Dermatofibroma is a commonly occurring cutaneous entity usually centered within the skin's dermis. Dermatofibromas are referred to as benign fibrous histiocytomas of the skin, superficial/cutaneous benign fibrous histiocytomas, or common fibrous histiocytoma. These mesenchymal cell lesions of the dermis clinically are firm subcutaneous nodules that occur on the extremities in the vast majority of cases and may or may not be associated with overlying skin changes. They are most commonly asymptomatic and usually relatively small, less than or equal to 1 centimeter in diameter. The term "fibrous histiocytoma" refers more to the morphologic appearance of the cell populations that comprise these lesions rather than strictly describing the cellular lineage.

There are two theories of the genesis of dermatofibromas: (1) reactive process vs. a clonal, and (2) neoplastic proliferation. Evidence exists for both of these theories. Often, patients who present with a dermatofibroma relate a history of possibly inciting local trauma at the site, such as from an insect bite or superficial puncture wound from thorns or wood splinters. Dermatofibromas occur in people of all ages, although more commonly during the ages of the 20s to 40s, and develop more frequently in females than males, with as high as a 2:1 female to male predominance according to some reports. They are a benign tumor, although there have been cases of local recurrence, and even more rarely, distant metastases have been reported. When considering the differential diagnosis of these lesions, it is vitally important to distinguish dermatofibromas from dermatofibrosarcoma protuberans, a similar-appearing but more aggressive cutaneous neoplasm.[1][2][3][4]

The development of dermatofibromas remains a controversial topic. Patients who are diagnosed with dermatofibroma often endorse a history of local trauma. This history argues for a reactive process. However, this history of local trauma is not reliably reported and only found in about a fifth of cases. More often, there is no inciting event or trauma, and the dermatofibroma develops spontaneously. Interestingly, although they will develop spontaneously, spontaneous regression is not commonly observed, which may support an argument against a primarily reactive process in favor of a clonal or neoplastic model.

Some studies have identified clonal markers in dermatofibromas cells by analyzing X-chromosome inactivation, which argues for a monoclonal pattern, and possibly favors a neoplastic process as the origin of development for dermatofibromas. Others propose that dermatofibromas are heterogeneously derived. They represent both a reactive and neoplastic process within the same lesion, wherein the histiocytoid component may be neoplastic, and the fibrous portion may result from reactive fibroblastic proliferation. 

Most dermatofibromas occur as solitary lesions, only clinically presenting as multiple simultaneous lesions in 10% of cases. Interestingly, although there are multiple histologic benign variants of dermatofibromas, in patients with two or more simultaneously occurring dermatofibromas, the lesions tend to appear histopathologically similar.[5][6][4]

Dermatofibromas are a common skin lesion in almost all populations, with one study reporting that they account for 3% of all dermatopathology laboratory specimens. The overall worldwide incidence is difficult to determine, as the vast majority of patients are asymptomatic, and therefore many patients may never seek care for these lesions. These lesions are most common in patients in their 20s to their 40s. Most studies indicate a female predominance, with an incidence ranging from equivocal to only slightly higher in females too as high as twice as frequent in females than males.[5][6][4]

Dermatofibromas have many histologic variants, including cellular, epithelioid, lipidized, aneurysmal, monster cell, and atypical (to name only a few), but the most frequently diagnosed is the common fibrous histiocytoma or common dermatofibroma. 

The common dermatofibroma is characterized histopathologically by a localized proliferation of spindle-shaped fibrous cells admixed with histiocytoid cells within the dermis. This proliferation is usually nodular in appearance, with spiculated but moderately defined borders that may have a pushing appearance regarding the surrounding tissue. The spindle cells will form focally what is referred to as a “storiform” pattern, which describes a multi-centric whorling appearance of the elongated nuclei. There may be intermixed capillaries and lymphocytes or multinucleated giant cells. These proliferations are usually contained within the dermis, but it is not uncommon to observe a small portion of the lesion dipping down into the subcutaneous tissue along septal lines. A helpful and distinguishing characteristic is the presence of trapped collagen bundles or “collagen balls” within and between the fascicles of spindled fibrous cells. These entrapped collagen collections are more commonly found forming at the periphery of the lesion.

The overlying epidermis is usually separated by a clearly delineated and unaffected zone of separation, the "Grenz zone." Typical reactive epidermis changes include hyperkeratosis and acanthosis. The epidermis will also often exhibit elongated rete ridges diving into the dermis with hyperpigmented basal keratinocytes, referred to as the “dirty feet” sign. 

Arguably the most important entity to distinguish dermatofibromas from is the similar-appearing, but much more worrisome, dermatofibrosarcoma protuberans (DFSP). These lesions are typically much more cellular, have marked storiform pattern, invade and involve the subcutis deeply, often entrapping fat as it dives and proliferates along the subcutaneous septae.[7][8][9]