Hypothesis: Pelvic irradiation adversely affects anal sphincter function after proctectomy with coloanal anastomosis for low rectal and middle rectal (<10 cm from the anal verge) tumors. Design: Case-control study. Setting: Private, tertiary care referral center. Patients: Patients treated for low rectal adenocarcinoma between January 1, 1994, and October 31, 1999. Interventions: Anal manometric data were prospectively collected at the time of initial diagnosis and before ileostomy closure. Main Outcome Measures: Mean and maximum resting pressures (RPs) and squeeze pressures, threshold volume for sensation, and maximal tolerable volume. Results: Twenty-three patients in the surgery group and 19 in the chemoradiotherapy group were considered for analysis; 15 patients had preoperative radiotherapy and 4 had postoperative radiotherapy. At the time of ileostomy closure, RPs were significantly lower in the chemoradiotherapy group than in the surgery group (32.7 ± 17 vs 45.3 ± 18 mm Hg; P = .03). Squeeze pressures were not significantly different between the surgery and chemoradiotherapy groups (108.7 ± 56.7 vs 102.0 ± 52.6 mm Hg; P = .69). The ratios of postresection to preresection RPs were also significantly lower in the chemoradiotherapy group (0.49 ± 0.29) than in the surgery group (0.76 ± 0.22) (P = .005). Eight to 12 weeks after proctectomy with coloanal anastomosis, a 24% decrease in RP was noted in the surgery group. The addition of adjuvant pelvic irradiation decreased RP by another 27%. Conclusion: Adequate shielding of the anal sphincter should be performed for low rectal cancers whenever a sphincter-preserving procedure is considered.
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