Dermatofibroma (benign fibrous histiocytoma)

INTRODUCTION — Dermatofibroma, also known as benign fibrous histiocytoma, is a common soft tissue lesion due to the proliferation of benign spindle cells within the dermis [1]. Dermatofibromas usually occur on the extremities and are more common in females. Various histologic subtypes of dermatofibromas exist. Treatment is usually not necessary.

This topic will discuss the clinical manifestations, diagnosis, and treatment of dermatofibroma. An overview of other benign skin lesions is provided separately. (See "Overview of benign lesions of the skin".)

EPIDEMIOLOGY — Dermatofibromas are one of the most common cutaneous lesions. The exact prevalence is difficult to measure since most patients do not seek care for these lesions due to their asymptomatic nature. Dermatofibromas usually appear in patients between 20 to 40 years of age (with a female predominance) but have been reported in all age groups [2,3].

PATHOGENESIS — The exact pathogenesis of dermatofibromas is debated and unknown. Most dermatofibromas are thought to be a reactive process secondary to local trauma including, but not limited to, insect bites, puncture wounds due to splinters, and ingrown hairs. However, some authors also believe that dermatofibromas are a spontaneous, benign, neoplastic process with evidence of clonal proliferation [1].

CLINICAL PRESENTATION — Dermatofibromas present as slow growing, pink or tan/brown to black, firm, dermal papules usually measuring <2 cm in diameter, most often located on the limbs (picture 1A) [4]. In individuals with darkly pigmented skin, dermatofibromas appear as dark brown to black papules (picture 1B). Rarely, a dermatofibroma may appear as an atrophic, depressed lesion [5].

The age of onset is usually in early to mid-adulthood, but dermatofibromas may also occur in children [3]. In most cases, dermatofibromas are solitary lesions, although multiple lesions can occur [2,6]:

●Multiple dermatofibromas – Multiple dermatofibromas are defined as >15 discrete lesions appearing in a few months on the extremities and trunk. Multiple dermatofibromas have been associated with underlying systemic diseases, including autoimmune diseases, malignancies, and human immunodeficiency virus (HIV) infection; immunosuppressive or immunomodulating therapies; and pregnancy [7,8].

●Multiple eruptive dermatofibromas – In rare cases, multiple dermatofibromas can appear over a period of a few months in different areas of the body [8]. In most cases, they are associated with an underlying autoimmune disorder or immunodeficiency [9-11], but they can be idiopathic [12,13].

●Multiple clustered dermatofibromas – Multiple clustered dermatofibromas are an extremely rare clinical variant of dermatofibroma. They present as a plaque composed of multiple (>15) reddish to hyperpigmented papules, most often located on the lower extremities or trunk [6,14]. There are several reports of congenital multiple clustered dermatofibromas [15-18].

HISTOPATHOLOGY — On routine histopathologic examination, common dermatofibromas show dermal proliferation of uniform spindle cells (fibroblast like and histiocyte like) separated from the overlying epidermis by a clear grenz zone. A variable mononuclear inflammatory infiltrate is also present. Collagen trapping may be noted in the periphery of the lesion. The overlying epidermis shows acanthosis with hyperpigmentation of the basal layer (dirty feet sign).

Immunohistochemical staining is positive for factor XIIIa, SMA, and vimentin and negative for CD34 and S100 (table 1).

RARE CLINICOPATHOLOGIC VARIANTS

Cellular dermatofibroma — Cellular dermatofibroma is an uncommon variant of dermatofibroma, representing less than 5 percent of dermatofibromas [19]. It is usually larger in size than the common dermatofibroma and may resemble early dermatofibrosarcoma protuberans (see "Dermatofibrosarcoma protuberans: Epidemiology, pathogenesis, clinical presentation, diagnosis, and staging"). Histologically, cellular dermatofibroma shows dense proliferation of fibrohistiocytic cells that display a swirling, storiform pattern with extension into the subcutaneous fat. Fat necrosis associated with lymphocytic infiltrate is often noted [20]. Cellular dermatofibroma has a higher risk of recurrence (approximately 10 percent) if incompletely excised [21].

Epithelioid dermatofibroma — Epithelioid dermatofibroma clinically presents as a polypoid, red papule or nodule with an epidermal collarette that may mimic amelanotic melanoma or nonulcerated pyogenic granuloma. Histologically, epithelioid dermatofibroma is composed of large, angulated, epithelioid cells with abundant eosinophilic cytoplasm that may be confused with an intradermal Spitz nevus [22]. However, in contrast with Spitz nevus, immunohistochemical staining of epithelioid dermatofibroma with S100 is always negative. Several studies have demonstrated ALK rearrangements and ALK overexpression in most epithelioid dermatofibromas, suggesting that they may be biologically distinct entities [23-25]. (See "Spitz nevus and atypical Spitz tumors", section on 'Immunohistochemistry'.)

Aneurysmal/hemosiderotic dermatofibroma — Aneurysmal dermatofibromas present as brown/blue papules, usually located on the lower legs. They may grow rapidly, ulcerate, and bleed. The clinical diagnosis is difficult, as these lesions can mimic melanoma, vascular tumors, or adnexal tumors (among others) [26]. Histologically, aneurysmal dermatofibroma is a spindle-cell tumor with cleft-like, blood-filled spaces; focal hemorrhage; and hemosiderin deposition [22]. Hemosiderotic dermatofibroma is considered by some an early stage of aneurysmal dermatofibroma. The lesion is composed of numerous small vessels, extravasated red blood cells, and intra- and extracellular hemosiderin deposition.

