Eman BahraniPanitta SitthinamsuwanTimothy H. McCalmontLaura B. PincusUniversity of California, San FranciscoFaculty of Medicine, Siriraj Hospital, Mahidol University2020-01-272020-01-272019-01-01American Journal of Clinical Pathology. Vol.151, No.6 (2019), 551-56019437722000291732-s2.0-85065669458https://repository.li.mahidol.ac.th/handle/20.500.14594/52264© 2019 American Society for Clinical Pathology. All rights reserved. Objectives: We observed keratoses with "clonal" nests present as numerous tiny collections, in which cells in "pagetoid" array are found, a configuration we termed microclonal seborrheic keratosis (MSK). To better distinguish MSK from pagetoid Bowen disease (PBD), we investigated use of immunohistochemical staining. Methods: Biopsy specimens of 26 MSKs, 17 PBDs, and 11 borderline cases were reviewed for histopathology and stained with p53, Ki-67, and p16. Results: High expression of Ki-67 and p16 was observed in 12 (80%) of 15 PBDs and in one (4%) of 23 MSKs. Low expression of p16 and high expression of Ki-67 were observed in 16 (70%) of 23 MSKs and in two (13%) of 15 PBDs. Expression of p16 was elevated in 12 (80%) of 15 PBDs and in three (13%) of 23 MSKs (P <. 0001). Conclusions: We describe a "microclonal" variant of seborrheic keratosis with morphology sometimes challenging to distinguish from PBD. High expression of p16 and Ki-67 or p16 alone favors the diagnosis of PBD over MSK.Mahidol UniversityMedicineArticleSCOPUS10.1093/ajcp/aqz001