Mohs’ micrographic surgery for cutaneous squamous cell carcinoma: practical considerations

Lesions of cutaneous squamous cell carcinoma (cSCC) vary in their biological aggressiveness and can be broadly divided into ‘low-risk’ and ‘high-risk’ groups based upon factors known to be associated with an increased tendency to local invasion, recurrence and nodal metastasis. 1-6 These factors include tumour size, 1,3,5-8 anatomical site (especially the lip, 6,9,10 ear, 6 genitalia, 10 and Marjolin’s tumours 1,10-12), poor histological differentiation, 1,6-8,13,14 depth of invasion, 1,6,8,13,14 perineural spread, 15,16 recurrence following previous treatment, 1,3,6,7 and host immunosuppression. 1,17 Relatively ‘low-risk’ cSCC tend to present early in sun-exposed sites and often respond well to treatments like curettage and cautery, 18-20 simple excision and cryosurgery. 21 However, more ‘high-risk’ lesions are more appropriately managed using radiation therapy, 18,19,22 radical excision 18,23,24 or Mohs’ micrographic surgery (MMS). 3,6,8,25-27 Unfortunately, some lesions lie beyond the scope of dermatological surgery, and metastatic lesions and cSCC invading bone or the orbit are best managed by surgical and oncological colleagues. Surgical excision of cSCC aims to completely remove the primary tumour and takes two main forms; radical excision with predetermined surgical margins and excision under total microscopic control (MMS). Radical excision relies upon clinical skill and judgement to predict how much tissue to excise in order to remove the lesion. The amount of clinically uninvolved tissue removed will generally increase for larger, deeper and recurrent lesions. 5-7,23 Intra- or post-operative vertical tissue sections are examined to give some estimate of the completeness of excision. In contrast, MMS uses horizontally orientated tissue sections for histologically controlled, staged tumour removal. 3,6,8,25-27 Patients undergoing MMS are counselled to expect a staged procedure beginning with resection of the obvious tumour together with a conservative margin of clinically uninvolved tissue. Mohs’ tissue sections are prepared and examined in order to seek out all malignant cells in the deep or peripheral surgical margins. Residual tumour, if present, may be grossly obvious or more worryingly, very difficult to identify, with small islands of malignant cells infiltrating local tissues and occasionally spreading along nerves. 15,16 This latter type of spread is often very subtle and is most likely to be detected by the comprehensive nature of horizontal Mohs’ sections. In contrast, standard vertical sections are much less likely to identify such tumours due to their random and two-dimensional nature, although modifications have been described. 28 Traditionally, MMS is performed using frozen sections, allowing staged tumour excision over a number of hours. However, despite careful preparation, subtle tumour spread may be obscured by inflammatory changes and can sometimes be difficult to identify on frozen sections. In this issue, Turner and colleagues 29 describe their results using permanent (formalin-fixed, paraffin-embedded, so-called ‘Slow-Mohs’) instead of frozen sections to treat 61 patients with cSCC. The logic behind this substitution is the technical superiority of permanent over frozen sections. The use of permanent sections with or without cytokeratin stains 30 in the management of the most difficult cSCC 3,26,31 and other skin tumours 31 has previously been recognized, but the article by Turner and colleagues raises the important question of when they are best used. There seems little justification (other than presumed cost-effectiveness) to use permanent sections routinely for all stages of MMS (as recommended by Turner and colleagues). Their data on 61 patients shows short- and medium-term cure rates (> 50% of patients were followed up for 3 years or less) to be as good but not superior to classical (frozen section) MMS. Consequently, if there is no difference in efficacy, the practicalities of the two approaches should be carefully examined. Using ‘Slow-Mohs’, Turner and colleagues found that tumour excision and reconstruction took place over a period of days or even weeks (median duration from first to last Mohs’ excision was 4 days [range 2–28 days]), with each stage requiring a separate hospital visit and a new local anaesthetic. Unfortunately, wounds left open for such periods are far more difficult to reconstruct than fresh wounds and some wounds (e.g. full-thickness eyelid or lip defects) cannot be safely left open. In contrast, most patients treated with classical (frozen section) MMS (appropriate for many “high-risk” cSCC 3,6,8,25-27) can have their tumours excised and their wounds repaired as a day case using one sustained local anaesthetic. For extremely high-risk cSCC, Mohs’ surgeons can elect to use classical (frozen section) MMS to rapidly excise all gross tumour in a matter of hours and then convert to using permanent sections. Although this approach does require the excision of a new Mohs’ tissue layer 3,26 and a few days delay, this will only be for the most difficult cases. Any perceived disadvantages must be weighed against the knowledge that no technique is infallible and many recurrent and extremely high-risk lesions may well demonstrate non-contiguous spread. 3 In such difficult cases, tissue conservation is not high on the agenda and further tissue resection offers an extra margin of safety. Only those patients in whom permanent sections reveal residual cSCC will need to undergo further resection, all others can proceed to wound reconstruction. This combination approach for selected cSCC lesions offers both the speed of frozen sections and the increased accuracy of permanent sections but can only be performed in suitably equipped Mohs’ micrographic surgery units staffed by properly trained and experienced personnel.