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A dermatofibroma, recognized in its common form as a fibrous histiocytoma, is a skin lesion characterized by oval-shaped cells that resemble histiocytes or spindle cells. Clinically, it often presents as a solitary, yellowish-brown, and slightly scaly nodule, typically measuring 0.5-1 cm in diameter, with the extremities, particularly the legs, being the most frequent location.

Clinical Presentation

The appearance of a dermatofibroma can vary, with the surface color ranging from tan to red. While usually small, some atypical cases may reach diameters of over 5 cm. Although solitary, some individuals may develop multiple dermatofibromas, and in rare instances, particularly in those with immune dysregulation or autoimmune conditions, over 15 nodules may occur.

A dermatofibroma is notably firm or hard to touch, exhibiting a firmness that surpasses most other benign skin growths. Its epidermis is attached, causing a characteristic dimple or Fitzpatrick sign when the skin around the lesion is gently pinched. This sign can also be elicited by sudden cold exposure, which causes the skin to contract.

While generally asymptomatic, a dermatofibroma may occasionally cause itching or pain, and in cases related to trauma, it may ulcerate.

Diagnostic Approach

The diagnosis of a dermatofibroma primarily relies on its clinical characteristics, including its firm texture and a positive pinch test. Dermoscopic examination typically reveals peripheral pigmentation with a central white area. In cases where the lesion mimics other conditions, an excision biopsy is necessary, ensuring that subcutaneous fat is also included.

Histopathological examination shows epidermal hyperplasia and basal cell layer hyperpigmentation, referred to as the 'dirty fingernail' sign. The tumor, located in the dermis, is non-encapsulated with visible whorls of spindle cells. However, various histological variants exist, such as cellular, aneurysmal, atrophic, and atypical dermatofibromas.

Immunohistochemical testing can identify antibodies to factor XIIIa within the tumor.

Differential Diagnosis

Given the overlapping clinical features with other conditions, it is crucial to consider a differential diagnosis. Conditions that may present similarly include:

Juvenile xanthogranuloma, which also has a firm texture

Dermatofibrosarcoma protuberans, which might appear as hard nodules on the trunk

Squamous cell carcinoma or atypical fibroxanthoma in cases of ulcerated and infected lesions

Other considerations for differential diagnosis encompass:

Malignant melanoma

Melanocytic nevus

Blue nevus

Spitz nevus

Keratoacanthoma

Keloid scar

In conclusion, while dermatofibromas are typically diagnosed through clinical examination and dermoscopy, histopathological evaluation and immunohistochemical testing are essential for confirmation, especially when differentiating from other skin lesions with similar presentations.