Morphologic Forms and Classification of Dermal Mitotic Figure Density in Primary Cutaneous Melanoma: A Retrospective Study.

New American Joint Committee on Cancer eighth edition staging parameters have removed mitotic rate as a stage T1 category criterion, but it remains embedded in the synopsis of primary cutaneous melanoma (CM). A paucity of data is available, characterizing atypical mitotic forms in CM. In this study, we classify the various morphologic forms of atypical mitoses, characterize mitotic figure density, and examine the correlation between atypical mitotic figures and Breslow depth. We performed a retrospective study of 185 thick (>0.8 mm) and thin (<0.8 mm) CM specimens. Metaphase mitotic figures represented the highest percentage of total mitotic figures in cases of thick melanoma (40%) and were the second most common in thin melanoma (18%). The average Breslow depth for melanoma harboring starburst mitoses was 2.85 mm, compared with the average Breslow depth of all thick melanoma cases, 1.88 mm. The average thickness of melanoma cases containing tripolar mitoses was 2.28 mm. Breslow depth correlated with the number of atypical mitotic figures in both thick and thin melanomas (the Pearson correlation test, r = +0.18, P < 0.01). Metaphase and prophase mitoses are a common finding in both thick and thin melanomas. Although atypical mitoses were indiscriminate, starburst and tripolar (ie, multipolar) mitoses were only inherent to cases of thick melanoma (stage T3). In sum, our study reveals a parallel relationship between the density of atypical mitotic figures and Breslow depth.