Mitosis in dermatofibroma: a worrisome histopathologic sign that does not necessarily equal recurrence

Background:  High mitotic rate is a worrying histopathologic feature that pathologists occasionally face when diagnosing benign fibrous histiocytoma (BFH), that otherwise seem to be benign. The clinical significance of this histologic feature has not been completely studied, because no studies on large series of BFH have been carried out, concerning mitotic rate as the only worrisome morphologic sign, in relation with recurrence. Methods:  In order to study how common a high mitotic rate is in BFH, which do not present any other worrisome morphological signs, we studied 200 cases from our archives and counted the number of mitosis per 10 high power fields (HPFs) in all of them. We selected those cases that had a mitotic number of 4 or more mitosis per 10 HPF. We also performed the immunohistochemical study with MIB-1 antibody (DakoCytomation, Denmark A/S). The clinical histories of the patients were also reviewed, in order to find out if there had been any recurrence. Results:  Four BFH presented four or more mitosis in ten HPF. None of them had presented any recurrence or metastasis, and the follow-up time was at least for 71 months. Conclusions:  We conclude that common BFH can occasionally present a high mitotic rate and still show a benign behavior.