An international survey characterizing Mohs tissue processing techniques and Mohs margin thresholds: how close is too close?

Respondents were provided three hypothetical Mohs surgery scenarios with the same parameters other than the tumour type on the cheek: namely a 20-mm nodular basal cell carcinoma (BCC) (Scenario 1), a 20-mm infiltrative BCC (Scenario 2), or a well-differentiated squamous cell carcinoma (Scenario 3). Respondents were asked if the first section (7 µm) was clear and complete but the second section (after 50 µm trimming) demonstrated a small focus of tumour, would they take another stage. Those wishing to take another stage were asked to define how many sections they required to call the tumour clear. These results are compiled to characterize the histological margin thresholds.

From practice, we identified heterogeneity in Mohs micrographic surgery (MMS) specimen tissue processing techniques and specifications, and in the Mohs surgeons' assessment of MMS specimen histological tumour clearance.

By surveying an international cohort of Mohs surgeons, we determined to characterize variation in margin threshold assessment (number of wafers/sections free of tumour to declare tumour clearance).

An online questionnaire was distributed to Mohs surgeons in the UK, European countries, Australia and New Zealand, assessing the background demographics of the surgeons and the technical factors involved in MMS tissue processing and posing three MMS scenarios to define margin thresholds.

In total, 114 consultant/attending-level Mohs surgeons responded, giving a response rate of 33.5% from 20 countries (including UK nations). The first scenario posed was a 20-mm cheek basal cell carcinoma (BCC) excised by MMS with a fully complete first wafer (7??m) clear of tumour and the second wafer (after trimming interval of 50??m) demonstrating a small dermal focus of nodular BCC; of the 58 surgeons, 16 (27.6%) would not take another stage. With a follow-up question, 16 of the 58 (27.6%) surgeons specified wanting three clear sections to declare tumour clearance. When the same scenario had a change to a 20-mm infiltrative BCC, 84.2% (48 of 57 surgeons) required a second MMS stage, with a follow-up question clarifying that a third (19 of 57) wanted three clear sections to determine clearance. For a well-differentiated 15-mm squamous cell carcinoma with the same factors there was no majority consensus, with the same proportion of surgeons (22.6%; 12 of 53) calling tumour clearance after one, two and three clear section(s) respectively. For MMS specimen processing specifications, routine sections/wafers of 5–10??m were reported by 77.4% of respondents (48 of 62) and for trimming interval values, 78.6% (48 of 61) specified a range between 20 and 200??m.

By surveying international Mohs surgeons, we highlight surgeon background characteristics, peer-compare assessment of margin thresholds for tumour clearance across three scenarios, and delineate tissue processing and intraoperative approaches.