Results of endoscopic intermuscular dissection for deep submucosal invasive rectal cancer: a three-year follow-up study

Background: Endoscopic intermuscular dissection (EID) is a promising new technique for managing rectal deep submucosal invasive cancer (D-SMIC), but long-term outcome data are currently lacking. Objective: This multicentre study evaluated the three-year oncological outcomes of EID, focusing specifically on patients with rectal D-SMIC who underwent active surveillance following the procedure. Design: Data from consecutive, prospectively recorded EID procedures for suspected rectal D-SMIC - based on optical diagnosis - performed at two academic centres between 2019 and 2023 were analysed. D-SMIC was defined as submucosal invasion of sm2-sm3 depth. Histological risk factors included poorly differentiated tumours (G3), lymphovascular invasion, high-grade tumour budding and positive or indeterminate resection margins (R1/Rx). Study outcomes included three-year rates of locoregional recurrence (intramural and nodal), distant recurrence (metastatic disease), non-salvageable recurrence, cancer-specific mortality and secondary rectal surgery. Cumulative incidence was estimated using the Aalen-Johansen method. Results: Among the 188 included cases, EID achieved an en bloc resection rate of 94.1% and R0 resection rate of 82.5%, respectively. Of the 177 procedures that were completed, 16% showed non-invasive histology (low-grade dysplasia/high-grade dysplasia; 20/177=11%) or superficial submucosal invasive cancer (sm1, 9/177=5%), and 31% (54/177) showed deeper (≥pT2) invasion. The remaining 94 D-SMIC cases (53%) represented the main target group. Of these, 37% (n=35) were classified as low risk (no histological risk factors), 34% (n=32) as intermediate risk (one risk factor) and 29% (n=27) as high risk (≥2 risk factors). Active surveillance was initiated in all low-risk patients, in 72% of the intermediate-risk cases and in 22% of the high-risk group. The remaining patients underwent completion surgery or adjuvant chemoradiotherapy. At three years, locoregional recurrence occurred in 7% (1/35, 95% CI 1% to 28%) of low-risk and 13% (2/15, 95% CI 2% to 35%) of intermediate-risk patients managed with active surveillance. All were successfully salvaged. Among the six high-risk patients under surveillance, locoregional recurrence was seen in two. No distant recurrences or cancer-specific deaths occurred in any D-SMIC group. Secondary rectal surgery was finally performed in 5.3%, 25.0% and 59.6% of the low, intermediate and high-risk groups, respectively. Conclusion: Despite the challenges associated with accurate preoperative staging, EID followed by active surveillance may offer a viable alternative to radical surgery for patients with low- and intermediate-risk rectal D-SMIC, avoiding rectal surgery in most cases while maintaining oncological safety.