Diminutive cancers of the colon and rectum: comparison between flat and polypoid cancers.

A comparative study of polypoid and flat colorectal cancers less than 20 mm in size was made. A review of the characteristics of 56 polypoid cancers and 29 flat cancers treated between April 1989 and January 1996 was performed. Both groups of cancers showed similar age and location distribution. Polypoid cancers were more likely to be well differentiated (38%) than were flat cancers (17%) (P < 0.05). Flat cancers were more likely to have reached the serosa (52% vs 12%; P < 0.01) and also demonstrated a higher frequency of lymph node involvement (41% vs p 9%) (P < 0.01) when compared to polypoid cancers. Flat colorectal cancers belong to a distinct subset which demonstrates greater biological aggressiveness than polypoid cancers of equivalent size.