Impact of the 2009 AJCC staging guidelines for melanoma on the number of mitotic figures reported by dermatopathologists at one institution.

Background: In 2009 the revised seventh staging system for melanoma recommended the use of mitotic count to separate stage T1a from T1b. However, careful scrutiny of cases may lead to an inadvertent selection effect, with consequent increased reporting of mitotic counts.

Methods: We investigated whether there is a significant increase in mitotic counts reported since 2009 for melanomas with a Breslow thickness of 1.0 mm or less. We conducted a retrospective, case-controlled study examining invasive melanoma cases at a large academic center. Mitotic counts were compared between pathology reports before 2009 (n = 61) and after 2009 (n = 125), with a subset of slides re-examined in a blinded fashion.

Results: Before the 2009 staging guidelines, 51% of cases had one or more mitosis reported compared to 38% after 2009 (p = 0.113). Blinded re-counting did not yield a significant difference when compared with the original pathology reports in either group.

Conclusions: There was not a significant difference in the number of mitoses reported after the implementation of the new guidelines.