DS21 Mohs surgeons had higher rates of detecting missed margins than consultant histopathologists when auditing Mohs slides

Abstract We aimed to determine whether there was any difference between the rate of concordance in audit of slides between Mohs surgeons and dermatopathologists. All digitally stored Mohs micrographic surgery audit cases from 2012 onwards were accessed and reviewed. In total 82 cases had been audited in 2024, during which time Mohs surgeons were the independent second opinion. An equal number (n = 82) of cases were then chosen from the period during which consultant histopathologists had been the auditor. The primary goal of the audit was to monitor for false negative margins, where the Mohs surgeon had called the margins clear but marginal tumour was seen at audit. In addition, we assessed slide quality and concordance with the histological diagnosis of debulked tissue. There were three cases of false negative margins detected by Mohs surgeons during 2024, but there were zero cases and 100% concordance between September 2021 and October 2022. Further analysis of audited cases up until 2012 (beginning of the current dataset in usage) showed that there were 515 cases audited and only two cases of a disagreement on audit. In one case this was established to be a labelling error and therefore was not related to slide interpretation. Mohs audits should be carried out by all departments offering this service, as outlined in the guidelines and standards document [British Society for Dermatological Surgery. Mohs standards 2020. Available at: https://bsds.org.uk/mohs-surgery/mohs-standards-2020 (last accessed 11 March 2025)]. We present evidence from 13 years of internal independent audit of Mohs surgery in a tertiary centre, focusing on a recent change to using Mohs surgeons rather than dermatopathologists as auditors. Our retrospective study demonstrated higher rates of false negative margin detection when audits were carried out by Mohs surgeons. We suggest that this relates to greater familiarity with interpreting horizontal frozen sections. In addition, dermatopathology colleagues’ workflow more commonly focuses on diagnoses and measuring or grading tumours, with Mohs surgeons’ focus being on margin clearance. Our study may suggest that a change in practice to Mohs surgeon’ auditing slides could offer the potential for fewer missed margins and recurrence. Furthermore, an internal audit process between Mohs surgeons may afford significant logistical and timesaving benefits to the audit system. A potential counter argument could be that it may create an increased workload for Mohs surgeons. However, with continually increasing pressure on both Mohs surgical and histopathology departments, considering this audit practice may be of significant benefit.