CK 8/18: the key to differentiating intracutaneous lesions with pagetoid features

Introduction: Neoplasms with pagetoid features are a category of rare lesions defined by the presence of atypical cells at different levels of the epidermis. The most important diseases within this category are mammary Paget disease (MPD), extramammary Paget disease (EMPD), Bowen’s disease, in situ melanoma, and pagetoid reticulosis. Aim: The aim of this analysis was to describe the importance of the cytokeratin 8/18 (CK 8/18) immunostaining in diagnosing MPD and EMPD and differentiating them from other lesions. Materials and Methods: A retrospective study was employed, based on the histopathological and immunohistochemical (IHC) characteristics of 30 cases that presented pagetoid features. The cases were processed and analyzed at the Department of Pathology, Mureş Clinical County Hospital, Târgu Mureş, Romania, from 2017 to 2020. Results: Five MPD cases, one EMPD case, one pagetoid reticulosis case, 10 Bowen’s disease cases, and 13 in situ melanoma cases were collected. Under Hematoxylin–Eosin staining, cells presented pale cytoplasm in MPD, EMPD, and in 25% of the melanoma cases. Hyperchromasia with nuclear enlargement was seen in all cases. Immunostaining with CK 8/18 was positive in all MPD and EMPD cases. Tests for CK7, p63, and CK AE1/AE3 were positive in MPD, EMPD, and Bowen’s cases. Tests for S100, SRY-box transcription factor 10 (SOX10), human melanoma black 45 (HMB45), and Melan A were positive in melanoma cases, while cluster of differentiation (CD)3, CD4, and CD8 tests were positive in the pagetoid reticulosis case. Conclusions: CK 8/18 is an IHC marker that can help establish the diagnosis of MPD and EMPD and differentiate them from other pagetoid neoplasms, ensuring the proper diagnosis and prognosis are provided.