My dad ( 50 years old) has had changes in his stool shape. what are the chances of changes in stool shape being colon cancer?

Possible: The clinical indications of a tumor growing in the distal bowel or simoid/rectosigmoid region is the appearance of blood, or narrowing of stool. Since the terminal large bowel functions to completely remove water from the GI tract, the initial site being cecum/rt. colon, if a tumor is obstructing lumen, the stool becomes narrowed. One requires colonoscopy to confirm source of problem.