Myxoid dermatofibroma

This report presents seven cases of a rare variant of dermatofibroma, characterized by marked mucinous stromal changes.

In this retrospective clinicopathological study, lesions preferentially occurred on the lower extremities of young to middle-aged females. Clinical (differential) diagnoses included: dermatofibroma (two cases), cyst, tophus, ganglion and neurofibroma. Histology revealed well circumscribed, faintly stained dermal to subcutaneous lesions which contained abundant stromal mucin. A variable amount of spindle-shaped cells, occasional lymphocytes and fine collagen fibres were also seen. Epidermal hyperplasia above as well as storiform arrangement of spindle cells and some sclerotic collagen at the periphery of the lesion indicated the fibrohistiocytic origin. Individual histopathological features included: bizarre giant cells, perifollicular arrangement and seas of mucin with pseudovascular clefts in one case each. Immunohistochemically, all lesions were positive for KiM1p (> 50% of the cells). Moreover, two lesions showed moderate reactivity for factor XIIIa (10-25%), one each with NK1C3 (> 90%) and smooth muscle actin (50%), but were otherwise negative with a broad panel of markers. Electron microscopy in one case revealed large pools of mucin interspersed by a moderate number of fibrocytes with fine fibrillary collagen.

This series confirms myxoid dermatofibroma as a rare variant of fibrohistiocytic tissue response; our series also indicates that many of these lesions might be misinterpreted as cutaneous myxoma.