Mitotic granuloma annulare: a clinicopathologic study of 20 cases

The finding of mitotic figures in granuloma annulare (GA) has not been emphasized in the literature. We describe 20 cases of a cellular, mitotically active variant of GA; we defined this group as cases having > or = 1 mitosis per 10 hpf. Clinically, the lesions could not be distinguished from typical, localized GA: there were 9 males and 11 females with a mean patient age of 49 +/- 15 years (mean +/- SD), compared to 45 +/- 20 years in a randomly selected control group of 60 patients with GA, and no unusual sites of predilection were noted. Histologically, a classic, palisading granuloma pattern predominated (18/20 cases). Lesions were located in the mid-dermis and tended to be more cellular than typical GA. The histiocytes comprising the lesion often had enlarged nuclei and prominent nucleoli. The number of mitoses per 10 hpf was 3.0 +/- 1.5 (range 1.0-7.2), control group 0.3 +/- 0.5; occasional atypical mitotic figures were observed. The proliferative nature of these lesions was confirmed using MIB-1 staining; the percentage of MIB-1 positive cells ranged from 5%-29% (mean 15 +/- 6%). Mitotic GA must be distinguished histologically from neoplastic processes, in particular epithelioid sarcoma. We conclude that histiocytes in clinically typical GA can exhibit an increased mitotic rate. Recognition of this variant is important in order to avoid overdiagnosis of a malignant condition.