UC Irvine

More breast cancers are diagnosed in the left breast than the right. The ratio (l/r) is called the laterality ratio. We analyzed 1.2 million cases of breast cancer diagnosed in the US between 1973 and 2010 and recorded by the Surveillance, Epidemiology, and End Results (SEER) program. We found that the laterality ratio depends upon the country of birth, but not race of the patient. We identified five countries of birth that had p-values larger than 0.995, while we expected to see less than 1. Those born in Japan (l/r=1.14, p=0.997), the Ryukyu Islands (l/r= 2.6, p=0.998), Laos (l/r=1.62, p=0.9999) and Algeria (l/r=2.1 p=0.9959) had significantly larger laterality ratios compared to the overall SEER population (l/r=1.04). Those born in Poland (l/r=0.92, p=0.997) had a laterality ratio significantly less than expected. We compared the laterality ratio calculated for tumors occurring in each quadrant of the breast for two immigrant populations: those born in Japan and those born in Poland. We found the only significant difference was in the laterality ratio of the upper outer quadrant. Thus, the birthplace effect appears to only occur in the upper outer quadrant of the breast. Finally, we found a small, but statistically significant, increase in the breast cancer laterality ratio with age, and decrease with birth year and year of diagnosis.