Left-sided breast cancer is more common than right-sided breast cancer, although researchers are currently unsure of the reasons behind this.

According to research, breast cancer is more common in the left breast than in the right breast.

The Surveillance, Epidemiology, and End Results Program performed an analysis that consisted of 881,320 people. The researchers found that left-sided tumors were observable in 50.8% of people and right-sided tumors were observable in 49.2% of individuals.

Although there are consistent reports of left-sided breast cancer occurring more than right-sided breast cancer, there is limited research into why this occurs.

This article looks at the possible theories for why left-sided breast cancer is more common than right-sided breast cancer. It also examines whether left or right-sided breast cancer affects the outcome.

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Experts are still not sure why left-sided breast cancer appears to be more common. Over the years, researchers have made various hypotheses to try to explain it, such as:

  • the larger size of the left breast
  • early detection of tumors in those who are righthanded
  • breastfeeding from the right breast more often than the left

Both sides of the breast share the same genetic and environmental risk factors for the development of breast cancer. However, the two sides of the breast may have differences in structures, including:

  • tissue structure
  • blood vessel supplies
  • lymphatic drainage

This may create differences in each side of the breast, which may play a role in which side breast cancer occurs.

Other factors that research has reported as impacting which side breast cancer occurs include:

  • the ratio of left-sided breast cancer to right-sided cancer may vary with age
  • that different areas of the breast may have different ratios relating to which side the cancer occurs in
  • there may be a link between inherited genes and which side of the breast the disease occurs in

It is important to remember that none of the theories below currently have enough research or scientific evidence to support them.

Size of the left breast

In general, the left breast may be larger than the right. Some research suggests there may be a link between breast size and breast cancer.

However, a 2019 review found no clear evidence directly linking breast size and breast cancer risk.

Early detection as a result of being righthanded

According to a 2022 article, another theory is that righthanded people may detect signs of breast cancer in the left breast more easily than the right, leading to earlier or more frequent diagnosis.

Being righthanded is more common than being lefthanded. Roughly 90% of people opt to use their right hand to carry out most tasks, and 10% prefer to use their left hand.

Breastfeeding using the right breast

The 2022 article also suggests another theory that people who breastfeed may feed from the right side of the breast more than the left.

Breastfeeding may lower the risk of breast cancer. Researchers state that for every 12 months of breastfeeding, the risk of breast cancer reduces by 4.3%.

Some research suggests that the side of the breast that cancer begins in may affect the outlook of breast cancer.

A 2022 large-scale analysis compared the type of breast cancer tumors and outlook in people with left-sided and right-sided breast cancer.

Researchers found that the following features occurred more in left-sided tumors than right-sided tumors:

  • poorly differentiated and undifferentiated tumors
  • hormone-negative and HER2-positive tumors
  • a higher number of grade 2 and 3 tumors

After taking into account the stage, grade, and hormone receptor status of the breast cancer, left-sided breast cancer showed more negative outcomes in some study participants.

In other participants, the research found no significant differences in outcome.

The study concluded that left-sided tumors may have a lower response to initial treatment with chemotherapy and may have a slightly worse outlook in the long term in comparison with right-sided tumors.

However, researchers still require further evidence to support these findings.

Other research shows different findings. A 2020 study of 228 people with breast cancer between 1999 and 2020 found that right-sided breast cancer had:

  • a higher rate of family history of breast cancer
  • more locally advanced and metastatic breast cancer
  • reduced 5-year survival rates in relation to the size and stage of breast cancer

The study found that left-sided breast cancer has associations with higher stages of early tumors.

The outlook of breast cancer may depend on many factors, including the stage, grade, and type of the disease. It may also depend on a person’s age at diagnosis and whether the cancer is recurring.

Left-sided breast cancer appears to be more common than right-sided breast cancer. However, the reasons for this are not clear and research is limited.

Some theories include the left side of the breast being larger, right-handed people checking their left breast more easily, and individuals preferring to breastfeed from the right breast. However, there is currently not enough evidence to support any of these theories.

Some research suggests that the side that breast cancer occurs in may affect the outlook. The size, grade, and type of tumor are also important factors in the disease’s outlook.