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Comment on: Effect of delay between nuclear medicine scanning and sentinel node biopsy on outcome in patients with cutaneous melanoma

Editor We read the article investigating delay between nuclear medicine scanning and sentinel node biopsy for melanoma with interest1, along with the related single-centre study2. The effect of delay between lymphoscintigraphy and surgery was studied, split at 12 h to define delayed or non-delayed scanning. The aim of the study is admirable, although we have concerns regarding the analysis and subsequent conclusions. First, dichotomizing ‘time’, a continuous variable, reduces power and overly simplifies associations between timing and surgery. For example, is there an optimal time window? Was a standard enhanced Tc99 dose used in ‘delayed’ scans (we use a 60 MBq dose for daybefore scans)? How was the relationship between Patente Bleu dye tagging and radiation tagging affected by delay? And can excessively reducing the time window to surgery potentially have unrecognized disadvantages? Second, were there systematic differences between the delayed and non-delayed cases that confound the observed association, such as surgical teams, nuclear medicine departments or centre? Interestingly, centre was included as a stratifying variable rather than a model covariate. The authors state ‘there were no survival differences between the hospitals’ without supporting data. While this suggests it would not violate the proportionality of hazards assumption, it is striking that one centre had 99⋅1 per cent delayed scans and another 98⋅4 per cent nondelayed scans. The multivariable model was changed from 2018: ‘ulceration’ was added without explanation. If that covariate was added, why was centre not? We believe this is important as the Gothenburg site may have had more differences besides a higher overall ulceration rate, higher Breslow thickness and with 99 per cent in the delayed group. Finally, the variation explained by the model is not clear, and could be described with regularized r squared: some associations may be consistent (leading to statistical significance) but not make a large impact (account for little variance).