“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.

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