P144 Diagnostic accuracy in melanoma: the number needed to excise, a single-centre experience

Abstract The number needed to excise (NNE) is an often-reported metric for assessing diagnostic accuracy in melanoma. It quantifies how many pigmented lesions need to be excised to diagnose one melanoma, with a lower NNE indicating fewer unnecessary excisions. Reported NNE values vary significantly across different clinical settings, ranging from as high as 83 for general practitioners to between 2.7 and 15 for experienced dermatologists. The malignant melanoma to melanoma in situ (MM:Mis) ratio is an additional measure that provides insight into how early melanomas are detected. By measuring both, we can gauge the ability to distinguish melanoma from benign lesions and the sensitivity in diagnosing early-stage melanoma. We set out to assess our department’s accuracy in diagnosing melanoma using NNE and MM:Mis. All atypical pigmented lesions excised or biopsied in a single centre in the Mid-West of Ireland over a 12-month period were retrospectively reviewed and the NNE and MM:Mis ratio calculated. From January to December 2023, 319 atypical melanocytic lesions were excised or biopsied. Of these, 69 were melanomas, 12 of which were of the lentigo maligna subtype. There were 51 cases of melanoma in situ or lentigo maligna, 60 dysplastic naevi and 139 benign naevi (including 10 Spitz naevi). The NNE was 2.66 and the MM:Mis was 2.1. The NNE from our review compares favourably with those reported in the literature, indicating a high level of diagnostic accuracy. Continuous evaluation and improvement of diagnostic accuracy in melanoma are crucial for optimizing resource use and minimizing patient morbidity. The ongoing challenge lies in reducing the excision of benign naevi while ensuring that no malignancies are overlooked.