts: Female Breast Cancer
• USPSTF: Breast Cancer Screening

How this page was made

An AI-assisted editorial system helped prepare this page. No named medical reviewer has reviewed it unless one is listed.

Cancer Explained is published by the National Cancer Information Foundation. It is not medical advice and does not suggest a test or treatment.

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