Combined dermatofibroma: co-existence of two or more variant patterns in a single lesion

Based on a series of 25 cases, we define and characterize combined dermatofibroma, a tumour comprising two or more variant patterns of dermatofibroma in a single lesion.

Dermatofibroma may present with a wide variety of architectural, cellular or stromal peculiarities. Architectural peculiarities include deep penetration, atrophy, collarette formation, fascicular to plexiform architecture, massive haemorrhage, prominent haemangiopericytoma-like vascularity and palisading; cellular peculiarities the presence of epithelioid cells, clear cells, granular cells, prominent myofibroblastic differentiation and atypical giant cells ('monster cells'); or stromal peculiarities such as prominent sclerosis, mucin, haemosiderin and cholesterotic deposits. In combined dermatofibromas two or more of these features are seen in complex or inhomogenous combination such as the silhouette of a deep penetrating dermatofibroma with an 'ordinary' storiform pattern in the upper and granular cell differentiation in the lower part of the lesion; or a dermatofibroma with ordinary features in the upper, prominent sclerosis in the middle and clear cells in the lower portion of the lesion; or the characteristic epidermal collarette and cells of epithelial cell histiocytoma with a plexiform ('neurothekeoma-like') architecture surrounded by a myxoid stroma with spindle-shaped to stellate cells. Clinically, these lesions preferentially occur on the lower extremities of young to middle-aged females, frequently with the diagnosis of a fibrohistiocytic lesion. Apart from one recurrence follow-up was uneventful in all other cases. Immunohistochemically, lesions are consistently positive with KiM1p, variably positive for factor XIIIa, smooth muscle specific actin and with KP1 (CD68), NK1C3 and E9.

Recognition of combined dermatofibroma allows the histopathologist to apply a confident benign label to unusual lesions which might otherwise elude diagnosis, or tempt description of 'new' entities and to avoid a misdiagnosis of malignancy.