A 63-year-old man undergoes a screening colonoscopy and is found to have a polyp in his sigmoid colon. Which type of polyp is most associated with malignancy?

Adenomatous polyps are common, occurring in up to 25% of the population older than 50 years of age in the United States. By definition, these lesions are dysplastic. The risk of malignant degeneration is related to both the size and type of polyp. Tubular adenomas are associated with malignancy in only 5% of cases, whereas villous adenomas may harbor cancer in up to 40%. Tubulovillous adenomas are at intermediate risk (22%). Invasive carcinomas are rare in polyps smaller than 1 cm; the incidence increases with size. The risk of carcinoma in a polyp larger than 2 cm is 35 to 50%. Although most neoplastic polyps do not evolve to cancer, most colorectal cancers originate as a polyp.