"Extramammary-Type" Paget Disease of the Breast.

BACKGROUND: Mammary Paget disease and extramammary Paget disease (EMPD) have different prognoses. Because they are indistinguishable on histopathological grounds, they must be distinguished on a topographical basis. OBJECTIVE: To study cases of Paget disease of the breast, which show similarities to EMPD. METHODS: Cases were selected by 2 different approaches: (1) those with an exceptionally good evolution and no evidence of subjacent tumor and (2) those expressing MUC5AC. RESULTS: Five cases were collected. All cases showed an indolent behavior with a known long clinical history in 2 cases (9 and 25 years, respectively) and a long follow-up in all cases but one (4-8 years). In all cases but one, surgery was performed, and no parenchymal tumor was found (either intraductal or infiltrating). All cases expressed cytokeratin 7 and MUC5AC without expression of MUC2, S100, or p40. LIMITATIONS: The short number of cases is a limitation of this study. In addition, case 5 is recent, so we have a very short follow-up. CONCLUSIONS: Some cases of mammary Paget disease behave like EMPD with slow progression and with no underlying associated tumor. Immunoexpression of MUC5AC might be a clue to identify them.