Rectal cancer cell. Coloured scanning electron micrograph (SEM) of a rectal carcinoma cell. Carcinomas are cancers that arise from the lining of an organ. The blebs (nodules) on the cell's surface are typical of cancer cells. Colorectal cancer is one of the most common cancers in the western world. Risk factors include a diet high in animal fat and low in fibre and a sedentary lifestyle.