Anatomic distribution of postoperative recurrence and radiotherapy target volume optimization in rectal cancer: A...

To characterize the anatomic pattern of postoperative recurrence in rectal cancer and determine whether irradiation of the distal mesorectum is necessary during preoperative radiotherapy, thereby informing safe target-volume reduction.

We retrospectively analyzed 2,530 patients with rectal cancer who underwent total mesorectal excision (TME) at our institution between 2006 and 2024, with protocolized postoperative follow-up. Survival was evaluated using Kaplan-Meier methods, and independent predictors were identified with Cox proportional hazards models. Recurrence topography was visualized using a registration-based, voxel-wise probabilistic atlas. We quantified the vertical distance from the caudal edge of each recurrence to the coccygeal tip plane. In sphincter-preserved patients, bowel function was assessed with the Low Anterior Resection Syndrome (LARS) score.

In the overall cohort, pelvic-floor recurrence occurred in 0.75%; no pelvic-floor recurrence was observed in the high rectal subgroup. Pelvic-floor recurrence was rare, and no statistically significant difference in survival outcomes was observed; however, this comparison was limited by the small number of events. Independent predictors of recurrence included family history of non-colorectal malignancy, poor differentiation, surgical approach, pT4 stage, pN positivity, and M1 disease. Compared with open surgery, laparoscopic surgery was associated with a lower risk of recurrence. After excluding pelvic-floor, anastomotic, and visceral recurrences, the most caudal in-field recurrence in mid/high rectal cancer remained above the coccygeal tip (mid rectum: 56.88 mm [45.21-68.56]; high rectum: 64.18 mm [45.35-83.01]). Long-term bowel function was favorable: 125 patients (88.65%) had LARS scores ≤ 20, 6 (4.26%) had scores of 21-29, and 10 (7.09%) had scores ≥ 30.

Pelvic-floor recurrence after rectal cancer surgery was rare in this cohort. The small number of events precluded a definitive survival analysis, and no statistically significant association with overall survival was detected (HR 0.986; 95% CI 0.139-7.018). In mid/high rectal cancer, the caudal boundary of in-field recurrence lies cranial to the coccygeal tip, suggesting limited benefit from routine distal mesorectal coverage. These anatomic recurrence data provide exploratory support for selective distal CTV de-escalation in mid/high rectal cancer, but prospective validation with treatment-plan-based dosimetric assessment is required before clinical implementation.