Triple-negative breast cancer (TNBC) is a fast-growing form of breast cancer. The American Cancer Society classifies TNBC as aggressive because it’s more likely than other types to have already metastasized (spread to other parts of the body) when it’s diagnosed. TNBC is also more likely to resist treatment and return after treatment than other types of breast cancer.

If you’ve received a triple-negative breast cancer diagnosis, it’s important to understand why this type tends to grow and spread quickly.

How Fast Does Triple-Negative Breast Cancer Grow?

The speed at which breast cancer grows and spreads typically depends on the type of cancer and how aggressive it is. Each person’s case is different. Some tumors grow faster than others, even among people with the same type of breast cancer.

In general, cancer cells in breast tissue need to divide about 30 times before a tumor is large enough to be felt. This process can take years, but it depends on the cancer’s subtype and grade. Triple-negative breast cancer tends to grow more quickly than other types. A 2016 study found that TNBC tumors grew by 1.003 percent per day, compared with 0.859 percent for HER2-positive tumors and 0.208 percent for hormone receptor (HR)-positive tumors.

Researchers found that TNBC metastatic breast cancers tended to be more aggressive and spread faster than other subtypes.

Other research has looked at tumor growth by measuring how quickly tumors double in size. A 2018 study used ultrasounds to track tumor volume in 265 people with breast cancer. About 64 percent of tumors grew over an average of 57 days. During the study, TNBC tumors increased more in volume than other types and had shorter doubling times than HR-positive tumors.

Your TNBC subtype may also affect how aggressive your cancer is. A study in JAMA Network Open looked at three rare subtypes of TNBC — medullary carcinoma, adenoid cystic carcinoma, and metaplastic carcinoma. These subtypes represent a small portion of triple-negative breast cancers, which themselves account for about 15 percent of all invasive breast cancers. The researchers found that metaplastic TNBC tended to be more aggressive and spread faster than the other subtypes.

If you want to know more about your specific subtype of TNBC, talk with your oncologist. They’ll go over your pathology report with you and explain what it means for your care.

Why Is Triple-Negative Breast Cancer Aggressive?

Triple-negative breast cancer tends to be more aggressive than other types of invasive cancer for four main reasons.

1. Triple-Negative Breast Cancer Doesn’t Respond to Hormone or HER2-Targeted Therapies

Most breast cancer cells have receptors — like estrogen receptors, progesterone receptors, or the HER2 protein — that help cancer grow. These cancers are often treated with hormone therapies that block these signals and slow down growth.

TNBC cells don’t have these receptors. Because of this, treatments that target estrogen or progesterone receptors or HER2 aren’t effective against TNBC. This limits treatment options.

2. Triple-Negative Breast Cancer Cells Tend To Be More Abnormal (Higher Grade)

During a biopsy, cancer cells are examined under a microscope and given a grade from 1 to 3 based on how different they look from healthy cells. Grade 1 cells look more like normal cells and usually grow slowly. Grade 3 cells look more abnormal and tend to grow and spread faster.

Invasive breast cancer types are first given a score between 3 and 9. That score is then used to assign a grade:

• A score of 3 to 5 — Grade 1
• A score of 6 or 7 — Grade 2
• A score of 8 or 9 — Grade 3

TNBC is a type of invasive breast cancer that’s often given the highest grade. These grade 3 cells are considered poorly differentiated — they look very different from normal cells. Grade 3 breast cancer cells tend to grow more quickly than lower-grade cells and spread faster throughout the breast and to other parts of the body.

3. Triple-Negative Breast Cancer Spreads More Easily

TNBC is more likely than some other types of breast cancer to spread to other parts of the body. Each breast cancer subtype tends to metastasize in its own way.

Among people first diagnosed with stage 1, 2, or 3 TNBC, almost 40 percent will eventually relapse after treatment and experience metastasis. Where the cancer spreads — such as to the brain, liver, bones, or lungs — can affect your prognosis (outlook).

As TNBC spreads, it may also develop mutations in different genes that make it resistant to treatments. This means that therapies that worked before may no longer be effective.

After treatment, many people with TNBC go into remission, meaning no signs of cancer can be found.

4. Triple-Negative Breast Cancer Is More Likely To Recur After Treatment

After treatment, many people with TNBC go into remission, meaning no signs of cancer can be found. However, around 40 percent of those with stage 1 to 3 TNBC will have a breast cancer recurrence, often within the first three to five years after diagnosis.

Why Is Triple-Negative Breast Cancer Harder To Treat?

Most breast cancer treatments work by targeting hormone receptors found on the outside of cancer cells. TNBC cells don’t have estrogen, progesterone, or HER2 receptors, so these treatments don’t work. Because TNBC is also more likely to spread to other parts of the body, it can also be more difficult to treat.

Treatments for Triple-Negative Breast Cancer

Despite the challenges described above, several approaches can help treat TNBC. Surgery is usually the first step for tumors that can be removed. Some people opt for breast-conserving surgery, like a lumpectomy. Others choose to have the entire breast removed with a mastectomy.

Chemotherapy may be given before surgery (called neoadjuvant therapy) to shrink the tumor or after surgery (adjuvant therapy) to destroy any remaining cancer cells. Radiation therapy is another adjuvant option if the tumor is large or cancer is found in the lymph nodes.

Some people opt for breast-conserving surgery, like a lumpectomy. Others choose to have the entire breast removed with a mastectomy.

Immunotherapy is sometimes used before surgery to help shrink large tumors. The U.S. Food and Drug Administration (FDA) has approved pembrolizumab (Keytruda) for treating people with TNBC.

Although targeted hormonal therapies don’t always work for TNBC, some drugs can be effective for people who have BRCA mutations. Poly (ADP-ribose) polymerase (PARP) inhibitors may be given to people with BRCA1 or BRCA2 gene mutations. PARP inhibitors include olaparib (Lynparza) and talazoparib (Talzenna).

Treatments for Recurrent Triple-Negative Breast Cancer

About one-quarter of people with TNBC will have a recurrence. When this happens, doctors may recommend a combination of treatments depending on where the cancer has returned and how it responded to previous therapies.

Chemotherapy, immunotherapy, and surgery may still be options. Newer treatment options are also available. For example, sacituzumab-govitecan-hziy (Trodelvy) is FDA-approved for people with metastatic or inoperable breast cancer who’ve tried at least two systemic therapies (like chemotherapy or immunotherapy). For people with metastatic disease, only one prior systemic therapy is needed before trying this medication.

Researchers continue to study new TNBC treatments in clinical trials. These medications may be used on their own or combined with existing treatments like chemotherapy.

It’s also important to take care of your overall health. Eating a healthy diet, managing other health conditions, and staying connected with your care team can all support your treatment and recovery.

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