Residual melanoma in wide local excision specimens after ‘complete’ excision of primary cutaneous in situ and invasive melanomas.

Wide local excision (WLE) to achieve adequate clearance margins is the standard initial definitive treatment for patients with biopsy-proven primary cutaneous melanoma. Residual melanoma in WLE specimens after prior complete excision-biopsy (CEB) is reported in 0-6.3% of cases. However, studies evaluating the prevalence, clinicopathological features and relevance of persistent disease in WLE specimens are limited. This study sought to determine the frequency of and clinicopathological characteristics associated with residual melanoma in WLE specimens performed after a CEB of primary cutaneous or acral melanoma (in situ or invasive) with clinically and histologically tumour-free margins, and assess its relevance. A review of the research database and pathology archives of a large Australian tertiary referral melanoma treatment centre was performed. Eligible patients were those for whom a definitive WLE was performed after CEB of a primary melanoma (in situ or invasive) with negative clinical and histological margins, between May 2013 and May 2015. All partial biopsies were excluded. Of 640 eligible patients, 510 (79.7%) had invasive melanoma and 130 (20.3%) had melanoma in situ. Residual disease was identified in 20 cases (20/640, 3.1%), of which three (15%) were melanoma in situ on CEB and 17 (85%) were invasive melanoma. On univariate analysis, the presence of residual disease in WLE specimens was associated with lentigo maligna (LM)/LM melanoma (LMM) subtype [odds ratio (OR) 10.33; 95% confidence interval (CI) 2.84-37.54; p=0.004], nodular melanoma (NM) subtype (OR 4.92; 95% CI 1.53-15.85; p=0.0076) and, for invasive tumours, higher tumour mitotic rate (mean 7.7, SD 7.51 vs 3.4, SD 4.83; OR 1.11; 95% CI 1.04-1.18; p=0.0014). Breslow thickness >4 mm was associated with a higher risk of residual disease (OR 7.30; 95% CI 1.88, 28.26; p=0.004). Cases with residual disease had primary tumours with a significantly larger diameter (median 14 mm, range 4-25) than those without residual disease (median 9 mm, range 2-60), (OR 1.07; 95% CI 1.03-1.11; p≤0.001) and were also more likely to be amelanotic (38% vs 14%), (OR 3.69; 95% CI 1.17, 11.60; p=0.026). Residual disease was associated with assessment of >3 slides of tissue (OR 6.98; 95% CI 1.54-31.62; p=0.0118) and complete blocking of the scar (OR 31.69; 95% CI 3.98-252.21; p=0.0011). Residual melanoma in WLE specimens is an infrequent occurrence. Risk factors for residual disease are LM/LMM and NM melanoma subtypes, higher mitotic rate, larger lesion diameter and amelanosis. Tumours with these features warrant more extensive pathological sampling. WLE after CEB for melanoma remains an important procedure to reduce local recurrence; however, limited pathological sampling of the WLE scar is probably appropriate for cases lacking high risk features.