Dermatofibroma (superficial benign fibrous histiocytoma) is a common cutaneous nodule of unknown etiology that occurs more often in women. Dermatofibroma frequently develops on the extremities (mostly the lower legs) and is usually asymptomatic, although pruritus and tenderness can be present. It is actually the most common painful skin tumor.  A number of well-described, histologic subtypes of dermatofibroma have been reported. Removal of the tumor is usually not typically required unless diagnostic uncertainty exists or particularly troubling symptoms are present.
This article discusses primarily cutaneous (superficial) dermatofibroma. Subcutaneous (deep) benign fibrous histiocytomas are also well documented and may have a more aggressive clinical course, as can tumors displaying cellular, aneurysmal (hemosiderotic), and atypical histologic variants of dermatofibroma.
In addition, benign fibrous histiocytomas are reported in bone, orbital, airway, gastrointestinal, splenic, genitourinary, and intracranial locations.
The precise mechanism for the development of dermatofibroma is unknown. Rather than a reactive tissue change, evidence that dermatofibroma may be a neoplastic process is demonstrated by its clonal proliferative growth.  Clonality, by itself, is not necessarily synonymous with a neoplastic process; it has been demonstrated in inflammatory conditions, including atopic dermatitis, lichen sclerosis, and psoriasis. Dermatofibroma tumorigenesis may be due to distorted protein kinase C activity. 
Results from immunohistochemical testing with antibodies to factor XIIIa, which label dermal dendritic cells, are frequently positive in dermatofibroma, while antibodies to MAC 387, which label monocyte-derived macrophages (histiocytes), show less consistent results. One study evaluated the expression in dermatofibroma of HSP47, a recently used marker for skin fibroblasts; CD68, a marker for histiocytes; and factor XIIIa. Most of the spindle-shaped cells in all 28 cases of dermatofibroma, irrespective of histologic variant, stained positively with HSP47, indicating that skin fibroblasts are a major constituent of dermatofibroma. Factor XIIIa–positive dendritic cells also are present, but the presence of CD68-positive histiocytes was inconsistent, especially between histologic variants.  CD14+ monocytes have been proposed as the cell of origin of dermatofibromas. 
The cell surface proteoglycan, syndecan-1,  and fibroblast growth factor receptor 2, involved in epithelial-mesenchymal cross-talk,  may play a role in the growth of dermatofibromas. Transforming growth factor-beta (TGF-beta) signaling might be a trigger of the fibrosis seen in dermatofibromas.  TGF-beta, along with other fibrinogenic factors, may be produced by mast cells, which have been reported to occur in abnormally high numbers in dermatofibromas. 
Gene fusions have been described in dermatofibroma tumorigenesis. [10, 11] ALK gene rearrangement and overexpression has been demonstrated in the epithelioid and atypical dermatofibroma variants. [12, 13]
Dermatofibromas are relatively common.
Frequency of dermatofibroma appears to be similar in all races.
Dermatofibroma can occur in patients of any age. In one series, 80% of biopsy specimens were from patients aged 20-49 years,  and in another the mean age of the patients was 42.18±13.72 years. 
Typical superficial dermatofibromas are considered benign lesions, and the prognosis for patients with this condition is excellent. However, discomfort from pain or itching may be significant.
Deep, cellular, aneurysmal (hemosiderotic), and atypical variants, which are notoriously more locally recurrent (≤20% of cases), can rarely metastasize. [16, 17, 18] (The deep subset of dermatofibroma that arises in the subcutaneous or deep soft tissue may undergo further classification.  ) Such variants, or any indeterminant dermatofibroma, might be regarded as potentially malignant lesions.  In these exceptional cases, pulmonary and nodal metastases were most commonly seen, some patients developed multiple satellite nodules, and deaths have occurred.  The primary tumors tended to be large and cellular, but aggressive behavior is not entirely predictable, although early or frequent recurrences of the tumor should raise concern.  Array-based comparative genomic hybridization (CGH) may prove useful in identifying these higher-risk variants. [21, 22, 17]
The epithelioid variant has also been reported to metastasize, although demonstration of ALK gene rearrangement may indicate it is a biologically distinct tumor. 
In a study of common dermatofibromas with an increased mitotic rate but no other worrisome features, none recurred or metastasized. 
Spontaneous regression has been reported,  and this may yield postinflammatory hypopigmentation, although this appears to be quite rare.