Previously, in Parts I and II of this series of articles, we showed through historical and contemporary literature review that there has been long-standing controversy and confusion among investigators regarding the nature of so-called colonic adenoma. [1,2] In Part III of this series, based on histopathologic reassessment, we came to the conclusion that what has long been called colonic adenoma or adenomatous polyp is not adenoma, namely, a benign tumor of glandular origin, but is carcinoma in situ (CIS), a malignant neoplasm of epithelium that remains confined to that epithelium. [3]

CIS of the skin and of the breast are two well-established and recognized lesions. In this, Part IV of this series, we will compare so-called colonic adenoma with CIS in the skin and in the breast. Through this comparison, one will see that (1) many, if not all, cytomorphologic attributes used to diagnose CIS in the skin and in the breast are present in so-called colonic adenoma, further reinforcing our conclusion that the latter is not adenoma, but is CIS; and (2) CIS is a unifying concept that can be applied to any epithelium.