The Effect of Laterality and Primary Tumor Site on Cancer-Specific Mortality in Breast Cancer: A SEER Population-Based Study

Background: Reduced overall survival has been observed in patients with left-sided versus right-sided breast cancer due to cardiac toxicity after radiotherapy. However, the effect of laterality and primary tumor site on breast cancer-specific mortality (BCSM) remains unclear. Patients and Methods: We analyzed data from 305,443 women ages 20- to 79-years-old diagnosed with breast cancer between 1990 and 2009. The data were obtained from the population-based Surveillance, Epidemiology, and End Results (SEER) program of the U. S. National Cancer Institute. The survival outcomes with regard to laterality and primary tumor site were compared using univariate and multivariate (Cox proportional hazards regression model) methods. Results: In the multivariate analysis, BCSM was affected by the primary tumor site (P<0.0001) but not laterality (P = 0.331). The combined effect was piecewise: using the left upper-outer quadrant as the reference, the BCSM hazard ratio (HR) was not significant in the right upper quadrant (P = 0.755) and the right central portion (P = 0.329). The BCSM HR was slightly increased in the left central portion as well as the left and right lower-outer quadrants (HRs from 1.136 to 1.145; P<0.0001). The BCSM HR was significantly increased in the upper-inner and lower-inner quadrants (HRs from 1.242 to 1.372; P<0.0001) on both sides. Laterality only impacted BCSM in patients with breast cancer located in the central portion (HR, 1.100; P = 0.013, using the right side as the reference). Conclusion: Patients with tumors in the upper-outer quadrant of both sides and the right central portion have a better prognosis than patients with tumors at other locations. Laterality is not regarded as a prognostic factor in breast cancer.