Cancer in Man's Best Friend As in humans, cancer in dogs is common, but the types differ.

I remember that, once my patient was asleep, I began the operation, removing the entire rectal cancer encased in healthy surrounding tissue, the most likely approach to cure.  Then, I reconnected the two ends of the intestine, allowing my patient to once again move his bowels normally (something which for weeks he had been unable to do).
Like so many with rectal cancer, my patient had begun experiencing cramping and a change in his bowel habits, including pencil-thin stools, often coated in a thin layer of blood.  And the cramping pain.  Pain obviously resulting from a partial blockage of the large intestine.

http://cancer.about.com/od/glossary/fl/Cancer-in-Manrsquos-Best-Friend.htm