Precancerous changes in the anal canal epithelium in resection specimens

Systematic histological examination for precancerous changes was carried out on 139 anal canals resected for anal or colo-rectal tumours or for inflammatory bowel disease. Severe squamous dysplasia or carcinoma in situ were found in 13 of 16 anal canals harbouring variants of squamous carcinoma but not in others. The lesions were mainly or totally located to the anal transitional zone (ATZ) in 11 cases and only in 2 cases exclusively to the squamous zone below the dentate line. Dysplastic areas separate from the tumour were found in 6 of the 13 cases. Junctional melanocytic activity and atypical melanocytic hyperplasia located in the ATZ as well as the squamous zone were present in 1 of 3 cases of malignant melanoma. The findings underline the close relationship between dysplasia and invasive carcinoma in the anal canal. Further they support the concept that basaloid, mucoepidermoid and some ordinary squamous carcinomas arise in the ATZ, and that this may also be the point of origin for malignant melanoma. A search for precancerous changes for anal canal tumours should therefore include the whole ATZ, i.e. in some cases up to 2 cm above the dentate line and it should be borne in mind that the lesions may be spotty.