Background: Squamous cell carcinoma in situ (SCCIS) has more subclinical lateral extension than invasive squamous cell carcinomas (SCC). Objective: To determine whether it takes a greater number of Mohs stages for clearance of SCCIS compared with SCC and whether the difference in final defect size and clinical size is larger in SCCIS than SCC. Methods: All Mohs micrographic surgery cases of SCCIS and SCC performed between January 2011 and December 2021 were identified. Number of Mohs stages were recorded and difference in final defect size and initial clinical size were calculated for SCCIS and SCC. Results: 4,363 cases were included, 1,066 SCCIS and 3,297 invasive SCC. The initial clinical size, final defect size, and the size difference were similar between SCCIS and SCC groups. However, SCCIS underwent more Mohs stages to achieve tumor clearance than invasive SCCs (1.5 +/- 0.7 vs 1.4 +/- 0.7 respectively, p < .001). In fact, 71% of SCCs were cleared after 1 Mohs stage compared with 61.1% of SCCIS. Conclusion: These findings support that SCCIS has more subclinical lateral extension and therefore is appropriate for Mohs surgery.
