Mohs Micrographic Surgery for Granular Cell Tumor Using S‐100 Immunostain

background. Granular cell tumor (GCT) is an uncommon tumor of neural origin. Most commonly it is found in the head and neck region and rarely on the foot. The pathologic diagnosis is often confirmed by immunohistochemical staining for S‐100 protein. The standard treatment is complete excision. Two previous cases of GCTs treated by Mohs micrographic surgery (MMS) have been reported in the English literature. The immunohistochemical stain S‐100 was not used in these cases. objective. Describe the use of S‐100 immunohistochemical stain on MMS frozen sections in clearing a GCT of the foot with extension along nerves without involvement of perineurium. methods. Tissue was embedded and cut using standard Mohs surgical methodology. Frozen sections were stained with hematoxylin and eosin and with an immunoperoxidase method for S‐100 protein using a Ventana automated stainer. results. MMS in combination with S‐100 was successfully performed on a GCT on the plantar surface of our patient's foot. The S‐100 immunohistochemical stain was helpful in tracking the extension along nerves of the GCT which was not evident with hematoxylin and eosin alone. conclusion. Using S‐100 immunohistochemical stain on the MMS frozen sections can increase the diagnostic accuracy for complete removal of GCT.