Subtypes of Breast Cancer

Triple-negative Breast Cancer (TNBC)

TNBC accounts for 10-15% of all breast cancers and is considered more aggressive than other types. TNBC does not have estrogen receptors (ER), progesterone receptors (PR), and HER2 overexpression. Because it lacks these features, it does not respond to hormone or HER2-targeted therapies. As a result, it is typically treated with chemotherapy, immunotherapy, surgery, and radiation. However, new targeted therapies are being developed.

Who is more likely to have TNBC?

Certain groups are more likely to develop TNBC:

• More common in younger people
• Black and Hispanic populations are more likely to be diagnosed
• Linked to BRCA1 mutations (60-80% of BRCA1-positive breast cancers are TNBC).
• Higher risk of recurrence than other breast cancers

TNBC Subtypes

TNBC consists of multiple subtypes. The most common is basal-like TNBC, which makes up about 80% of cases and is often linked to BRCA1 mutations. Some subtypes respond differently to treatment, and research continues into targeted therapies.

Systemic Treatments for TNBC

While TNBC does not respond to hormone or HER2-targeted therapies, several systemic treatments are available. These drugs travel throughout the body and are given as pills or intravenous (IV) infusions.

Chemotherapy
Chemotherapy is the main treatment for TNBC, as it remains the most effective option for many patients. It works by killing or slowing the growth of cancer cells (though it may also affect healthy cells), leading to side effects like hair loss.

Immunotherapy
Immunotherapy boosts the immune system to recognize and attack cancer cells. Immunotherapy for TNBC typically targets PD-L1 proteins.

• Atezolizumab (Tecentriq)
• Pembrolizumab (Keytruda)

Antibody-Drug Conjugates (ADCs)
ADCs combine chemotherapy with a targeted antibody, helping deliver the drug directly to cancer cells while sparing healthy tissue.

• Sacituzumab govitecan (Trodelvy)

PARP Inhibitors
These drugs prevent cancer cells from repairing their damaged DNA. This type of drug is known to work well for people with BRCA1 or BRCA2 mutations.

• Olaparib (Lynparza)

Clinical Trials

Research into new targeted therapies for TNBC is ongoing. If standard treatments are not effective, your doctor may recommend a clinical trial as an option.

Use our PatientPath tool to explore treatment options tailored to your specific diagnosis.

HER2-Low and TNBC

Even though TNBC is considered HER2 negative, about 30% to 50% of cases are HER2-low, meaning they have small amounts of HER2 protein. Emerging research suggests that some HER2-low TNBC cases may respond to HER2-targeted therapies, like antibody-drug conjugates.

Ask your doctor whether this applies to your cancer and if it affects your treatment options.

Learn more about HER2-low breast cancer and its potential treatment implications here.

Resources and Support

• Webinar: Advancements in Care for TNBC
• Questions & Experts: Medical Oncologist, Dr. David Cescon, Answers Your Questions About TNBC
• MedSearch: Information on TNBC treatments
• Project Report: TNBC Project

Resources from partner organizations:

• Triple Negative Breast Cancer Foundation: Understanding Triple Negative Breast Cancer
• Breastcancer.org: Triple Negative Breast Cancer (TNBC)
• Expose TNBC: Raising awareness and providing education on TNBC.
• Uncovering TNBC: Information and resources dedicated to understanding TNBC.

Additional reading:

• Advocacy Guide: Increasing Research on HER2-Low Breast Cancer
• Advocacy Guide: Accessing Genetic Testing in Canada
• Olive Branch of Hope: Supports individuals of African-Caribbean heritage and other culturally diverse backgrounds affected by cancer.
• Every Breast Counts: A resource hub for Black communities in Canada, created by Women’s College Hospital and Olive Branch of Hope.

Medical Review by Roochi Arora, MD, FRCPC, August 2025

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