Adenoma--carcinoma sequence or "de novo" carcinogenesis? A study of adenomatous remnants in a population-based series of large bowel cancers.

Although it is well known that colorectal cancers can arise on a preexisting adenoma or de novo, the relative importance of these two pathways is still highly controversial. The authors studied the proportion of cancers with adenomatous remnants in a nonselected population-based series of 1630 resected colorectal cancers, so that they could estimate by subsite the importance of the adenoma-carcinoma sequence. Four factors appeared to be related independently to the presence of adenomatous tissue within cancers in a multiple logistic model: tumor extension, growth pattern, location, and size. It appeared that infiltrating and ulcero-infiltrating tumors, which represented 39.8% of all resected colorectal cancers, very rarely displayed adenomatous tissue (0.5%), whereas it was more common in fungating and ulcero-fungating cancers (25.8%; P less than 0.001). In these exophytic cancers, the presence of adenomatous tissue was related very closely to the tumor size and extension, and it was seen in as many as 83% of small cancers (less than 2 cm) limited to the mucosa or submucosa. Right colon cancer showed consistently fewer adenomatous remnants than left colon or rectal cancer. These figures suggest that there are roughly two types of colorectal cancers, one of the infiltrating or ulcero-infiltrating type, which usually would arise de novo and account for approximately 40% of all colorectal cancer cases, and the exophytic type, which would mainly follow an adenoma-carcinoma sequence, although some might be de novo cancers, in particular in the right colon.