Pan B, et al. - Researchers analyzed relevant studies within recent 10 years (2008-2018) published in PubMed, Embase, and Web of Science, to better understand the survival of unilateral breast cancer (UBC), bilateral breast cancer (BBC), synchronous BBC (SBBC), and metachronous BBC (MBBC). They also investigated the impact of interval time on the prognosis of SBBC and MBBC. Overall 15 studies with 72,302 UBC and 2912 BBC were analyzed. As for overall survival (OS), a worse prognosis was demonstrated by BBC and SBBC, while MBBC showed similar or non-superior survival as UBC. With different interval time used, change in the OS comparison between SBBC and MBBC was reported. According to findings, the longer the interval time used, the worse the survival of SBBC. The worst prognosis was observed for SBBC with interval of 3-12 months between the two cancers. Use of 6 months to distinguish SBBC from MBBC was associated with similar OS shown by these two clinical entities.