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Triple-positive breast cancer is a type of breast cancer that occurs when breast cancer cells use estrogen receptors, progesterone receptors and the human epidermal growth factor 2 (HER2) protein to grow. Triple-positive breast cancer is a subtype of HER2 positive breast cancer; it accounts for about 10% of all breast cancer diagnoses.

We spoke with breast medical oncologist Jason Mouabbi, M.D., to learn more about triple-positive breast cancer, including how it’s diagnosed and the latest research advances.

What does it mean when breast cancer is triple-positive?

Normal breast cells have receptors for the two hormones, estrogen and progesterone. When these receptors attach to the estrogen and progesterone hormones, it promotes cell growth. Some breast cancer cells use these hormone receptors to fuel the cancer to grow. This is known as hormone receptor positive breast cancer. 

The HER2 protein is involved in cell growth and can be found in normal breast cells. Breast cancer cells that contain a higher-than-usual amount of HER2 are considered HER2 positive.   

“About 20% of breast cancer diagnoses are HER2 positive,” says Mouabbi. “Of those, about half are also estrogen-receptive and progesterone-receptive. So, patients with breast cancer cells that contain estrogen receptors, progesterone receptors and high levels of HER2 are referred to as having triple-positive breast cancer.”   

How is triple-positive breast cancer diagnosed?

Triple-positive breast cancer has the same symptoms as other types of breast cancer.

“A lump is the most common symptom,” says Mouabbi. “Sometimes, very early-stage breast cancer can only be detected on a screening mammogram. That’s why it’s so important to get your breast cancer screenings.”

If a mammogram detects a suspicious lump, your doctor will biopsy the lump. This is when tissue from the tumor is taken and inspected under a microscope. If the tumor is determined to be cancerous, your doctor may also do immunohistochemistry (IHC) tests to determine whether the cancer is driven by the estrogen receptor, progesterone receptor and/or the HER2 receptor.

“With an IHC test, we’ll stain the tissue and look at it under a microscope,” explains Mouabbi. “We’re staining for the estrogen receptor, progesterone receptor and high levels of HER2. If all three of them come back positive, the patient is diagnosed with triple-positive breast cancer.”  

Is triple-positive breast cancer aggressive?

HER2 positive breast cancer grows quickly and is considered more aggressive than hormone receptor positive breast cancers. Remember, triple-positive breast cancer is a subtype of HER2 positive breast cancer.

“HER2 positive breast cancer is the most treatable type of breast cancer, and it is the most responsive to treatment,” says Mouabbi. “So, yes, it’s more aggressive, but it’s the most responsive to therapies we currently have.”

How is triple-positive breast cancer treated?

Treatment for triple-positive breast cancer depends on the stage. It is treated comprehensively, using techniques to treat both HER2 positive breast cancer and hormone receptor positive breast cancer.

Patients with stage I tumors that are smaller than 2 centimeters will usually be treated with surgery first, followed by chemotherapy in combination with HER2-targeted therapy. They may receive radiation therapy, then hormone therapy (also known as endocrine therapy).

Patients with more advanced cancers are typically treated with a combination of chemotherapy plus HER2-directed therapy, followed by surgery. They’ll be evaluated to see if they need radiation therapy, and then they’ll receive hormone therapy to keep the cancer from coming back.

Is triple-positive breast cancer curable?

Curability for triple-positive breast cancer depends on many factors, including the stage of the disease.

“Because it’s very responsive to the kinds of therapies we have, triple-positive breast cancer has a more favorable prognosis,” says Mouabbi. “Patients with triple-positive breast cancer who respond to treatment have excellent five-year survival rates.”

What research is being done to advance triple-positive breast cancer treatment?

MD Anderson is looking to change the standard of care for triple-positive breast cancer by eliminating chemotherapy as one of its treatments. MD Anderson breast medical oncologist Vicente Valero, M.D., is leading the ADEPT clinical trial, which is studying a combination of HER2-directed therapies and hormonal therapy as a treatment after surgery for stage I triple-positive breast cancer patients.

“For patients with early stage I cancer who have an excellent prognosis, we think that chemotherapy isn’t necessary, and we want to eliminate the use of chemotherapy in those patients,” says Mouabbi. “So, the patients would have surgery, possible radiation and then HER2-targed therapy in combination with hormonal therapy. Because they would receive no chemotherapy, the treatment will be better tolerated with fewer side effects. We think this is going to change the way we manage very early-stage triple-positive breast cancer.”

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