Assessment of mitotic rate reporting in melanoma

Background: In patients with cutaneous melanoma, mitotic rate (MR) historically has been reported as the number of mitoses per high-power field (hpf) or per 10 hpf. The most recent revision of the American Joint Committee on Cancer melanoma staging system now incorporates MR and specifies that MR should be reported as mitoses per mm², with a conversion factor of 1 mm² equaling 4 hpf. However, because many pathologists continue to report MR in hpf units, we sought to compare the 2 conventions for reporting MR; this is important now that MR is used for staging and prognostic information. Methods: A retrospective analysis was performed of a database that combined patients from a large multicenter study and our single-institution melanoma database. All patients with pathology reports that included MR were included. For patients with MR reported in hpf units, MR was converted to mitoses per 10 hpf. Statistical analysis was performed to test differences in Breslow thickness (BT), ulceration, sentinel lymph node (SLN) status, and overall survival (OS) (log-rank test) between the mitoses per mm² group versus the mitoses per 10-hpf group. Results: A total of 1,148 patients were identified; of these, 759 were reported as per mm² and 389 were reported in hpf units. When patients were subdivided into categories of MR of 0, 1, or more than 1, there was no statistically significant difference in mean or median BT, ulceration, or SLN positivity within categories between patients with MR per mm² versus patients with MR reported per 10 hpf. There was also no difference in OS between groups. Subdividing into smaller categories of MR of 0, 1, 2, 3, 4, 5, or more than 5 did not yield different results. Conclusions: Although the American Joint Committee on Cancer staging system reports a conversion factor for MR of 1 mm² equals 4 hpf, no clinically meaningful differences in predictors of prognosis (BT, ulceration, SLN positivity) or OS were seen between groups when a conversion factor of 1 mm² equaling 10 hpf was used. Therefore, for practical purposes, MR reported per 10 hpf approximates MR per mm².