Bowel cancer, gross specimen. Adenocarcinoma is a type of colorectal cancer arising from glandular epithelial tissue. Adenocarcinomas (seen here as the large brown areas at left and lower right) will have developed from adenomas, the benign version of the cancerous tumour. The tumour has a typically smooth edge and central fissures/ulceration. Adenocarcinomas account for about 90-95% of all colorectal cancers.