Dermatofibroma (Fibrous Histiocytoma):

Clinical features:

Typically firm papule on extremities of young adults, may be pigmented

 Histologic features:Proliferation of fibroblasts centered in mid-dermis with storiform growth pattern

Spindle cells trap dense collagen into balls

Cells are plump with angular nuclei, mitoses may be present ；Epidermal hyperplasia common, sometimes with follicular or sebaceous induction

Histologic variants presenting diagnostic challenges:

Deep DF ;

Granular cell DF ;

Angiomatoid DF ;

DF with atypical cells (Atypical cutaneous fibrous histiocytoma) ;

Clinical features: Seen in Cowden's disease (multiple hamartoma syndrome) or sporadically Small pearly papules or nodules, any site

Histologic features: Well-circumscribed hypocellular bundles of collagen ;Collagen lies in parallel arrays, separated by spaces containing mucin (plywood grain pattern) ;IP staining +CD34, actin, vimentin

Epithelioid Cell Histiocytoma

Clinical features:

Typically polypoid lesions, most common on extremities of adults. Clinical impression often includes pyogenic granuloma, nevus, and basal cell carcinoma.

Histologic features:Polypoid and centered in papillary dermis, usually with epidermal collarette
Epidermis flattened with effaced rete ridges ;Even distribution of large angular epithelioid cells which tend to interdigitate ;Abundant eosinophilic cytoplasm, large eccentrically placed nuclei ;Minor component of spindle cells, abundant small blood vessels ;IP staining positive for factor XIIIa, vimentin, negative for S100, HMB45, CD34

Differential diagnosis:

1.Desmoplastic (dermal) Spitz nevus: nested, not interdigitating cells, S100 positive

2.Malignant melanoma: cellular atypia, epidermal involvement, S100 positive

3.Histiocytoid hemangioma: enlarged endothelial cells, CD31 positive

4.Reticulohistiocytoma: more abundant cytoplasm, secondary elements

5.Atypical fibroxanthoma: more atypia, abnormal mitoses, sun-damaged skin of elderly