Importance: Use of digital whole-slide imaging (WSI) for dermatopathology in general has been noted to be similar to traditional microscopy (TM); however, concern has been noted that WSI is inferior for interpretation of melanocytic lesions. Since approximately 1 of every 4 skin biopsies is of a melanocytic lesion, the use of WSI requires verification before use in clinical practice. Objective: To compare pathologists' accuracy and reproducibility in diagnosing melanocytic lesions using Melanocytic Pathology Assessment Tool and Hierarchy for Diagnosis (MPATH-Dx) categories when analyzing by TM vs WSI. Design, Setting, and Participants: A total of 87 pathologists in community-based and academic settings from 10 US states were randomized with stratification based on clinical experience to interpret in TM format 180 skin biopsy cases of melanocytic lesions, including 90 invasive melanoma, divided into 5 sets of 36 cases (phase 1). The pathologists were then randomized via stratified permuted block randomization with block size 2 to interpret cases in either TM (n = 46) or WSI format (n = 41), with each pathologist interpreting the same 36 cases on 2 separate occasions (phase 2). Diagnoses were categorized as MPATH-Dx categories I through V, with I indicating the least severe and V the most severe. Main Outcomes and Measures: Accuracy with respect to a consensus reference diagnosis and the reproducibility of repeated interpretations of the same cases. Results: Of the 87 pathologists in the study, 46% (40) were women and the mean (SD) age was 50.7 (10.2) years. Except for class III melanocytic lesions, the diagnostic categories showed no significant differences in diagnostic accuracy between TM and WSI interpretation. Discordance was lower among class III lesions for the TM interpretation arm (51%; 95% CI, 46%-57%) than for the WSI arm (61%; 95% CI, 53%-69%) (P =.05). This difference is likely to have clinical significance, because 6% of TM vs 11% of WSI class III lesions were interpreted as invasive melanoma. Reproducibility was similar between the traditional and digital formats overall (66.4%; 95% CI, 63.3%-69.3%; and 62.7%; 95% CI, 59.5%-65.7%, respectively), and for all classes, although class III showed a nonsignificant lower intraobserver agreement for digital. Significantly more mitotic figures were detected with TM compared with WSI: mean (SD) TM, 6.72 (2.89); WSI, 5.84 (2.56); P =.002. Conclusions and Relevance: Interpretive accuracy for melanocytic lesions was similar for WSI and TM slides except for class III lesions. We found no clinically meaningful differences in reproducibility for any of the diagnostic classes.
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