Nuances of cytokeratin‐7 immunohistochemistry interpretation during Mohs micrographic surgery for anogenital extramammary Paget's disease

Mohs micrographic surgery (MMS), or modified peripheral-margin Mohs in coordination with other specialties, has become the standard surgical treatment for anogenital extramammary Paget's disease (AEMPD) with local recurrence rates of 3.3%1 with intraoperative cytokeratin-7 (CK7) immunohistochemistry (IHC).1, 2 We aim to discuss the nuances of CK7 interpretation on frozen sections (FS) when performing MMS for AEMPD, based on the treatment of 110+ cases at the authors' institution. Apart from Paget cells (PCs), other anogenital histological structures stained with CK7, including eccrine and sebaceous glands, serve as positive immunostaining controls (Figure 1a,b). MMS with CK7 is generally reliable for achieving clear margins on anogenital skin. However, when approaching deeper-cephalad mucosal tissue of the anus and vagina,3 other cell populations, such as anal glands and anal transitional zone mucosa, also stain positive.4 In this scenario, the micrographic surgeon may observe a diffuse epithelium IHC staining pattern, precluding reliable identification of PCs (Figure 2a,b). While cervical squamocolumnar and endocervical glands are known to be CK7 positive, the described diffuse IHC staining may be observed even prior to reaching these deeper-cephalad structures. This likely represents an artifactual staining that would be best assessed on permanent sections and additional staining. The lack of cell-specific staining or a classic clustered or pagetoid distribution of CK7 positive structures (e.g., single cells, diffuse pattern) might represent a red flag for the loss of CK7 stain interpretability in deeper mucosa of the anogenital region. Another nuance of CK7 interpretation on Mohs FS relates to the presence of vulvar Toker cells (TCs).5 While TCs have been postulated as precursors of PCs, normal sparse TCs stain positive for CK7, representing a potential source of confusion during margin assessment. Even though TCs usually appear as single cells located in the basal epidermis (Figure 2c), they may group in cases of hyperplasia, appearing as clusters of CK7-positive cells.5 Additionally, the differentiation between TCs and PCs based on cell morphology (e.g., atypia, pleomorphism)5 may be difficult in FS. This is an uncommon finding in MMS cases for AEMPD but tends to occur when the Mohs surgeon performs a separate "inner moat" (an internal margin around one or more central critical structures on deeper mucosal tissue).3 The significance and prognostic value of the number and distribution of TCs (single scattered vs. clusters of cells) is unknown. One of the main advantages of an inner moat is that it provides completely pre-determined clear boundaries for multidisciplinary colleagues who remove residual tumor.3 This, in turn, reduces time under anesthesia and the risk of postoperative positive margin discrepancy after reconstruction.3 The primary disadvantage is the unknown of whether the inner margin can be cleared with Mohs FS until after the procedure has started. However, one must also consider patient and team tolerance—both in the discomfort of retraction of anogenital tissue and the added processing time under local or tumescent anesthesia.3 In conclusion, the majority of AEMPD cases treated with traditional or modified MMS achieve clear margins, with low recurrence and good tolerability. However, for the more complex cases (very large, bilateral, approaching critical structures or anogenital mucosal tissue), there exists the dilemma of whether or not to attempt the inner moat due to potential artifactual diffuse CK7 staining or TCs of unknown significance. If an internal margin around critical structures is considered, further discussion with the patient and multidisciplinary treatment team is necessary on expectations, nuances of mucosal staining patterns, and care goals. If indeterminate staining patterns are encountered, we recommend discontinuing MMS and relying on permanent sections. We thank Dr. Jerry D. Brewer for the images.