EDUCATION AND TRAINING
Diploma of Expert Practice in Mohs Histological Procedures: introducing a new IBMS examination within cellular pathology
Guy Orchard introduces the latest addition to the IBMS portfolio of Diploma of Expert Practice examinations.
In 2010, worldwide data showed that there were 80,000 deaths from skin cancer. Of this total, 49,000 were due to malignant melanoma, the most aggressive and often fatal form of skin cancer. The remaining cases were due to non- melanoma skin cancer, largely, although not exclusively, composed of squamous cell carcinomas.
What is striking is the significant steady
increase in death rates from skin cancers over the past two decades. In 1990 the death rate for skin cancer were recorded at 51,000 cases worldwide, since when we have seen an increase in skin cancer death rates of 36%. This figure is alarming when the public knowledge of the risk factors for skin cancer are all too familiar.
The effects of ultraviolet (UV) light, in particular the UVB part of the spectrum, are the most significant contributing factor to skin cancer incidence. Not only are members of the public more aware but they are also exposed to worldwide campaigns to warn of the risks associated with UV exposure, whether natural or artificial in the form of sunbeds. It should also be noted that death rates for skin cancer are not the only parameter worth monitoring. What is perhaps more significant are the data on the incidence of skin cancers. In the UK alone, 84,500 non-melanoma cases were recorded in 2007; however, this does not reflect the real incidence as a large number of non-melanoma cases are simply not reported as they are not always presented in a direct clinical setting. In comparison, there were 10,672 cases of malignant melanoma in the UK in 2007. Clearly, in terms of incidence, non-melanoma skin cancer is by far the major contributor, albeit with a more favourable survival outcome.
TUMOUR TYPE AND CLEARANCE There are two main forms of non-melanoma skin cancer, squamous cell carcinoma (SCC) and basal cell carcinoma (BCC). Both typically occur on sun-exposed sites, most notably the
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head, face, arms and legs. The main treatment for both is based on surgical removal, and it is the BCC that represents the predominant tumour type in this group. Fortunately, metastatic spread from BCCs is extremely rare, whereas SCCs will metastasis to other sites far more readily, especially if left untreated and allowed to invade local tissue sites. In the case of BCCs and SCCs, local tissue invasion does destroy tissue architecture, and surgical removal with adequate clearance margins is a prerequisite for successful cure rates.
Cellular pathology laboratories have become used to receiving wide excisions of non-melanoma skin cancers for assessment. Classically, the clinician requires confirmation of tumour type and also an indication of clearance of tumour margins. Sometimes these excisions can involve significant removal of skin tissue. As these tumours commonly occur on facial sites, the cosmetic impact for the patient of wide tissue removal can be quite traumatic.
MOHS TECHNIQUES Alternative options for the surgical removal of such tumours, most significantly on the face, saw the introduction of Mohs micrographic surgery techniques. These involve the removal of skin tumours such as BCC and SCC with minimal tissue margin clearance. More recently, Mohs has also been used to remove malignant melanomas, most notably
the lentigo maligna melanoma variant which commonly occurs on the face. However, the technique can be used for the removal of many other tumour types, such as dermatofibrosarcoma protuberans (DFSP), microcystic adnexal carcinoma, merkel cell carcinoma, Paget’s disease and atypical fibroxanthoma.
The method involves a rapid microscopic assessment of frozen section tissue in a controlled surgical procedure, involving inking of all margins of the tissue. Mohs is now widely regarded as the best procedure for obtaining complete margin control clearance during removal of skin cancer. Popularly used in the USA, the term ‘complete circumferential peripheral and deep margin assessment’ (CCPDMA) defines precisely what the Mohs procedure entails. The cure rate for tumour removal is high (97–99.8%), although this does depend on tumour type and anatomical site. The technique was devised in 1938 by
Frederick Mohs in the USA; however, it did not become widely popular until the 1980s, and was first introduced to the UK in 1985 as a recognised surgical procedure at St John’s Hospital for Diseases of the Skin (now St John’s Institute of Dermatology), at GSTS Pathology, London. The use of the Mohs technique has since grown in popularity and several large centres around the UK and in Ireland now offer Mohs as a routine service procedure.
Anatomical map displaying tissue removal from the scapula of a patient. THE BIOMEDICAL SCIENTIST 731
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