To better estimate the likelihood of depressed-type adenoma developing into more advanced colorectal cancer, we assumed that a given colorectal cancer had a depressed type origin when depressed non-polypoid-type cancerous glands remained microscopically identifiable. Among 100 adenomas, depressed non-polypoid-type glands were found in 15, and lesion size in tumor with depressed non-polypoid type did not differ from that in others. Among 100 submucosally invasive cancers, depressed non-polypoid-type glands were found in 14, and lesion size in tumor with depressed non-polypoid-type did not differ from that in others. Among 100 cancers invading the muscularis propria, depressed non-polypoid-type glands were found in 15; these were significantly smaller lesions than those without depressed non-polypoid-type cancers (p<0.05). Among 100 more deeply invasive cancers, depressed non-polypoid-type grands were found in 16, with no difference in size from other similarly invasive lesions. We concluded that the rate of depressed-type adenoma progression to invasive cancer was about 15% and that depressed non-polypoid-type cancers are likely to invade the muscularis propria even when smaller than cancers of other types.

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