Abstract Background: Minimally invasive breast surgery (MIS), including robotic and endoscopic mastectomy, has emerged as an alternative to conventional mastectomy (CM). However, robust prospective multicenter data comparing postoperative complications between MIS and CM have been lacking. Methods: The MARRES study (NCT004585074) is a multicenter, prospective cohort was conducted at 18 institutions in South Korea. Women with breast cancer or BRCA1/2 mutation carriers who underwent total, skin-sparing, or nipple-sparing mastectomy followed by immediate breast reconstruction were enrolled. The primary endpoint is the rate of postoperative complications of Clavien-Dindo grade ≥3 between the MIS and CM groups. A prespecified non-inferiority analysis with 1:1 matching was conducted according to the protocol. Results: A total of 1,914 women (2,140 cases) were enrolled. After applying eligibility criteria, 1,875 women were included in the analysis: 584 underwent MIS and 1,291 underwent CM. A total of 2,095 cases were analyzed, including 678 MIS cases (52 endoscopic and 626 robotic) and 1,417 CM cases. Among them, 615 cases (90.7%) in the MIS group and 1,358 cases (95.8%) in the CM group were performed for breast cancer, while the remaining cases involved benign disease or risk-reducing mastectomy in BRCA1/2 mutation carriers. Meanwhile, out of 1,875 women analyzed, 253 women (13.5%) underwent bilateral mastectomy, of which 116 women (45.8%) were diagnosed with bilateral breast cancer. Among women with unilateral breast cancer (N=1,616), those undergoing MIS were younger (median age, 46.0 vs. 47.0 years; p=0.0003) and more frequently aged 50 years (69.9% vs. 61.4%; p=0.0012). The MIS group had a higher prevalence of normal to mild breast ptosis (74.7% vs. 52.1%; p0.0001) and more frequently presented with early-stage disease, including preoperative diagnoses of DCIS (37.8% vs. 28.9%; p=0.001). In terms of tumor biology, the MIS group had higher rates of progesterone receptor positivity (72.8% vs. 66.6%; p=0.033), HER2 negativity (71.1% vs. 68.6%; p=0.002), and low Ki-67 expression (41.0% vs. 30.0%; p0.0001). Postoperative complications of Clavien-Dindo grade ≥3 were significantly lower in the MIS group than in the CM group (11.2% vs. 19.3%; p0.0001). Overall complication rates were also lower in the MIS group, with a higher proportion of patients experiencing no complications (Grade 0: 71.2% vs. 61.7%; p0.0001). Early postoperative complications (within 30 days) occurred less frequently in the MIS group than in the CM group (25.7% vs. 33.5%; p=0.0003). No significant difference was observed in late complications (beyond 30 days) between the two groups (10.8% vs. 13.5%; p=0.073). In the prespecified 1:1 matched analysis, Clavien-Dindo grade ≥3 complications were reported in 9.5% of MIS cases and 18.0% of CM cases, with an absolute difference of -8.5%, satisfying the predefined non-inferiority margin of 5%. Conclusion: This study indicates that MIS is not inferior to CM followed by reconstruction regarding severe complication rates; furthermore, MIS shows better outcomes of postoperative complication rates in the early postoperative period. MIS is safely administered in breast surgery in terms of surgical outcomes. Citation Format: H. Park, J. Ryu, J. Lee, H. Kim, S. Lee, H. Lee, Y. Kang, J. Kim, H. Choi, K. Kim, H. Shin, A. Han, Y. Lee, J. Cheun, M. Lee, J. You, S. Bae, Y. Kim, N. Kim. Postoperative Complication Rates of Minimally Invasive Mastectomy Compared to Conventional Mastectomy: A Prospective Multicenter Cohort Study (MARRES) abstract. In: Proceedings of the San Antonio Breast Cancer Symposium 2025; 2025 Dec 9-12; San Antonio, TX. Philadelphia (PA): AACR; Clin Cancer Res 2026;32(4 Suppl):Abstract nr RF1-01.
Park et al. (2026) studied this question.