Colon cancer is the result of cancerous (malignant) cells growing in the tissues of the colon. Typically, there are no early signs of the disease. When symptoms do appear, they can include blood in the stool, narrow stools, and diarrhea or constipation. Tests used to diagnose colon cancer include rectal exams, fecal occult blood tests (FOBT), and colonoscopies. Treatment options range from surgery, chemotherapy, radiation therapy, or a combination of these.