Predicting sentinel node positivity in melanoma patients: external validation of a risk-prediction calculator (the MIA nomogram) using a large European population-based patient cohort.

Background A nomogram to predict SN-positivity (the Melanoma Institute Australia (MIA) nomogram) was recently developed and externally validated using two large single-institution databases. However, there remains a need to further validate the nomogram's performance using population-based data. Objectives This study sought to address this using a European national patient cohort. Methods Cutaneous melanoma patients who underwent SN biopsy in the Netherlands between 2000 and 2014 were included. Their data were obtained from the Dutch Pathology Registry (PALGA). The predictive performance of the nomogram was assessed by discrimination (C-statistic) and calibration. Negative predictive values (NPV) were calculated at various predicted probability cut-offs. Results Of the 3049 patients who met the eligibility criteria, 23% (691) were SN-positive. Validation of the MIA nomogram (included parameters: Breslow thickness, ulceration, age, melanoma subtype and lymphovascular invasion) showed a good C-statistic of 0.69 (95% CI 0.66-0.71) with excellent calibration (R2 0.985, p=0.399). The NPV of 90.1%, found at a 10% predicted probability cut-off from having a positive SN biopsy, implied that by using the nomogram, a 16.3% reduction in the rate of performing a SN biopsy could be achieved with an error rate of 1.6%. Validation of the MIA nomogram considering mitotic rate as present/absent showed a C-statistic of 0.70 (95% CI 0.68-0.73). Conclusions This population-based validation study in European melanoma patients confirmed the value of the MIA nomogram in predicting SN-positivity. Its use will spare low-risk patients the inconvenience, cost and potential risks of SN biopsy while ensuring that high-risk patients are still identified.