Rapid frozen section immunostaining of melanocytes by microphthalmia-associated transcription factor

Mohs micrographic surgery (MMS) has increasingly become an accepted therapy for melanoma in situ on chronically sun damaged skin (CSDS). However, melanocytes are difficult to locate in frozen material on hematoxylin and eosin. In addition, determining the cut-off between the melanoma and the "atypical melanocytic hyperplasia" in CSDS can be challenging in frozen or formalin-fixed paraffin-embedded sections, with or without immunohistochemistry (IHC). In this article, we report the use of a rapid, 35-minute protocol using microphthalmia-associated transcription factor (MITF) IHC for identifying melanocytes in frozen tissue for its potential use in MMS. In contrast to melanoma antigen recognized by T cells (MART-1), MITF is a nuclear stain, which simplifies identification of melanocytes and quantification of melanocytic parameters. In this study, MITF IHC in frozen sections yielded equivalent melanocyte nuclear diameter and density measurements compared with formalin-fixed paraffin-embedded sections. Nuclear diameter measurements obtained with MITF were similar to that previously reported with MART-1, but the melanocyte density figures were lower. Reliable labeling of melanocytes in frozen sections required the use of diaminobenzidine (DAB) chromogen with Giemsa counterstaining and a buffer devoid of surfactant. Our experience with MITF IHC indicates that it is a dependable immunostain in frozen sections, and may prove to be useful in MMS as an adjunct to hematoxylin and eosin and MART-1 IHC for interpretation of margins for melanoma in situ on CSDS.