Histologic analysis of the irradiated anal sphincter

There is accumulating evidence, both quantitative and qualitative, that pelvic irradiation adversely affects anorectal function. However, histologic evidence of sphincter injury has not been demonstrated. This study was designed to perform histologic assessment of collagen deposition and nerve alteration in the internal anal sphincters of rectal cancer patients who underwent abdominoperineal resection after adjuvant chemoradiation therapy and to correlate the degree of histologic changes with the time interval between chemoradiotherapy and abdominoperineal resection.

Anal canal specimens were prospectively collected in patients undergoing abdominoperineal resection. Representative slides were cut transversely at the level of the dentate line. Using trichrome and S-100 protein staining, a single pathologist blinded to the patients' treatment assessed collagen deposition and nerve fiber densities in the internal anal sphincter, respectively.

Twelve patients received radiation for rectal cancer (chemoradiotherapy group) and six were treated by surgery alone, including four patients with rectal cancer (1 leiomyosarcoma) and two with Crohn's disease (control group). There was a trend toward increased fibrosis (replacement of >10 percent of normal structures by collagen) and nerve density in the chemoradiotherapy group compared with the control group (P = 0.08 and P = 0.05, respectively). Nerve density significantly increased as chemoradiotherapy to abdominoperineal resection interval increased (P = 0.04).

Pelvic irradiation results in damage to the myenteric plexus of the internal anal sphincter of patients with rectal cancer; these alterations seem to be time-dependent. A trend toward increased collagen deposition also was observed. Together, these results provide a morphologic basis, which concurs to previously described physiologic and clinical alterations in the anal sphincter of patients irradiated for rectal cancer.