The margin in skin cancer surgery

A skin cancer has a visible edge and an invisible one. The standard margin exists because two prospective studies measured the gap between them.

For basal cell carcinoma, 117 previously untreated, well-demarcated tumours were marked in 2 mm increments before being removed by Mohs surgery, so the subclinical extension could be read off the markings. For tumours less than 2 cm across, a 4 mm margin was needed to clear more than 95 per cent of them. For cutaneous squamous cell carcinoma the same method in a prospective series gave 4 mm for most tumours, and at least 6 mm for high-risk ones, defined there as 2 cm or larger, histological grade 2 or higher, invading the subcutaneous fat, or in a high-risk site. Those two papers, from 1987 and 1992, are the entire evidential basis for the numbers written in every guideline since.

Three things complicate the rim in practice. A specimen shrinks: one measurement series found reductions of the order of 70 to 80 per cent in measured margin width between the mark drawn on the skin and the figure the pathologist reports, so the surgeon's margin and the pathologist's margin are not the same quantity and the disagreement between them is arithmetic rather than blame. Dermoscopy moves the visible edge outwards: in one series 16.8 per cent of tumours had a visible border that fell short of the dermoscopic one. And the face has no spare skin, so margins on the nose and around the eyes are narrower than the evidence would like, which is why those are the sites where Mohs surgery earns its cost.

When tumour reaches the inked edge the excision is incomplete. Rates in ordinary practice run at about 2.6 to 15 per cent depending on the site and the series, and are highest on the nose, in aggressive histological subtypes and where invasion goes beyond the dermis. What follows depends on the tumour: re-excision or Mohs surgery for aggressive subtypes, central facial sites and squamous cell carcinoma, and in one long-term series of 23 incompletely excised facial basal cell carcinomas six recurred, all of them superficial multifocal, infiltrative or micronodular in subtype. In a frail patient with an incompletely excised low-risk basal cell carcinoma at a low-risk site, watching is a defensible choice and is made often.