Breslow Thickness Measurements of Melanomas Around American Joint Committee on Cancer Staging Cut-Off Points: Imprecision and Terminal Digit Bias Have Important Implications for Staging and Patient Management

Breslow thickness is the most important prognostic factor in patients with clinically localized primary cutaneous melanomas, and its accuracy has important implications for staging and management. A review of the Melanoma Institute Australia database and population-based data for the state of New South Wales, Australia, found an unexpectedly large number of melanomas reported as being exactly 1.0 mm thick. We sought to determine possible causes for this biologically implausible finding. The tumor thickness of 125 invasive cutaneous melanomas with a recorded Breslow thickness of 0.9-1.1 mm was remeasured and recorded by two pathologists. Concordance of measurements between the two pathologists was high (intraclass correlation coefficient 0.816, 95 % CI 0.733-0.873). The original measurements showed clustering at 0.9, 1.0, and 1.1 mm, whereas the review measurements did not. The original measurements staged 84 cases (72 %) as T1 and 33 (28 %) as T2, while the reviewed measurements staged 58 cases (50 %) as T1 and 59 (50 %) as T2 (p < 0.001). Our study demonstrated imprecision in Breslow thickness measurements and its significant impact on staging. Two potential sources of imprecision are failure to follow standardized thickness measurement guidelines and the phenomenon of terminal digit bias, not previously identified as a problem in this field. Educating pathologists about this phenomenon and the importance of utilizing ocular micrometers may improve the precision of melanoma thickness measurements around critical staging cut-off points. Clinicians must also be educated to appreciate that there is an inevitable margin of error with Breslow thickness measurements that should be considered when making management decisions.