Melanoma staging: Varying precision and terminal digit clustering in Breslow thickness data is evident in a population-based study

Abstract

Background: Errors in Breslow thickness reporting can give misclassification of T category, an important classifier in melanoma staging.

Objective: We sought to investigate precision (number of digits) and terminal digit clustering in Breslow thickness and potential consequences for T category.

Methods: All first primary and morphologically verified invasive melanomas in Norway between 2008 and 2015 were included. A smoothing model was fitted to estimate the underlying Breslow thickness distribution without digit clustering.

Results: Thickness was reported for 13,057 (97.5%) patients; the median was 1.0 mm (range, 0.09-85). It was reported as whole numbers (15.6%), to 1 decimal (78.2%) and 2 decimal places (6.2%)-thin tumors with more precision than thick tumors. Terminal digit clustering was found with marked peaks in the observed frequency distribution for terminal digits 0 and 5, and with drops around these peaks. Terminal digit clustering increased proportions of patients classified with T1 and T4 tumors and decreased proportions classified with T2 and T3.

Limitations: Breslow thickness was not reported in 2.5% of cases.

Conclusions: The Norwegian recommendation of measurement to the nearest 0.1 mm was not followed. Terminal digit clustering was marked, with consequences for T category. Pathologists, clinicians, and epidemiologists should know that clustering of thickness data around T category cut points can impact melanoma staging with consequent effect on patient management and prognosis.

Keywords: Breslow thickness; T category; melanoma; misclassification; precision; terminal digit clustering.