Myxoid dermatofibrohistiocytoma: an indolent post-traumatic tumor composed of CD34+ epithelioid and dendritic cells and factor XIIIa+ dendrophages.

Factor XIIIa+ dendrophages and CD34+ "deep dermal dendrocytes" are distinct subsets of embryonic dermal dendritic stem cells that persist in interstitial and adventitial sites in adult dermis. We encountered a unique myxoid dermal tumor composed of these two cell types. It arose after trauma to the thumb of a 49-year-old man and was locally excised. The patient is without recurrence at 18 months. The disc-shaped tumor was lobulated, yellow, and mucoid and involved the margins. A fibrillar myxoid stroma contained mast cells, wispy collagen with medium-to-small vessels, and loosely deployed small eosinophilic tumor cells. The tumor cells were amitotic and had oval or bean-shaped, bland nuclei; some cells were binucleated. The cells were epithelioid or dendritic with bipolar, stellate, and racquet-shaped cytosomes whose tapering cell processes blended with fibrillar collagen. Vacuolated epithelioid cells focally formed vessel-like luminal structure. All cells strongly expressed vimentin. Thirty percent of the tumor cells were elongated, dendritic factor XIIIa+ cells whose dendritic processes enshrouded mast cells or FXIIIa-negative tumor cells. A subset of the FXIIIa+ cells also expressed MAC387 and lysozyme. The other 70% of the cells were CD34+. Many CD34+ cells were epithelioid with strong membrane and vacuolar decoration. Some CD34+ epithelioid cells had globular cytoplasmic inclusions. Other CD34+ cells were dendritic with multipolar fibrobroblast-like cytosomes and weaker CD34+ membrane decoration. Actin and S-100 were negative. Ki 67 was expressed in 1% of the tumor cells. Double stains for CD34 and Ki 67 showed that both CD34+ cells and FXIIIa+ dendrophages were Ki 67+, as were many papillary dermal vessel endothelial cells. The composition of the tumor by mast cells, FXIIIa+ dendrophages, and CD34+ primitive cells recapitulates the dermal microvascular unit. We propose the descriptive term myxoid dermatofibrohistiocytoma (MD) for this novel tumor. It appears to be an unusual response by dermal dendritic cells, perhaps due to continued stimulation by post-traumatic cytokines. Clarification of its biology and nosology awaits identification and study of more cases.