The number of benign moles excised for each malignant melanoma: the number needed to treat


  • Conflict of interest: none declared.

Dr Susana Sidhu, The Welsh Institute of Dermatology, University Hospital of Wales, Cardiff, CF14 4XW, UK


Background.  The ratio of benign moles excised for each malignant melanoma (MM) diagnosed, i.e. the number needed to treat (NNT), may be a useful indicator of diagnostic accuracy and the efficient use of healthcare resources, and may have personal implications for the patient.

Aim.  To assess the NNT for a group of consultant dermatologists serving a population of 600 000, and to compare this with similar studies from other countries.

Methods.  This was a retrospective analysis of data on pigmented lesions excised over a 5-year period (2005–2009). The lesions were divided into three groups: benign naevi (BN), dysplastic naevi (DN) and MM. The NNT ratio was calculated as (BN + DN + MM)/MM.

Results.  In total, 4691 lesions were examined. The overall mean NNT was 6.3, with a range of 4.9–11.3 for each of nine consultant dermatologists. The mean NNT was 7.6 for female and 4.8 for male patients. There were more patients with BN (n = 3534; 75%) than with DN (n = 407; 9%) or MM (n = 750; 16%). The gender representation was similar in the DN and MM groups, but had a disproportionately female bias in the BN group (67% female, 33% male patients). Overall, there were more female patients in all three groups [2962 female patients (63%) and 1729 male patients (37%)].

Conclusions.  The NNT of 6.3 in this study compares favourably with NNT ratios from studies of dermatologists from other countries. This study may encourage other countries and individual doctors to assess their NNT ratios, as it may be an important indicator of the efficient use of resources and the avoidance of unnecessary surgery for patients.