Why Is Left-Sided Breast Cancer More Common?

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.

Female doctor with patient's test results

simon2579 / Getty Images

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.

Frequently Asked Questions

  • Is left or right breast cancer worse?

    It depends. While some studies have shown that left and right breast cancer have similar outcomes, other research has pointed to left-sided breast cancer as being more aggressive.

  • Do a lot of people get breast cancer in both breasts?

    Data suggests that cancer in both breasts occurs in 2.9% of breast cancer patients.

  • What causes breast cancer?

    Experts don't know exactly what causes breast cancer, but many different risk factors can increase its chances of developing. These can include factors like having a family history of the disease, age, and reproductive history.

  • Does breast cancer spread aggressively?

    Some cases of breast cancer spread faster and more aggressively than others. This growth rate likely depends on several factors, such as the cancer subtype, stage, grade, whether it's genetic, and the patient's age.

27 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.
  1. Narod SA. Bilateral breast cancers. Nat Rev Clin Oncol. 2014;11(3):157-166. doi:10.1038/nrclinonc.2014.3

  2. Busk T, Clemmesen J. The frequencies of left- and right-sided breast cancer. Br J Cancer. 1947 Dec;1(4):345-51. doi: 10.1038/bjc.1947.31

  3. Sughrue T, Brody JP. Breast tumor laterality in the United States depends upon the country of birth, but not race. PLoS ONE. 2014;9(8): e103313. doi:10.1371/journal.pone.0103313

  4. Al Saad, S., Al Shenawi, H., Almarabheh, A. et al. Is laterality in breast cancer still worth studying? Local experience in Bahrain. BMC Cancer. 2022;22(968). doi:10.1186/s12885-022-10063-y

  5. Zeeneldin AA, Ramadan M, Elmashad N, Fakhr I, Diaa A, Mosaad E. Breast cancer laterality among Egyptian patients and its association with treatments and survival. J Egypt Natl Canc Inst. 2013 Dec;25(4):199-207. doi:10.1016/j.jnci.2013.09.003

  6. Barbara RC, Piotr R, Kornel B, Elżbieta Z, Danuta R, Eduardo N. Divergent impact of breast cancer laterality on clinicopathological, angiogenic, and hemostatic profiles: A potential role of tumor localization in future outcomes. J Clin Med. 2020 Jun 2;9(6):1708. doi:10.3390/jcm9061708

  7. Haque R, Yood MU, Geiger AM, Kamineni A, Avila CC, Shi J, Silliman RA, Quinn VP. Long-term safety of radiotherapy and breast cancer laterality in older survivors. Cancer Epidemiol Biomarkers Prev. 2011 Oct;20(10):2120-6. doi:10.1158/1055-9965.EPI-11-0348

  8. Correa CR, Litt HI, Hwang WT, Ferrari VA, Solin LJ, Harris EE. Coronary artery findings after left-sided compared with right-sided radiation treatment for early-stage breast cancer. J Clin Oncol. 2007 Jul 20;25(21):3031-7. doi: 10.1200/JCO.2006.08.6595.

  9. Abdou Y, Gupta M, Asaoka M, et al. Left sided breast cancer is associated with aggressive biology and worse outcomes than right sided breast cancer. Sci Rep. 2022;12(13377). doi:10.1038/s41598-022-16749-4

  10. Centers for Disease Control and Prevention. What are the risk factors for breast cancer?.

  11. Islami F, Liu Y, Jemal A, et al. Breastfeeding and breast cancer risk by receptor status-a systematic review and meta-analysisAnn Oncol.2015;26(12):2398-407. doi:10.1093/annonc/mdv379

  12. Amer MH. Genetic factors and breast cancer laterality. Cancer Manag Res. 2014 Apr 16;6:191-203. doi:10.2147/CMAR.S60006

  13. Ekbom A, Adami HO, Trichopoulos D, Lambe M, Hsieh CC, Pontén J. Epidemiologic correlates of breast cancer laterality (Sweden). Cancer Causes Control. 1994 Nov;5(6):510-6. doi:10.1007/BF01831378

  14. Paulson KG, Iyer JG, Nghiem P. Asymmetric lateral distribution of melanoma and Merkel cell carcinoma in the United States. J Am Acad Dermatol. 2011 Jul;65(1):35-9. doi:10.1016/j.jaad.2010.05.026

  15. Boxer Wachler BS. Assessment of levels of ultraviolet A light protection in automobile windshields and side windows. JAMA Ophthalmol. 2016;134(7):772–775. doi:10.1001/jamaophthalmol.2016.1139

  16. American Cancer Society. What is breast cancer?.

  17. Carmichael AR, Bendall S, Lockerbie L, Prescott R, Bates T. The long-term outcome of synchronous bilateral breast cancer is worse than metachronous or unilateral tumours. Eur J Surg Oncol. 2012 Jun;28(4):388-91. doi:10.1053/ejso.2002.1266

  18. National Cancer Institute. Definition of multifocal breast cancer.

  19. Utaalioglu BO, Bilici A, Kefeli U, Şeker M, Oncel M, Gezen C, Gumus M, Demirelli F. The importance of multifocal/multicentric tumor on the disease-free survival of breast cancer patients: single center experience. Am J Clin Oncol. 35(6):580-6. doi:10.1097/COC.0b013e31822d9cd6

  20. Jobsen JJ, van der Palen J, Ong F, Riemersma S, Struikmans H. Bilateral breast cancer, synchronous and metachronous; differences and outcome. Breast Cancer Res Treat. 2015;153(2):277-283. doi:10.1007/s10549-015-3538-5

  21. Sakai T, Ozkurt E, DeSantis S, et al. National trends of synchronous bilateral breast cancer incidence in the United States. Breast Cancer Res Treat. 2019;178(1):161-167. doi:10.1007/s10549-019-05363-0

  22. Kheirelseid EAH, Jumustafa H, Miller N, et al. Bilateral breast cancer: analysis of incidence, outcome, survival and disease characteristics. Breast Cancer Res Treat. 2011;126(1):131-140. doi:10.1007/s10549-010-1057-y

  23. American Society of Clinical Oncology. Breast cancer - metastatic: introduction.

  24. American Cancer Society. What is cancer recurrence?

  25. American Society of Clinical Oncology. Breast cancer - metastatic: statistics.

  26. American Cancer Society. What causes breast cancer?.

  27. Nakashima K, Uematsu T, Takahashi K, et al. Does breast cancer growth rate really depend on tumor subtype? Measurement of tumor doubling time using serial ultrasonography between diagnosis and surgeryBreast Cancer. 2019;26(2):206-214. doi:10.1007/s12282-018-0914-0

By Cristina Mutchler
Cristina Mutchler is an award-winning journalist with more than a decade of experience in national media, specializing in health and wellness content.