Pagetoid Spread in Basal Cell Carcinoma, the Missing Counterpart with Novel Morphologic Features: a Case report and Literature Review

Summary: Pagetoid spread is a hallmark feature of in-situ counterparts of invasive malignancies, including squamous cell carcinoma in situ (SCCIS), melanoma in situ (MIS), and extramammary Paget disease (EMPD). However, despite the commonness of basal cell carcinoma (BCC), its in-situ counterpart is not well understood. Pagetoid spread in BCC has never been reported until a recent study of 5 such cases in 2023. Herein, we report a 79-year-old man with an ill-defined plaque on the scalp. Excision showed basal cell carcinoma with superficial and nodular patterns. Focally there was pagetoid spread of single cells and small groups emanating from the basal layer. The cells reached stratum granulosum without parakeratosis. They imparted an epithelioid morphology with large, pale cytoplasm and perinuclear clefting. There were no mitotic figures. In contrast to classic BCC, the nucleoli were prominent. These cells were positive for MOC31, while negative for Cam5.2 and SOX10. Morphologically, our pagetoid BCC was most similar to EMPD given the large pale cytoplasm. The lack of parakeratosis and frequent mitotic figures was unusual for SCCIS. Given its unusual morphology, pagetoid BCC may raise diagnostic issues, especially in small samples. We further expand the nascent literature, reporting novel morphologic features aiding differential diagnosis.