Anorectal substructure dosimetry and low anterior resection syndrome after neoadjuvant radiotherapy for rectal cancer: an exploratory analysis

Abstract Low anterior resection syndrome (LARS) is a frequent complication after sphincterpreserving surgery for rectal cancer, often exacerbated by neoadjuvant radiotherapy. While radiotherapy is a recognized risk factor, the specific contribution of dose to anorectal substructures remains unclear. This retrospective study evaluated whether dosimetric parameters of key anorectal components are associated with postoperative dysfunction. We analyzed 25 patients treated with long-course neoadjuvant radiotherapy followed by surgery, with available dose–volume histogram data and LARS assessment performed at least 6 months after restoration of bowel continuity. The primary endpoint was dichotomized LARS (no LARS vs any LARS). Dose metrics for the internal anal sphincter (IAS), puborectalis muscle (PRM), external anal sphincter (EAS), levator ani muscle (LAM), and a composite IAS–PRM structure were analyzed. High-dose parameters showed the strongest associations with LARS. Among single structures, IAS V50, IAS maximum dose, PRM V50, and IAS V45 were the most significant predictors. The composite metric (IAS+PRM V50) demonstrated a strongassociation with dysfunction (median 0 vs 77.5; p = 0.000062; ρ = 0.82). Lower-dose and mean-dose metrics were less informative. EAS and LAM parameters showed weaker but still significant correlations. These findings suggest that LARS is more strongly associated with high-dose exposure to structures involved in passive continence and anorectal support, particularly the IAS–PRM complex. This supports a functional-unit approach to anorectal toxicity. Although limited by small sample size and retrospective design, the results identify clinically relevant candidate dosimetric parameters. If validated, reducing high-dose exposure to the IAS–PRM complex may improve functionaloutcomes without compromising oncological treatment.