The prognostic comparison among unilateral, bilateral, synchronous bilateral, and metachronous bilateral breast cancer: A meta‐analysis of studies from recent decade (2008‐2018)

BACKGROUND: The incidence of bilateral breast cancer (BBC) is increasing nowadays comprising 2%‐11% of all breast cancer (BC). According to the interval time between the first and second cancer, BBC could be divided into synchronous (SBBC) and metachronous (MBBC). However, this interval time is quite different across studies. It remains controversial whether the survival of BBC, SBBC, and MBBC is similar or worse compared to that of unilateral breast cancer (UBC), and whether the survival of SBBC is similar or worse compared to MBBC. To better understand the survival of UBC, BBC, SBBC, and MBBC and how the interval time would influence the prognosis of SBBC and MBBC, we performed this meta‐analysis on studies from recent 10 years (2008‐2018). METHODS: Databases of PubMed, Embase, and Web of Science were searched for relevant studies within recent 10 years. Hazard ratio (HR) was adopted to evaluate the difference of overall survival (OS) of UBC, BBC, SBBC, and MBBC. HR of OS comparisons were performed between BBC vs UBC, SBBC vs UBC, MBBC vs UBC, and SBBC vs MBBC with 3, 6, 12 months as the interval time, respectively. RESULTS: There were 15 studies of 72 302 UBC and 2912 BBC included in the meta‐analysis. The summary HR of OS comparison between BBC vs UBC was 1.68 (95% CI: 1.28‐2.20), SBBC vs UBC was 2.01 (95% CI: 1.14‐3.55), MBBC vs UBC was 3.22 (95% CI: 0.75‐13.78). When 3, 6, 12 months were used as the interval time, the summary HR of the OS comparison between of SBBC vs MBBC were 0.64 (95% CI: 0.44‐0.94), 1.17 (95% CI: 0.84‐1.63) and 1.45 (95% CI: 1.10‐1.92), respectively. CONCLUSION: BBC and SBBC showed worse prognosis in terms of OS compared to UBC while MBBC manifested similar or non‐superior survival as UBC. The OS comparison between SBBC and MBBC changed with different interval time used. The longer the interval time used, the worse the survival of SBBC. SBBC with interval of 3‐12 months between the two cancers had the worst prognosis. When 6 months was used to differentiate SBBC from MBBC, these two clinical entities showed similar OS.