To diagnose rectal cancer, doctors may use several methods:
Colonoscopy: A procedure that allows the doctor to view the inside of the rectum and colon using a flexible, lighted tube. Biopsy: During a colonoscopy, a tissue sample may be taken for laboratory analysis. Imaging tests such as CT scans, MRIs, and PET scans to determine the extent of the cancer. Blood tests to check for specific markers associated with cancer.