Assessment of tumor mitotic rate in primary cutaneous malignant melanomas 1 mm or less in thickness

Background Tumor mitotic rate in thin melanomas is recognized as a powerful, independent prognostic factor predicting survival. In nonulcerated cases, the presence of any dermal mitotic activity upstages the disease to pT1b. The extent to which tissue should be histologically examined to assess mitogenicity, however, has not been studied. Objective We sought to determine whether in staging thin melanomas, there is a significant benefit in examining numerous tissue sections containing invasive disease. Method In all, 71 cases of thin cutaneous melanomas diagnosed between January 2012 and June 2013 were identified after a search performed on the Pathlab database. The slides were retrieved and reviewed retrospectively, comparing the identification of the first dermal tumor mitotic figure, if present, at 4 check-points: the first, third, fifth, or tenth tissue section examined. Results A statistically significant difference in identification of the first dermal mitotic figure was found in examining 1 versus 3 tissue sections ( P  = .0411). No significant difference was found in examining numerous tissue sections. Limitations This was a retrospective study from a single institution with a limited number of participants. Conclusion In staging thin melanomas without ulceration, the optimal number of sections to assess is 3. No additional benefit is gained by examining numerous tissue sections.