Formalin-fixed tissue Mohs surgery (slow Mohs) for basal cell carcinoma: 5-year follow-up data

Background Mohs surgery using a formalin‐fixed tissue technique (slow Mohs) was used to treat 1090 basal cell carcinomas (BCCs) occurring in 1000 patients without Gorlin syndrome in a prospective, open nonrandomized trial of therapy carried out in a university dermatology department. Objectives To record outcomes and 5‐year cure rates in these patients. Results Five multirecurrent BCCs could not be cleared and Mohs surgery was abandoned. In the remaining 1085 BCCs, after debulking, clearance was achieved with a mean of 1·7 stages and 7·7 blocks. Logistic regression analysis showed that large tumour size and position on the nose were the only factors that significantly predicted the risk of tumour excision requiring more than two Mohs stages. Wounds were managed by the Mohs surgeon (n = 917), by an oculoplastic surgeon (n = 117) and by a plastic or other surgeon (n = 56). The mean interval between the first Mohs excision and Mohs clearance, regardless of the closure technique, was 4·2 days (range 0–44) and the mean interval between tumour clearance and defect closure was 1·9 days (range 0–49). Reconstruction of 117 periocular defects by the oculoplastic surgeon was done at a mean of 0·8 days (range 0–6) after Mohs clearance. Reconstruction by the plastic surgeon was done at a mean of 10 days (range 0–49) after Mohs clearance. Five‐year follow up was possible in 750 BCCs. Within this group 21 tumour recurrences were identified, giving a 5‐year cure rate of 97·2% for all patients, 97·8% for primary BCC and 95·3% for recurrent BCC. The mean tumour recurrence time was 2·5 years (range 0·6–4·99) following Mohs excision. There was a higher risk of recurrence for big (four of 78; 5%) and recurrent (nine of 193; 5%) BCCs compared with other indications for Mohs surgery. Conclusions The essential difference between frozen sections and formalin‐fixed sections is the greater use of an automated laboratory system to process the smaller size and greater number of blocks produced using the latter process. An effective Mohs service based on formalin‐fixed sections requires a minimum of three half‐day operating sessions together with a pathology laboratory able to provide results routinely within 24–48 h. A formalin‐fixed tissue Mohs service is useful for certain skin tumours and requires less technician time than a frozen section service but this advantage may not outweigh the inconvenience to the patient.