Breast cancer typically develops in one breast (known as unilateral) but can also occur in both breasts (known as bilateral).
Researchers have consistently found that breast cancer is more common in the left breast than in the right, though they haven't pinpointed an exact reason behind this observation.
This article provides an overview of the evidence showing left-sided breast cancer to be more common.
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Left-Sided Breast Cancer
The breast is one of the body's major paired organs. This means it's an organ that comes in a pair, or set of two, like the lungs, kidneys, and ovaries.
Even though these organs are a pair, the tissue structures in each can be different. This is why many people get breast cancer in just one breast.
For reasons that are still being explored, data shows that breast cancer develops more frequently in the left breast than it does in the right breast.
Statistics
Decades of research have suggested that when people develop unilateral breast cancer, it is roughly 5% to 13% more likely to grow in the left breast than in the right breast.
While many studies have shown that survival outcomes are similar for both left- and right-sided breast cancer, some findings have suggested that left-sided breast cancer may lead to worse survival rates and higher chances of cancer recurring (coming back).
Moreover, left-sided breast cancer may pose additional heart-related risks for people undergoing radiation therapy, particularly for older patients, such as heart damage and worse survival outcomes. That's because of the location of left-side tumor—on the side where the heart is located.
Possible Theories
There's no apparent reason breast cancer occurs more often in the left breast, but experts have floated some theories. The main hypotheses include the following:
Still, none of the theories have been widely or broadly accepted. Older research has also pointed out that left-sided breast cancer also is more common in men, which would rule out some of the potential reasoning.
Some skin cancers (like melanoma) are also more common on the left side rather than the right. While this can be explained at least in part to sun exposure while driving (where the left side of the upper body is facing the window), researchers believe there may be other potential reasons related to asymmetry in the body.
Location of Breast Tumors
Breast cancer is a type of cancer that starts in the breast. It can begin in one breast develop in both breasts.
One-Sided
Cancer in one breast is more common. It's also linked to better prognosis and survival rates than having breast cancer in both breasts.
That said, it's still possible to have more than one tumor in the same breast. When it originates from the same tumor, it's known as multifocal breast cancer. When more than one tumor develops separately, it's multicentric breast cancer.
In Both Breasts
It's not common to develop breast cancer in both breasts, but it is possible and occurs in roughly 2.9% to 11% of breast cancer patients.
People who develop bilateral breast cancer are typically younger, have smaller breast tumors, and are diagnosed earlier.
Metastasis Characteristics
Metastatic breast cancer is breast cancer that spreads to other parts of the body, usually the bones, brain, lungs, or liver.
Experts believe it develops when breast cancer cells that stay in the body after initial treatment are triggered again. However, they're still investigating precisely why that sometimes happens.
Of the more than 290,000 people who are diagnosed with breast cancer each year in the United States, it's estimated that about 6% of patients have metastatic breast cancer.
Summary
For decades, data has shown that people are more likely to be diagnosed with breast cancer in the left breast than in the right breast. But despite these findings, researchers haven't determined exactly why this is the case.
Potential reasons behind this statistic include larger left breast size, more frequent self-screening of left breast, and right-side breastfeeding preferences. However, experts are still exploring these links, and more research is needed.
A Word From Verywell
Breast cancer is one of the most common cancers among women in the United States. This is why it's so important to stay on top of your routine self-exams and checkups with a healthcare provider if accessible to you.
As soon as you notice any abnormal changes to your breasts—particularly if you have a known family history of breast cancer—reach out to a healthcare provider as soon as possible to discuss concerns.
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