What Is Bilateral Breast Cancer?

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Bilateral breast cancer is when tumors develop in both breasts of the same patient. Compared to unilateral (one side) breast cancer cases, bilateral breast cancer patients tend to be younger and the tumors smaller and of an earlier stage at diagnosis.

According to the Surveillance, Epidemiology, and End Results (SEER) database (1973–2014), bilateral cancers occurred in 1.4% of breast cancer patients in 1975 to 2.9% in 2014. 

Other data sets show the incidence range of 1%–11% of breast cancer patients, depending on how the cases are defined and the time period of surveillance. It is difficult to know how often this type of cancer actually occurs because there isn't optimal evidence differentiating between women who have had one or two primary cancers.

Unfortunately, data are limited on how to best manage bilateral breast cancer, what the best treatments are, and prognosis (outcome). However, it is a relatively uncommon cancer.

A middle-aged, mixed race woman, sits on an exam table in a doctors office.

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Bilateral breast cancers are categorized as follows as synchronous and asynchronous, or metachronous:

  • Synchronous bilateral breast cancer (SBBC) is when cancers develop at the same time, generally within three to 12 months.
  • Asynchronous, or metachronous bilateral breast cancer (MBBC) is when the cancers develop at different times. There is no standard definition for when the second cancer in the opposite breast is diagnosed. Some clinicians say at least one year apart but others argue a shorter time interval is appropriate.

Both cancers are considered primary cancers and not one that has metastasized (spread).


Symptoms of synchronous bilateral breast cancer consists of cancer signs and symptoms in both breasts. Symptoms may include:

  • Swelling of all or part of a breast
  • Skin dimpling
  • Breast or nipple pain
  • Nipple retraction (turning inward)
  • Nipple or breast skin that is red, dry, flaking, or thickened
  • Nipple discharge
  • Swollen lymph nodes

Causes and Risk Factors

There is little known about the risk factors for bilateral breast cancer. Some research indicates that possible risk factors include:

  • Regular alcohol consumption
  • Being 30 years old or younger at the first diagnosis
  • Presence of a BRCA1, BRCA2, or CHEK2 gene mutation
  • Family history of breast cancer in any first-degree relative (such as mothers or sisters) or second-degree relative (such as grandmothers or aunts)
  • Living in a metropolitan area
  • Lobular cancers rather than ductal cancers (lobular cancers begin in the milk-producing glands of the breast)
  • Estrogen receptive-negative (ER-negative) cancers
  • Scatter radiation

There may also be some factors that help protect patients from developing bilateral breast cancer. These include:

  • Age of first period being age 13 and older
  • Experiencing three or more pregnancies

Traditional risk factors for breast cancer that do not seem to increase risk for bilateral breast cancer include cigarette smoking, hormone replacement therapy, or oral contraceptives.


Diagnosis of synchronous bilateral breast cancer is based on signs and symptoms of cancer in both breasts. At the time of physical examination, the cancer may not be noticeable in both breasts. If cancer is detected in one breast, standard practice is to follow up with a bilateral mammography, which may detect signs of cancer in the opposite breast (asynchronous bilateral breast cancer).

While mammography is the standard screening tool for breast cancer, mammography also has its limitations. Mammograms vary in sensitivity and have been shown to miss detection of bilateral breast cancer. In one study, the mammography detected only 19% of bilateral breast cancers compared with 72.7% of unilateral breast cancers.

Further imaging by MRI (magentic resonance imaging) screening may be indicated to identify and diagnose bilateral breast cancer. However, bilateral MRI is not the current standard of practice. This is because MRI screenings have not been proven to reduce breast cancer deaths.

If you believe you are at high risk, speak with your doctor about screening for bilateral breast cancer.


There are several treatments that can potentially reduce the risk of developing cancer in the opposite breast, such as:

  • Tamoxifen is a drug that acts both like estrogen and antiestrogen. A daily dose is typically taken in tablet form for five to 10 years. It is associated with a 50% risk reduction in bilateral breast cancer. A standard course of tamoxifen treatment is five years, and the associated decrease in risk lasts 15 years. Some studies indicate a shorter treatment term may provide as much protection.
  • Hormonal therapy and aromatase inhibitors have been shown to provide protection when used after stopping tamoxifen treatment. Hormone therapy works by blocking or removing hormones associated with cell growth, with the aim of stopping the cancer cells from growing. Similarly, aromatase inhibitors reduce the levels of hormones in the body. One study showed that they reduce the risk of developing cancer in the opposite breast by 43%.
  • Adjuvant chemotherapy is continued chemotherapy (cancer drug) treatment beyond the removal of a tumor, with the purpose of preventing recurrence. It is associated with a 27% risk reduction in developing cancer in the opposite breast.
  • Bilateral mastectomy is the surgical removal of both breasts and is associated with a 95% risk reduction in developing bilateral breast cancer.

Considerations for Bilateral Mastectomy

Increasingly, women with unilateral breast cancer are treated with bilateral mastectomy (removal of both breasts). The reason is to stop the risk of death from developing bilateral breast cancer, or cancer in the opposite breast. However, this treatment is controversial because bilateral mastectomy is not proven to decrease death from breast cancer.

It's important to speak with your healthcare provider and assess your risk of developing bilateral breast cancer and whether this surgical intervention is warranted.


There is conflicting evidence about the survival rate for bilateral breast cancer. Some reports suggest higher mortality when compared with unilateral breast cancer, while others report similar survival rates.

A 2007 study of a Swedish population of women analyzed the difference in prognosis between synchronous and metachronous bilateral breast cancer. It found that women were 3.9 times more likely to die of metachronous bilateral breast cancer compared with women with unilateral breast cancer if:

  • They were younger than 50.
  • Cancer developed in the opposite breast within five years of initial diagnosis.

However, the same study found that if women develop bilateral breast cancer more than 10 years after the first diagnosis, they have a similar prognosis as women with unilateral breast cancer.

A 2018 study suggests that developing synchronous bilateral breast cancer has a worse prognosis and higher mortality rate than unilateral breast cancer as a result of having simultaneous cancers.

A Word From Verywell

Despite the limited and conflicting evidence on the treatment and prognosis for bilateral breast cancer, it is still a relatively uncommon diagnosis. If you believe you are at risk, speak with your doctor about screening for bilateral breast cancer. Early detection and tailored treatment will improve your prognosis.

11 Sources
Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
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By Rebecca Valdez, MS, RDN
Rebecca Valdez is a registered dietitian nutritionist and nutrition communications consultant, passionate about food justice, equity, and sustainability.