Prospective study of formalin-fixed Mohs surgery and haematoxylin and eosin stains with control contralateral biopsies for lentigo maligna: 5-year follow-up results

Summary Background There is little consensus on the optimum form of surgical management for lentigo maligna (LM). Currently, because malignant melanocytes spread down adnexal structures, full‐thickness skin removal is the only surgical option. Interpretation of Mohs histological specimens is difficult because of the presence of abnormal melanocytes in otherwise normal sun‐damaged skin. Objectives To investigate Slow Mohs for surgical excision of LM, to see whether the use of control contralateral skin biopsies would enable the end point of excision to be more easily interpreted and to investigate factors that influence the subclinical amelanotic extensions of LM. Methods The Slow Mohs technique for formalin‐fixed tissue was used in 74 patients with LM. Before surgery LMs were classified as well defined, poorly defined, incompletely excised or recurrent. Control biopsies were taken from healthy skin of the contralateral side. Specimens were processed in formalin, stained with haematoxylin and eosin (H&E) and the results read at 24–48 h. The excision margin required for complete excision was measured and patients were followed for a minimum of 5 years to exclude recurrence. Results On average the final excision margin required was 6·7 mm. Margins were significantly greater for ill‐defined, recurrent and incompletely excised LM compared with well‐defined LM. The presence of depigmented patches preoperatively did not correlate with the excision margin, but LMs showing nesting required significantly wider excision margins. There were seven (12%) recurrences at a mean 4·4 years after surgery in the group with 5‐year follow‐up. Recurrence occurred only in recurrent and ill‐defined primary LM. Conclusions The use of Slow Mohs formalin‐fixed tissue and H&E section staining, even with comparator biopsies, does not provide sufficient discrimination to identify residual disease confidently. What's already known about this topic? There is no established optimal surgical management for lentigo maligna (LM). LM may spread beyond the visible pigmented border. What does this study add? Some 62% of LMs contain amelanotic extensions. Excision margins of 6 mm were sufficient for 96% of well‐defined LMs but insufficient for ill‐defined, recurrent or incompletely excised lesions. LMs showing nesting require significantly wider excision margins than those without melanocyte nesting. Contralateral control biopsies do not help to distinguish between involved and uninvolved tissue.