Why Is Breast Cancer More Common in the Left Breast?

A large new study found that breast cancer occurs slightly more often in the left breast. Published on 4 August 2022 in Scientific Reports, the study also suggests that left-sided breast tumours may have more aggressive biology than those arising in the right breast.

Many explanations have been proposed, including the left breast being larger, right-handed people finding it easier to detect signs of cancer in the left breast, and breastfeeding people potentially using the right breast more often. However, there is currently no strong evidence supporting any of these theories.

The study analysed 881 thousand patients with breast cancer in the SEER database, which contains US cancer statistics. Researchers found that tumours were left-sided in 50.8% of patients and right-sided in 49.2%.

Although reports consistently show that left-sided breast cancer occurs more often than right-sided breast cancer, research into why this happens is limited.

This article examines possible explanations for why left-sided breast cancer is more common. It also considers whether having cancer in the left or right breast affects disease outcomes differently.

Several hypotheses may explain why left-sided breast cancer appears more common, including:

• the larger size of the left breast
• earlier tumour detection in right-handed people
• more frequent use of the right breast for breastfeeding, which may be the most likely explanation

Both breasts share the same genetic and environmental risk factors for breast cancer. However, structural differences between the two sides may include:

• tissue structure
• blood vessel branching
• lymphatic drainage

Possible Theories

It is important to remember that none of the following theories currently has sufficient research or scientific evidence to support it.

Left Breast Size

The left breast may generally be larger than the right. Some research suggests a possible relationship between breast size and breast cancer.

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

Earlier Detection as a Result of Right-Handedness

According to a 2022 article, another theory is that right-handed people may find it easier to detect breast cancer signs in the left breast than the right, leading to earlier or more frequent diagnosis.

Right-handedness is more common than left-handedness. Roughly 90% of people prefer to use their right hand for most tasks, while 10% prefer their left.

Breastfeeding From the Right Breast

A 2022 article proposed another theory: people who breastfeed may use the right breast more often than the left.

Breastfeeding reduces breast cancer risk. Researchers report a 4.3% reduction in risk for every 12 months of breastfeeding.

Does Left-Sided Breast Cancer Have a Worse Prognosis Than Right-Sided Breast Cancer?

Some research suggests that the side in which breast cancer starts may affect the course of the disease.

A large 2022 analysis compared breast tumour types and their effects on disease outcomes in people with left- and right-sided breast cancer.

The researchers found that the following characteristics were more common in left-sided than right-sided tumours:

• Poorly differentiated and undifferentiated tumours,
• Hormone-negative and HER2-positive tumours
• More grade 2 and 3 tumours

Considering breast cancer stage, grade and hormone receptor status, some studies showed worse outcomes for left-sided breast cancer.

The Scientific Reports study discussed here concluded that left-sided tumours may respond less well to initial chemotherapy and have a slightly worse long-term prognosis than right-sided tumours.

However, the researchers still need more evidence to support these findings.

Earlier, smaller studies have also reached different conclusions. A 2020 study of 228 people who had breast cancer between 1999 and 2020 found that right-sided breast cancer was associated with a higher rate of family history of breast cancer and more advanced-stage disease.

Summary

Left-sided breast cancer appears more common than right-sided breast cancer. The reasons are unclear, and research is limited. Possible explanations include the left breast being larger, right-handed people finding it easier to examine the left breast, and a preference for breastfeeding from the right breast. However, there is currently insufficient evidence to support any of these theories.

Some research suggests that the side in which breast cancer develops may affect the disease course. Tumour size, grade and subtype are also established, important factors affecting prognosis.