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Colonoscopies can find polyps that are usually benign but grow into cancer on rare occasions. Gastroenterologists remove polyps, protecting patients from possible malignant growth. But the progression from polyp to cancer takes years, and frequent colonoscopies are expensive and come with the serious risk of puncturing the wall of your colon. If you're at average risk for colon cancer, tell your doctor you want to wait a decade between tests.