Supplementary Material for: Breast Conservation Therapy vs. Mastectomy Survival in Young Breast Cancer Patients: Systematic Review & Meta-Analysis

Background: While young breast cancer patients exhibit lower overall survival (OS) than older groups, it remains unclear whether OS differs between breast-conserving therapy (BCT) and mastectomy specifically in this younger population. Although BCT carries a higher recurrence risk than mastectomy in young patients, OS appears similar across all ages. This meta-analysis therefore directly compares OS and breast cancer specific survival (BCSS) in young patients treated with BCT versus mastectomy. Methods: Our review included 6 studies that compared OS between BCT and mastectomy in patients aged 40 years or younger diagnosed with stage I-III breast cancer. The endpoint was OS/BCSS, and only studies presenting hazard ratios (HRs) were included in the analysis. Meta-analyses used inverse-variance random effects (REML with Hartung–Knapp CIs); heterogeneity was assessed with Q and I². Leave-one-out, fixed-effect, and DerSimonian–Laird models tested robustness; Egger’s test explored small-study effects. Analyses were performed in RevMan 5.3 and Stata 17, with p<0.05 or 95% CIs excluding 1.00 considered significant. Results: OS showed no difference between BCT and mastectomy (pooled HR =0.99, 95% CI 0.88–1.10, I²= 54%). BCSS favored BCT with a small but significant reduction in mortality (HR 0.92, 95% CI 0.86–0.98, I²=0%). Subgroup analyses of OS suggested stage dependence: benefit with T1–2 (HR 0.93, 0.87–0.99) and N0–1 (HR 0.93, 0.87–0.99), but no advantage in T1–3 or N2–3 disease (both HR 1.19, 0.99–1.44). Leave-one-out and model checks did not change inference; unadjusted estimates suggested benefit but this attenuated after adjustment. Conclusion: In women ≤40 years with stage I–III breast cancer, BCT with radiotherapy achieves OS comparable to mastectomy and may yield a modest BCSS benefit, particularly in lower-burden disease (T1-2 or N0–1). Given the observational nature of the evidence and residual heterogeneity, results should be interpreted cautiously and individualized clinical decision-making is recommended; randomized or prospectively adjusted data are still needed.