KEY POINTS
- The exploratory retrospective analysis included 25 patients treated with long-course neoadjuvant RT followed by sphincter-preserving surgery for rectal cancer. LARS was assessed at least 6 months after restoration of bowel continuity.
- Investigators separately contoured the internal anal sphincter (IAS), external anal sphincter, puborectalis muscle (PRM) and levator ani, and additionally created a combined IAS+PRM structure intended to approximate a functional continence unit.
- 15/25 patients (60%) had some degree of LARS, while 10 had none. Prescribed neoadjuvant doses were 45 Gy in 3 patients, 50.4 Gy in 11 and 54 Gy in 11.
- Among individual structures, the strongest association was IAS V50: median 0% in patients without LARS versus 80% in those with LARS, with ρ=0.88 and p<0.000001. IAS maximum dose and PRM V50 were also strongly associated with dysfunction.
- The composite IAS+PRM V50 also separated the groups markedly: median 0% versus 80%, with ρ=0.824 and p<0.000001. Lower-dose and most mean-dose metrics showed considerably weaker associations.
- The pattern supports a possible functional-unit concept, in which injury to passive continence and anorectal support structures may matter more than treating the anal canal as a single OAR. The puborectalis may therefore be an under-recognized structure in rectal RT planning.
- These results should not be converted into a clinical V50 constraint. The cohort was small, numerous dosimetric variables were tested without multiplicity correction, major surgical confounders were incompletely modeled, and no manometric or other objective functional validation was available.
CLINICAL TAKEAWAY
The study raises a practical planning hypothesis: the internal anal sphincter and puborectalis may deserve explicit attention when optimizing neoadjuvant rectal RT. The very strong V50 associations are intriguing, but they need independent validation before the IAS–PRM complex is treated as a formal OAR or given a dose constraint.