Breast cancer is the second most common cancer in women. You probably know there are many types of breast cancer, but it wouldn’t be surprising if you had never heard of metaplastic breast cancer. That’s because it’s an extremely rare form of invasive breast cancer.

In this article, we’ll go over the characteristics of metaplastic breast cancer, address some easily confused terms, and review treatment options.

Research indicates that metaplastic breast cancer makes up about 0.2 to 5 percent of all breast cancers. There are fewer than 10,000 cases a year in the United States.

Many breast cancers begin in the milk ducts. Under a microscope, the cancer cells resemble ductal cells, but they look abnormal.

Metaplastic breast cancer also begins in the milk ducts. But it looks very different under a microscope. There may be some abnormal ductal cells. But there are also one or more other types of cells in the cancer tissue that aren’t usually found here, like those that makeup skin or bone.

Metaplastic breast cancer is usually, but not always triple-negative. This means it lacks estrogen receptors (ER), progesterone receptors (PR), and human epidermal growth factor 2 (HER2).

Metaplastic tumors tend to be high grade. This means the cancer cells look a lot different from normal cells and reproduce at a high rate. Unlike other types of breast cancer, it’s more likely to be metastatic and spread to the lungs or bones than the lymph nodes.

Symptoms of metaplastic breast cancer are the same as most other types of breast cancer. These may include:

  • a lump or thickening on the breast
  • change in the size or shape of the breast
  • puckering or dimpling of the skin
  • nipple turning inward
  • nipple discharge
  • breast pain

Cancer begins with damage to DNA that allows abnormal cells to grow out of control. It’s not clear exactly why an individual develops breast cancer.

According to the National Institutes of Health (NIH), there are no known inherited genetic predisposing risk factors for metaplastic breast cancer. The cause for this type of cancer is not known.

Known risk factors for any breast cancer include:

  • age — most breast cancers are diagnosed after age 50
  • inherited gene mutations, like BRCA1 and BRCA2
  • personal or family history of breast or ovarian cancer
  • first period before age 12 and menopause after age 55
  • physical inactivity
  • overweight or obesity after menopause
  • taking hormone replacement therapy or certain oral contraceptives
  • drinking alcohol

Because metaplastic breast cancer is rare, the exact incidence of BRCA1 and BRCA2 mutations isn’t known in this type of cancer.

Metaplastic breast cancer is diagnosed the same way as other types of breast cancer. This may include:

A biopsy is the only way to confirm a breast cancer diagnosis. After taking a sample of tissue from the tumor, a pathologist examines it under a microscope. The presence of multiple types of cells can suggest that it’s metaplastic breast cancer. Your biopsy results will contain additional information, like:

  • ER, PR, and HER2 status
  • tumor grade

Treatment is like that of other types of breast cancer, including both local and systemic therapy. Most metaplastic breast cancers are also triple negative, which means they can’t be treated with hormone therapy. But they’re more likely to be locally advanced than triple-negative breast cancer and may not respond as well to chemotherapy.

Your treatment plan will be based on the features of your cancer, like:

  • tumor size
  • tumor grade
  • hormone receptor (HR) status
  • HER2 status

Surgery

Surgery for breast cancer may involve:

  • Breast-conserving surgery, also called lumpectomy, is a procedure in which the surgeon removes the tumor and a margin of healthy tissue around it.
  • Mastectomy is a surgery that’s done to remove the entire breast.

The type of surgery you choose depends on several factors. These include the size and number of tumors and personal preference.

Chemotherapy

Chemotherapy destroys cancer cells throughout the body and can reduce the risk of spread and recurrence. Chemotherapy can take place before or after surgery.

Radiation therapy

Radiation therapy usually follows breast-conserving surgery to target any cancer cells that may have been left behind. It can also be used following a mastectomy. Radiation can be directed toward the tumor site or nearby lymph nodes.

Drug therapy

Deciding which drug therapy to use depends on the characteristics of the cancer.

Hormone therapy is used to treat HR-positive breast cancers. These drugs help block or stop hormones from fueling the cancer. Metaplastic breast cancer is more likely to be HR-negative, in which case hormone therapy isn’t an option.

Most metaplastic breast cancer is also HER2-negative. But if yours is HER2-positive, you may benefit from targeted therapies like:

  • monoclonal antibodies
  • antibody-drug conjugate (ADC)
  • kinase inhibitors

Gene profiling may help doctors provide more targeted treatments. Researchers have identified various molecular abnormalities that could lead to more targeted therapies. One example of this is a protein called PD-L1.

Research published in 2021 looked at the combination of chemotherapy drugs and pembrolizumab, a PD-L1 antibody. Positive responses to this combination therapy were observed in tumors with intermediate PD-L1 expression.

A 2021 case report involved a 72-year-old woman with stage 4 triple-negative metaplastic breast cancer. Her cancer tested positive for PD-L1. She was treated with pembrolizumab for 2 years, during which time she also had surgery. After 32 months, scans showed no evidence of disease, and she maintained a good quality of life.

Metaplastic breast cancer has a poorer prognosis than other breast cancer types. It has twice the risk of recurrence and shorter disease-free and overall survival than non-metaplastic triple-negative breast cancer.

Research shows that between 2010 and 2014, women diagnosed with metaplastic breast cancer were more likely to be older at diagnosis. They also had more co-morbid conditions (other conditions that occur at the same time) than women with other breast cancers.

At a median follow-up of 44.5 months, overall survival rates were:

  • stage 1: 85 percent
  • stage 2: 73 percent
  • stage 3: 43 percent

The 3-year overall survival rate for metastatic disease (stage 4) was 15 percent. Outcome was not affected by hormone or HER2 status. Worse outcomes were associated with:

  • increasing age
  • advanced stage
  • lymphovascular invasion
  • axillary lymph node dissection vs sentinel node dissection
  • no radiation
  • no chemotherapy

Many factors affect your outlook. After reviewing your tests and medical history, your oncologist can provide you with a more individual prognosis and an idea of what you can expect from treatment.