Also known as Mohs' micrographic surgery. A method of removing skin cancers, usually basal cell carcinomas, squamous cell carcinomas and occasionally lentigo maligna and dermatofibrosarcoma protuberans (DFSP). Conventional excision entails a vertical incision encompassing the entire skin, whereas in Mohs' surgery, successive horizontal layers of skin are excised with microscopic examination of the margins to determine whether further excision is required and, if so, allowing planning of what area to excise in the next stage.
The approach preserves healthy tissue and is therefore useful in tumours in the face where structures which are difficult to reconstruct may be preserved, e.g. the eyelids. Where structures have to be sacrificed, this is done so with the knowledge that it is involved with tumour. The rates of complete excision are higher compared with conventional excision. Where reconstruction is required, this can be performed safe in the knowledge that further excision is unnecessary.
Mohs originally employed chemosurgery by applying a paste of zinc chloride mixed with Bloodroot extract (Sanguinaria canadensis). This mixture penetrated the skin, destroying tumour and fixing tissue. Microscopic examination of this already fixed tissue permitted assessment of the margins and further application of the paste accordingly.
Modern Mohs' uses local anaesthetic and cautery. Microscopic examination is usually done immediately after each stage so that the entire surgery can be done in a single attendance. The microscopic sections of tumour are therefore cut as frozen sections where the tissue is embedded in a resin that is cut in a microtime within a cryostat. The tissue is then processed conventionally with formalin fixation and paraffin embedding for corroboration with the original frozen section report. One exception is Mohs' surgery for DFSP: the spindle cell appearence of these cells can be difficult to interpret on the poorer quality frozen section specimens. A 'slow' Mohs' approach is used instead where the tissue is formalin fixed and processed conventionally. The surgery therefore takes place over several days rather in a single setting.
At present, only a handful of hospital in the UK offer Mohs' surgery.