Why Is Breast Cancer More Likely to Occur on the Left Side of the Body Than on the Right?

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Breast cancer continues to hold mysteries that scientists don’t understand. And while there may be more important mysteries out there than why breast cancer is more likely to occur on the left side than the right side, we likely won’t know for sure how vital the answer to this question is until we find it.

For now, what we know is that breast cancer is more likely to occur in the left breast than in the right, accounting for about 53 percent of unilateral breast cancer cases. An investigation into data from the Icelandic Cancer Registry found that, out of 2011 unilateral breast cancer cases occurring from the years 1948 to 1987, 1139 of those cases were in the left breast, an excess of 13%. Similarly, an analysis of 18 U.S. studies found that breast cancer occurred in the left breast 4% more often than the right.

We don’t know why this happens, but we do have some plausible theories.

It’s entirely possible that the increased incidence of breast cancer on the left side of the body is a coincidence. But then again, maybe it’s not. One thought is that the left breast tends to be larger than the right breast, meaning there is more tissue on that side to develop breast cancer in. One study showed a slight connection between the chances of cancer and left- or right-handedness. These and potentially dozens of other factors could be at play in determining which side of the body breast cancer originates in.

This information doesn’t mean much to most of us right now, but further research could use it to find out something new about what causes breast cancer. Knowing the increased likelihood of cancer on the left side could pave the way for new theories and discoveries. For example, maybe cancer can be caused by some sort of activity that individuals always do in the same way, using the same side of the body. We have only to make the connection between that activity and this data, and we could begin researching the connection and potentially preventing more cancer cases.

It’s also interesting, and potentially important, to note that breast cancer that occurs on the right side of the body is probably more likely to be genetic rather than environmentally influenced. The data study in Iceland showed that people with right-side primary breast cancer more often had a first-degree relative with breast cancer as well.

This may not seem like especially relevant information at first, but there are some potential situations in which it could be useful. If you’re diagnosed with breast cancer in your right breast, others in your family may be more likely to develop cancer as well and could be alerted to this fact before their cancer gets too far along.

Similarly, imagine that you’ve tested positive for the BRCA-1 gene mutation, and your doctor finds a lump in your right breast, where it is more likely to be gene-related. You now have one more reason to get the lump biopsied to ensure that it’s not cancer. Your doctor could give you a better understanding of your likelihood of this being cancer, partially based on which breast the lump occurs in.

Could there be other characteristics of right-side or left-side breast cancer that could lead us to more accurate diagnostic tests, better treatments, or new ways to prevent cancer? Only time and research will tell.

Want to learn more? Read on for 10 more things you didn’t know about your breasts.

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Elizabeth Nelson is a wordsmith, an alumna of Aquinas College in Grand Rapids, a four-leaf-clover finder, and a grammar connoisseur. She has lived in west Michigan since age four but loves to travel to new (and old) places. In her free time, she. . . wait, what’s free time?
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