Abstract
Background: Extramammary Paget disease (EMPD), pagetoid squamous cell carcinoma in situ (PSCCIS), and Paget disease of the breast (PD) are intraepidermal carcinomas with overlapping histopathologic features. CK7 and CAM5.2 stains are frequently utilized to distinguish PSCCIS from EMPD and PD. However, some cases of PSCCIS can stain positively for CAM5.2 and CK7, indicating a potential pitfall with these stains. p63 has been shown to distinguish PSCCIS from EMPD. We assessed p63 staining in PD and compared it to p63 staining of PSCCIS and EMPD. Methods: A retrospective search for 15 examples each of PSCCIS, EMPD, and PD with remaining tissue in the paraffin block was performed. The diagnosis was confirmed by a board-certified dermatopathologist and immunostaining for p63, CK7, and CAM5.2 was performed. Staining >55% was scored as positive. Staining <55% was scored as negative and an approximate percentage of positive cells was recorded. Results: Diffuse nuclear expression for p63 was detected in 100% (15/15) of PSCCIS cases, 0% (0/15) of PD cases, and 0% (0/15) of EMPD cases. CK7 and CAM5.2 stains were positive in 100% of PD. CAM5.2 was positive in 100% of EMPD and CK7 was positive in 93% of EMPD. CAM5.2 was positive in 0% of PSCCIS biopsy specimens, but partial staining was seen in 20%. CK7 was positive in 13%, but partial staining was seen in 47%. Conclusions: p63 immunostaining is a highly sensitive and specific method for differentiating between PSCCIS and PD or EMPD. While CAM5.2 and CK7 are also useful ancillary stains in this differential diagnosis, false-positive and false-negative staining occurs with these two markers. © 2023 John Wiley & Sons A/S. Published by John Wiley & Sons Ltd.