Other — Other uncommon variants of dermatofibroma include:

●Lipidized dermatofibroma – Lipidized dermatofibroma presents as an exophytic, yellowish nodule, usually larger than common dermatofibromas and most often located on the lower legs (ankle-type fibrous histiocytomas) [27]. Histologically, this variant demonstrates foamy histiocytes with hyalinization of the collagen. There are no reports of recurrence after excision.

●Atypical dermatofibroma (dermatofibroma with monster cells) – Atypical dermatofibroma is a rare histologic variant characterized by large, pleiomorphic cells with large, bizarre, and hyperchromatic nuclei (also referred to as "monster cells").

●Metastasizing dermatofibroma – Metastasizing benign dermatofibroma is an exceedingly rare variant that may have the clinical and pathologic features of cellular, aneurysmal, or atypical dermatofibroma [28-30]. These lesions are usually larger than common dermatofibromas (>3 cm) and tend to recur locally after excision. In a series of 16 cases, metastases involved the lungs, lymph nodes, soft tissues, and liver, and six patients died of disease [28].

DIAGNOSIS — The diagnosis of dermatofibromas is usually made clinically, based on clinical appearance and history. If the lesion is longstanding, it should have no history of rapid change. Dermatofibromas characteristically exhibit the "dimple sign," where the lesions dimple centrally when lateral pressure is exerted (picture 2A-B).

Dermoscopic examination may support the clinical diagnosis. It typically shows a peripheral, delicate pigment network with a central, scar-like area (picture 3).

Rapidly changing lesions, symptomatic lesions, and lesions showing atypical features should be biopsied to rule out atypical variants or other neoplasms. (See 'Rare clinicopathologic variants' above and 'Differential diagnosis' below.)

DIFFERENTIAL DIAGNOSIS — The clinical differential diagnosis for dermatofibromas is broad and includes the following:

●Dermatofibrosarcoma protuberans – Dermatofibrosarcoma protuberans is a locally aggressive, slow growing tumor that can be mistaken for a benign dermatofibroma (especially at early stages) (picture 4A-B). Histologic differentiation is key as dermatofibrosarcoma protuberans has a higher mitotic rate and displays positive staining for CD34 and negative staining for factor XIIIa (opposite of a benign dermatofibroma) (table 1). Dermatofibrosarcoma protuberans lesions tend to be large, with an average size >5 cm. (See "Dermatofibrosarcoma protuberans: Epidemiology, pathogenesis, clinical presentation, diagnosis, and staging".)

●Intradermal nevi – Intradermal nevi (picture 5) are benign, melanocytic tumors that are softer than dermatofibromas and do not exhibit the "dimple sign" when pinched. (See "Acquired melanocytic nevi (moles)".)

●Basal cell carcinoma – Early nodular basal cell carcinoma (BCC) typically presents as a pink, skin-colored, or pigmented papule, most often located on the face (picture 6A-B). Dermoscopy shows arborizing vessels, blue-gray globules and dots, and small erosions (picture 7). (See "Basal cell carcinoma: Epidemiology, pathogenesis, clinical features, and diagnosis".)

TREATMENT — No treatment is usually required for asymptomatic, static, common dermatofibromas. Surgical excision is not advised for common dermatofibromas, as it results in a scar that will likely appear worse than the original lesion. For patients who desire treatment for cosmetic concerns, we suggest cryotherapy with liquid nitrogen rather than surgical excision. Cryotherapy has been tried with some success for protuberant lesions with resolution or flattening of the lesion with good cosmetic results [31,32].

Atypical variants have a higher risk of recurrence, and a complete excision with clear margins should be performed. (See 'Rare clinicopathologic variants' above.)

PROGNOSIS — Dermatofibromas are usually benign, static, asymptomatic lesions that exhibit little to no growth. Spontaneous resolution is rarely reported and is the exception.

Atypical variants (eg, cellular, epithelioid, aneurysmal) may recur locally after surgical excision and, in exceptionally rare cases, metastasize.

SUMMARY AND RECOMMENDATIONS

●Definition – Dermatofibroma, also known as benign fibrous histiocytoma, is a common soft tissue lesion due to the proliferation of benign spindle cells within the dermis.

●Clinical presentation – Dermatofibromas present as slow growing or static, pink or tan/brown to black, firm, dermal papules usually measuring <2 cm in diameter, most often located on the limbs (picture 1A-B). Lesions are usually solitary. The development of multiple lesions has been associated with autoimmune disorders, immunodeficiency, or malignancy. (See 'Clinical presentation' above.)

Rare clinicopathologic variants of dermatofibroma include cellular, epithelioid, aneurysmal/hemosiderotic, lipidized, and atypical dermatofibromas. (See 'Rare clinicopathologic variants' above.)

●Diagnosis – The diagnosis is usually made clinically, based on clinical appearance and history. Dermatofibromas characteristically exhibit the "dimple sign," where the lesions dimple centrally when lateral pressure is exerted (picture 2A-B). Dermoscopy typically shows a peripheral, delicate pigment network with a central, scar-like area (picture 3). Rapidly changing, symptomatic, or atypical lesions should be biopsied to rule out atypical variants or other neoplasms. (See 'Diagnosis' above.)

●Treatment – Treatment differs for common versus atypical lesions. (See 'Treatment' above.)

•No treatment is usually required for asymptomatic, static, common dermatofibromas. However, for patients who desire treatment for cosmetic concerns, we suggest cryotherapy with liquid nitrogen rather than surgical excision (Grade 2C).

•For lesions that were biopsied and confirmed histologically as atypical variants, we suggest surgical excision rather than observation or cryotherapy (Grade 2C). Clear margins must be obtained; otherwise, recurrence is common.