eP363 - Comparative Outcomes of Robotic vs. Open Proctectomy for Rectal Cancer at High Risk of Positive Circumferential Resection Margin

Purpose/Background: Robotic proctectomy (RP) is increasingly popular, but concerns persist regarding its effectiveness when compared with open proctectomy (OP) for locally advanced rectal cancer with a high risk of circumferential resection margin (CRM) involvement. This study compares surrogate cancer outcomes following RP vs. OP in this subpopulation.Methods/Interventions: A retrospective review of adult patients undergoing RP or OP for rectal cancer with curative intent was conducted between August 2012 and June 2023. Patients at high risk of CRM+ were selected based on the MRI-based definition from Mercury I and II trials: mesorectal fascia (MRF) distance of ≤ 1 mm or 3 out of 4 among “unsafe low rectal cancer plane (mrLRP)”(extension into the intersphincteric plane and/or ≤1 mm from the levator ani and/or with involvement of adjacent structures such as the external sphincter), anterior quadrant tumor invasion, tumor height < 4.0 cm from the anal verge, and mrEMVI+. Demographic, neoadjuvant treatments, tumor characteristics, and procedure-related data were analyzed. The RP group included converted cases in an intent-to-treat analysis. The primary outcome was the rate of involved (y)pCRM (≤1mm), while secondary outcomes included mesorectal grading and rates of distal margin involvement.Results/Outcomes: Out of 413 patients, 125 (30%) underwent OP vs. 288 (70%) RP. OP was significantly associated with a more significant proportion of cT4 tumors (39.3% in the OP group vs. 24.8% RP counterparts, P = 0.021), a more significant proportion of multivisceral/concomitant resections (40.8% versus 18.4%, P < 0.001) and less frequent Total Neoadjuvant Therapy use (17.1% in OP vs. 47.1% in RP, P = 0.001). Sphincter preservation was not significantly associated with rates of pCRM+ (8.6% vs. 12.8%, p= 0.163).RP was less commonly associated with pCRM+ (7.3% vs. 17.6%, P = 0.002). RP remained associated with decreased risk of pCRM+ after adjustment for cT stage, neoadjuvant therapy type, and multivisceral resection (OR 0.4, 95% CI 0.2 to 0.7, P =0.006).Mesorectal grading was comparable (incomplete mesorectum in 6% RP patients vs. 11.8% OP patients, p = 0.327). There were no cases of positive distal resection margin in either group.Conclusion/

Discussion: In a selected subpopulation of patients with locally advanced rectal cancer at high risk of pCRM+, RP is an effective approach and could be pursued when technically possible as an alternative to OP.