Colon cancer. Light micrograph of an adenocarcinoma of the colon. The tumour cells have made several acini containing mucinous material and inflammatory cells.Almost all colon cancers are primary adenocarcinomas, which are the third most common cancer in both men and women. Colon cancers are the most common gastrointestinal (GI) carcinomas and have the best prognosis. Magnification: x100 when printed at 10 centimetres wide.