Location of Tumors on the Breasts

Left Breast More Common

Among California women, cancer is slightly more common in the left breast than in the right. Scientists have not been able to figure out why this is so, but it is true for women from all ethnic groups and all age groups. It is also true regardless of how far the disease has progressed when it is detected. Across the nation, tumors are also slightly more common in the left breast.

Side of the Body Where California Women Had Breast Tumors, 1988–1999

Figure 7

Figure 8

Upper, Outer Quadrant Most Likely

The most common place on the breast for a California woman to have a tumor is the upper, outer quadrant, where 36 percent of tumors are found. The percentage of tumors found in other locations on the breast are given in the figures below and on the next page. The percentages are very similar for women from all ethnic groups and all ages.

Figure 9

Figure 10

How This Research Was Done

Researchers used information about breast tumors that physicians and other health professionals reported to the California Cancer Registry during the years 1988–1999. Information in this section comes from Chapter 9 of Breast Cancer in California, 2003, “Laterality, Detailed Site, and Histology of Female Breast Cancer, California, 1988–1999,” by Sandy L. Kwong, M.P.H.