Triple-negative breast cancer (TNBC) is a form of breast cancer that grows rapidly. According to the American Cancer Society, TNBC is considered an aggressive type of cancer because it’s more likely to have spread to other parts of the body (known as metastasis) by the time it’s diagnosed. TNBC is also more likely to return after treatment compared with other kinds of breast cancer.

If you’ve been diagnosed with TNBC, it’s important to understand the factors that contribute to its rapid growth and ability to spread.

How Fast Does Triple-Negative Cancer Grow?

The speed at which breast cancer grows and spreads typically depends on the type of cancer it is. Each person’s case is also different, and some people may have tumors that grow more quickly than others’ do.

In general, breast cancer cells need to divide around 30 times before a tumor can be felt with the fingers. Each cell division takes from one to two months, which means that tumors take three to five years to develop. However, this is an average, and the triple-negative form is known to grow more quickly than other types of breast cancer.

One study found that TNBC tumors increased in size by 1.003 percent per day, compared with 0.859 percent for hormone receptor (HR)-negative/human epidermal growth factor receptor 2 (HER2)-positive tumors and 0.208 percent for certain types of HR-positive tumors.

Other researchers have looked into how quickly different types of breast cancer grow by looking at their doubling times, or how long it takes the tumor to grow twice as large. A study found that in 265 people with breast cancer, 64 percent of tumors grew over the course of (on average) 57 days. Those with TNBC saw their tumors increase more in volume than others’ during the study. The doubling times of these tumors were also shorter than those of HR-positive tumor types.

The subtype of TNBC you have can also influence how aggressive your cancer is. One study looked at three rare subtypes of breast cancer — medullary carcinoma, adenoid cystic carcinoma, and metaplastic TNBC. When viewed under a microscope, the cancer cells of each subtype have slightly different characteristics. Researchers found that TNBC metaplastic breast cancers tended to be more aggressive and spread more quickly compared to other subtypes.

If you’re interested in learning more about your specific subtype of TNBC, you can talk to your oncologist. They will go over your pathology report with you to talk about your case.

Why Is Triple-Negative Breast Cancer Aggressive?

Compared to other types of invasive cancer, TNBC tends to be more aggressive. The reasons for this include a lack of hormone receptors along with certain changes in the cancer cells. Fortunately, oncologists and researchers continue to look for more effective breast cancer treatments as a way to treat this aggressive form of cancer.

1. Triple-Negative Breast Cancer Doesn’t Need Hormones To Grow

Most breast cancer cells have hormone receptors on their surface that are used to receive growth signals from hormones. These include estrogen receptor (ERs), progesterone receptor (PRs), and HER2 protein. HR-positive cancers rely on the presence of hormones to help them grow and divide, so they get larger more slowly.

However, TNBC cells don’t express ER, PR, or HER2 receptors. This means that the cancer cells can continue to divide and spread more quickly because they don’t need hormones to tell them to grow.

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

Breast cancer cells taken from a biopsy and examined under a microscope are usually given a grade. This number (between 1 and 3) refers to how similar the cancer cells are to normal cells. The lower the number, the more normal the cells look, so higher grades mean cells look less normal.

Invasive breast cancer types are given scores from 3 to 9 and then assigned to grade 1, 2, or 3. TNBC is a type of invasive breast cancer that tends to be given the highest grade. These grade 3 cells are considered “poorly differentiated” and look extremely 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 body.

3. Triple-Negative Breast Cancer Spreads Easily

TNBC also is more likely to spread beyond the breast more often than some other types of breast cancer. Each breast cancer subtype tends to spread — or metastasize — from the breast and lymph nodes to a different part of the body, including the brain, liver, bones, and lungs. In people who are originally diagnosed with localized TNBC (only in the breast), around 25 percent will eventually relapse with cancer in other parts of the body.

As the cancer moves throughout the body, it can develop mutations in different genes that make it resistant to treatments. This means therapies that worked before may no longer be effective.

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

After cancer treatment, many people with TNBC go into remission, meaning their cancer is gone. However, around 40 percent of those with stage 1 to 3 TNBC will have a recurrence. TNBC tends to come back within the first three to five years after diagnosis. Read more about recurrence rates in TNBC.

Why Is Triple-Negative Breast Cancer Harder To Treat?

Most breast cancer treatments focus on targeting the hormone receptors on the outside of cancer cells. This keeps the hormones from binding to the receptors, blocking signals that tell cancer cells to grow and divide. TNBC cells don’t have these receptors, so these treatments don’t work, leaving oncologists with fewer options.

Because TNBC spreads to other parts of the body more often, it can also be more difficult to treat.

Treatments for Triple-Negative Breast Cancer

Despite the challenges described above, several approaches are available to help treat TNBC. Surgery is usually the first option to remove tumors that are small enough. Some people opt for breast-conserving surgery (such as a lumpectomy), while others choose to have the entire breast removed (mastectomy).

Chemotherapy may be given before surgery to help shrink the tumor (known as neoadjuvant therapy) or afterward to help clear any remaining cancer cells (adjuvant therapy). Radiation therapy may also be given as an adjuvant therapy after surgery if the tumor is large or cancer is found in the lymph nodes.

Immunotherapy may be given 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 work for TNBC, other drugs may be used. 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).

Treating Recurrent Triple-Negative Breast Cancer

Since almost one-quarter of people with TNBC will have a recurrence, doctors and researchers continue to look for new types of treatment. In some cases, surgery, immunotherapy, and chemotherapy can be used again. New medications such as antibody-drug conjugates can also be given. One example is sacituzumab govitecan (Trodelvy), which is FDA-approved for metastatic and/or inoperable breast cancer.

New drugs are also being investigated in clinical trials with the goal of treating more TNBC cases. These medications may be used alone or combined with existing treatments like chemotherapy.

Talk With Others Who Understand

MyBCTeam is the social network for people with breast cancer and their loved ones. On MyBCTeam, more than 60,000 members come together to ask questions, give advice, and share their stories with others who understand life with breast cancer.

Are you living with TNBC? Do you still have questions about why it tends to be more aggressive? Share your experiences in the comments below, or start a conversation by posting on your Activities page.