Factors associated with residual tumor at time of Mohs micrographic surgery for basal cell and squamous cell carcinomas

Background: Residual tumor is not always clinically apparent following biopsy of cutaneous carcinomas, which may prompt patients to question the need for definitive treatment. Objective: We investigated the percentage of cases in which residual tumor was histologically present at the time of Mohs micrographic surgery (MMS) for basal cell carcinomas (BCCs) and squamous cell carcinomas (SCCs) and investigated factors associated with residual tumor. Methods: We examined 483 MMS cases performed for biopsy-proven BCC (n = 287) and SCC (n = 196) between October 2022 and April 2023. Single-stage MMS specimens were step-sectioned en face to exhaust the block. Univariate and multivariable logistic regression models were created. Results: Residual tumor was identified in 83.3% of BCC and 66.8% of SCC at the time of MMS (P = .01). In patients clinically appearing tumor-free following biopsy, residual histologic tumor was identified in 68.2% of BCC and 41.5% of SCC. Residual tumor was significantly more likely in men (P = .04), high-risk sites (P = .002), smaller biopsy sizes (P = .0003), and larger preoperative sizes (P < .0001). Limitations: Single center, retrospective cohort. Conclusion: The majority of patients with BCC and SCC have residual histologic tumor at the time of MMS, oftentimes even when tumor is not clinically apparent. Multiple factors impact the presence/absence of residual tumor.