Mucinous endometrial epithelial proliferations: A morphologic spectrum of changes with diverse clinical significance

Background: Mucinous differentiation of the endometrium can occur in a spec trum of changes ranging from benign (metaplasia) to malignant (adenocarcino mas with mucinous differentiation). Mucinous proliferations with simple arc hitecture are generally considered benign; however, more complex growth pat terns have an uncertain biologic behavior, particularly when these changes are focal and/or are encountered in biopsy or curettage material. The dispa rity between the degree of cytologic atypia and the neoplastic potential ma kes their interpretation difficult in routine practice. We categorized and prospectively studied a series of these lesions based upon their degree of architectural complexity and correlated them with follow-up curettings and hysterectomies over a period of three years. Methods: Mucinous proliferatio ns of the endometrium were subdivided into three categories (A, B, or C) ba sed upon increasing degrees of architectural complexity. Type A were mucin- containing epithelial cells, present singly or in small tufts, within archi tecturally benign glands or involving the endometrial surface. Type B proli ferations were more complex, consisting of mucin-containing epithelial cell s forming small pseudoglands with rigid, punched out spaces and no supporti ng stroma. Conspicuous cytologic atypia or architectural features such as a filiform growth pattern characterized type C alterations. One hundred two curettings and 36 hysterectomies from 52 patients were reviewed. Results: P atient's ages ranged from 39 to 71 years (median, 55 yr); 41 patients (80%) were over age 50. Twenty patients (40%) were receiving hormone replacement therapy. Nineteen type A, 17 type B, and 16 type C mucinous endometrial pr oliferations were analyzed. Excluding those cases in which a conventional c oexisting precancerous lesion was also present in the initial endometrial s ample, the percentages of endometrial carcinoma following a curettage diagn osis of types A to C, respectively, were 0, 64.7%, and 100%. Carcinomas fol lowing type B alterations were all well-differentiated and all were confine d to the endometrium or inner third of the myometrium. Conclusion: Mucinous endometrial proliferations comprise a spectrum subdivisable into biologica lly meaningful subsets. A high percentage of type B alterations were found to have endometrial adenocarcinoma on follow-up; however, all were well-dif ferentiated and showed either no or minimal invasion. This finding suggests that the absence of cytologic atypia in complex mucinous lesions identifie s subsets of lesions at low concurrent risk for deeply invasive cancer. The presentation of type B lesions as predominantly microglandular surface les ions without co-existing atypical hyperplasias suggests that a subset of we ll-differentiated adenocarcinomas arise via neoplastic alterations in surfa ce epithelium.