Background  In the Netherlands basal cell carcinomas (BCC) are eligible for Mohs microscopic surgery (MMS) if certain criteria are fulfilled.

Objective  To study the MMS indication criteria practised at the department of dermatology of the Erasmus University Medical Center, Rotterdam and to identify predictive factors for extensive subclinical tumour spread among BCCs eligible for MMS.

Methods  Pre-operative patient and tumour characteristics were derived retrospectively between January 2nd 2006 and December 28th 2009 from 1174 patient records, accounting for 1464 BCCs. Multivariate logistic regression models were used to calculate crude and adjusted odds ratios (OR) with 95% confidence intervals (CI) for one vs. two or more stages and for narrow (≤2 stages) vs. extensive subclinical spread (≥3 stages).

Results  H-zone location [adjusted OR 1.51 (95% CI 1.16–1.96)], recurrent tumour [adjusted OR 1.50 (95% CI 1.11–2.02)], aggressive subtype [adjusted OR 1.25 (95% CI 1.01–1.56)] and tumour size ≥11 mm [adjusted OR 1.53 (95% CI 1.20–1.96)] were significantly associated with two or more stages. Predictive factors for extensive subclinical spread were recurrent tumour [adjusted OR 2.26 (95% CI 1.61–3.17)], tumour size ≥21 mm [adjusted OR 1.69 (95% CI 1.13–2.51)] and location in the H-zone [adjusted OR 1.68 (95% CI 1.15–2.46)].

Conclusion  ‘Rotterdam’ indication criteria used for MMS are appropriate. Predictors for extensive subclinical spread are important for patients’ and surgeons’ expectations prior to the operation about time span, defect size, reconstruction and possible associated morbidity.