Left sided breast cancer is associated with aggressive biology and worse outcomes than right sided breast cancer.
Breast cancer is more common on the left side than the right side.
Analysis of survival outcomes and clinical characteristics of 881,320 patients with breast cancer recorded by the Surveillance, Epidemiology, and End Results (SEER) program.
Our findings suggest that left sided breast cancer is associated with aggressive biology and worse outcomes compared to right sided breast cancer.
Women are slightly more likely to be diagnosed with breast cancer in the left breast than in the right, with the ratio of left to right side tumors ranging between 1.05 to 1.2.
SEER database, 881,320 patients:Left sided and right sided tumors were seen in 50.8% and 49.2% of patients respectively.
Poorly differentiated and undifferentiated tumors, as well as hormone negative and HER2 positive tumors were significantly more prominent on the left side compared to the right side.
In the SEER population, left sided breast cancer demonstrated poorer OS compared to right sided breast cancer.
Left sided tumors continued to show poorer outcomes even after adjusting for grade, stage, and hormone receptor status.
Tumors that are ER and/or PR negative appear to have worse outcomes on the left side.
Cell proliferation and cell-cycle related gene sets including G2M checkpoint, Mitotic spindle, E2F targets and MYC targets, were significantly enriched in left sided tumors.
Of the 865 genes that are significantly highly expressed on the left side, included BRCA1, BRCA2, BRIP1, CHEK2, FANCC, PALB2, TP53 and MSH6.
Of 155 patients that underwent neoadjuvant chemotherapy, there was a lower rate of pathologic complete response (pCR) in response to neoadjuvant chemotherapy with left sided tumors as compared to right sided tumors (15.4% versus 29.9% respectively.
SEER analysis confirms the persistence of left breast cancer predominance; with a slightly higher prevalence of ER/PR negative and HER-2 positive tumors on the left.
The significant differences seen in clinicopathological variables by laterality appear to have no clinical relevance and are largely driven by the large sample sizes.
There is a slight survival disadvantage to left sided breast cancer compared to right side with a hazard ratio of 1.05.
In another SEER analysis, Bao et al. showed slightly worse breast cancer specific mortality in the left central portion of the breast.
Most other previous studies showed no difference in survival outcomes by breast cancer laterality.
A single study of 5,459 breast cancer patients in Egypt which showed significantly worse survival in left sided breast cancer compared to right.
That being said, patients with left sided breast cancer treated with radiation therapy were at greater risk of cardiovascular complications, and historically had worse outcomes.
Several cell proliferation gene sets are found to be significantly enriched on the left side, indicating distinct underlying biologic characteristics between the two sides.
An institutional analysis of the neoadjuvant cohort, where left sided tumors were less likely to achieve a pCR compared to right sided tumors.
Left sided breast tumors have a more proliferative genomic profile, lower responses to neoadjuvant chemotherapy, and slightly worse long-term outcomes compared to right sided breast cancer.