Yes, colon cancer can be caused by changes in bowel habits, including changes in shape and caliber.
A sigmoidoscopy and barium enema are adequate tests to screen for cancer, but as you suspected, a colonoscopy would be the definitive test.
If you a truly concerned about colon cancer, I would pursue the colonoscopy. There are some institutions that do a "no-sedation" colonoscopy, but you have to be a motivated patient.
Another alternative would be a CT, or "virtual," colonoscopy, but not all institutions offer the test.
These options can be discussed with your personal physician.
This answer is not intended as and does not substitute for medical advice - the information presented is for patient education only. Please see your personal physician for further evaluation of your individual case.
Kevin Pho, M.D.
www.kevinmd.com
www.twitter.com/kevinmd
I am still wondering if I had the sigmoidoscopy recently if I could have full blown colon cancer so soon after the test? Isnt barium enema a good test? Also would all of my stool be thin all the time or just sometimes? Is loose stool a symptom of colon cancer because my doctor says it is constipation that can signal colon cancer. Thanks Diana