Mohs' micrographic surgery using frozen sections alone may be unsuitable for detecting single atypical melanocytes at the margins of melanoma in situ

Summary Background It remains questionable whether micrographic surgery with frozen sections is an appropriate technique for excision of melanoma in situ (MIS) of the lentigo maligna type. Advocates of the technique have interpreted MIS as being histologically defined by nests and contiguous atypical melanocytes on the basal layer. Others, however, have viewed the periphery of MIS as consisting of scattered single atypical melanocytes, a finding that may be difficult or impossible to establish on frozen sections. Objectives To examine the reliability of micrographic surgery using frozen sections interpreted by an experienced Mohs' surgeon, in the excision of MIS. Methods From a total of 154 specimens, frozen sections from the 50 specimens with margins that were considered difficult to interpret were thawed, sent for routine processing and then examined ‘blind’ by a dermatopathologist. Results Using the dermatopathologist's report on paraffin‐embedded sections as a reference point, the sensitivity and specificity of frozen sections were calculated to be 59% and 81%, respectively. Conclusions Using these histological criteria, micrographic surgery with frozen sections alone is unreliable in the excision of MIS.