Malignant potential in flat elevations.

Ninety-nine colonoscopically removed flat elevations were examined. They were divided into two groups: Group 1--flat elevations 1 to 5 mm in diameter (55 cases)--and Group 2--flat elevations 6 to 10 mm in diameter (44 cases). Group 2 had a higher percentage of adenomas (86.4 percent) than Group 1 (67.3 percent). In adenoma cases (flat adenomas), Group 2 tended to show a higher degree of dysplasia. The rates of mild and moderate dysplasia were 83.8 percent and 16.2 percent in Group 1 vs. 1.70 percent and 13.2 percent in Group 2, respectively. Group 2 contained six cases (15.8 percent) of intramucosal carcinoma (severe dysplasia), while there were no cancer cases in Group 1. Both malignancy rate and degree of dysplasia were size dependent; the larger the lesion the more severe the dysplasia. Our study also revealed that small flat elevations tend to contain higher numbers of nonneoplastic lesions than do larger lesions. Increased detection of nonneoplastic lesions seems to have contributed to an overall decline in the malignancy rate of flat elevations in recent reports.