Nine retrospective non-randomised studies (n=3,739, SSM 1,104 and NSSM 2,635) were included in the review. The Maxwell test statistic was not significant, which indicated high levels of agreement between reviewers and the McNemar test result showed an even spread of agreement. Median follow-up ranged from 15.6 to 67 months. Seven studies attained a score of six or more stars on the Newcastle-Ottawa scoring system, which indicated higher quality studies. In studies that reported on tumour grade, SSM was performed less often for patients with higher tumour grades than NSSM.
There were no significant differences observed between SSM and NSSM groups in local recurrence (OR 1.25, 95% % CI 0.81 to 1.94; seven studies, n=3,436). There were no significant differences observed in postoperative complications between SSM and NSSM groups (OR 0.81, 95% CI 0.57 to 1.16; three studies, n=789).
There were significantly fewer incidences of distant relapse observed for patients who received SSM compared to those who received NSSM (OR 0.67, 95% CI 0.48 to 0.94; five studies, n=2,122). There were no significant differences observed when patients who received nipple-sparing SSM were compared to those who received NSSM (OR 0.83, 95% CI 0.45 to 1.52; two studies, n=401).
There were no significant differences between SSM and NSSM groups for disease stage and axillary lymph node status. Meta-regression analyses showed that study quality scores, differences in presentation stage and proportion of in situ disease did not influence local recurrence or distant relapse.
The was no statistically significant heterogeneity reported across the studies for any outcome. There was no evidence of publication bias from the funnel plots.