A retrospective, multicenter analysis of the predictive value of mitotic rate for sentinel lymph node (SLN) positivity in thin melanomas

Background: There is a paucity of studies to substantiate whether the presence of a single mitosis justifies sentinel lymph node (SLN) biopsy (SLNB) in thin melanomas. Objective: We sought to determine if mitotic rate is associated with SLNB outcome when taking into account other prognostic factors. Methods: All cases of melanoma that underwent SLNB in the province of Alberta, Canada, between 2007 and 2013 were reviewed through a provincial tumor database. Results: A total of 1072 patients fulfilled inclusion criteria. When analyzing all melanomas regardless of thickness, mitotic rate was a good predictor of SLN status. When stratified by Breslow thickness, only intermediate melanomas (1.01-2.0 mm) demonstrated a significant relationship between mitotic rate and positive SLN status (P = .010). For melanomas 1 mm or smaller, mitotic rate was not associated with SLN status. A statistically significant interaction was identified between Breslow thickness and mitotic rate such that for decreasing Breslow depth, the effect of mitotic rate on SLNB status diminished (P = .028). Limitations: The study was retrospective in nature. There is underlying variability in mitotic rate reporting methods over time, and between different dermatopathologists. Conclusions: Mitotic rate does not have unequivocal utility in predicting SLNB status in thin melanomas. There is a significant interaction between mitotic rate and Breslow depth, such that the predictive value of mitotic rate on SLN positivity may be dependent on Breslow thickness.