Dermatofibroma and Fibrohistiocytic Lesions

01The classical DF presents on the lower extremities of young to middle-aged adults as a firm, slightly hyperpigmented papule with the dimple sign on lateral compression. Dermoscopically, a central white scar-like patch (corresponding to dermal fibrosis with reduced epidermal melanin) sits within a delicate, regular peripheral pigment network (corresponding to acanthotic rete ridges with basal hypermelanosis). Zaballos's foundational study of 412 lesions identified ten dermoscopic patterns: the prototype (central white plus peripheral network) accounted for the largest share, with multiple peripheral white patches, central white network, and homogeneous brown patches as the next most frequent variants.

02The dimple sign (Fitzpatrick sign) is a clinical feature with dermoscopic correlates. Lateral compression of the lesion produces inward depression because the dermal fibrosis tethers the overlying epidermis. Under dermoscopy, the same maneuver may reveal the white scar-like center moving with the dermis rather than sliding over it. The sign is highly specific but not pathognomonic: deep DF and some scars can produce similar tethering.

03Atypical dermatofibromas form a heterogeneous group. Cellular DF is histopathologically more cellular than classic DF and clinically larger, sometimes ulcerated; dermoscopically it often shows polymorphic vessels, central red-blue homogeneous areas, and may lack the typical white patch. Aneurysmal DF (also called hemosiderotic) contains pseudovascular blood-filled spaces and may appear as a blue, purple, or even nodular dark-red lesion clinically; dermoscopically it often shows red-blue or purple lacunar areas resembling angiokeratoma or vascular malformation. Atypical (pseudosarcomatous) DF carries minor histopathologic atypia (occasional mitoses, mild pleomorphism) but behaves benignly; it may show unusual dermoscopic features but is most often diagnosed on histology after excision of an unusual lesion.

04Epithelioid cell histiocytoma (also called epithelioid fibrous histiocytoma) is now recognized as a distinct entity rather than a DF variant. It typically presents as a polypoid or pedunculated reddish papule on the lower extremity, sometimes with a collarette. Dermoscopically it shows red-pink homogeneous areas, polymorphic vessels, and absence of the classic DF features. The 2015 discovery of recurrent ALK gene rearrangements (most often SQSTM1-ALK and VCL-ALK; also DCTN1, PRKAR2A, and ETV6 partners) cemented the entity as biologically distinct, and ALK immunohistochemistry is now part of the differential workup. Recently, ROS-1 fusions and rare FN1-EGFR rearrangements have also been described in cases with epithelioid morphology.

05Deep DF (fibrohistiocytoma profundum, deep benign fibrous histiocytoma) is a rare subcutaneous variant that presents as a firm subcutaneous nodule, usually larger than classic DF, with a higher recurrence rate (about 20% locally) and a preference for the head, neck, and lower extremity. Dermoscopy is limited because the lesion is mainly subcutaneous; surface findings are non-specific (homogeneous skin-color or pink with telangiectasia). Imaging or excisional biopsy is usually required. The differential includes dermatofibrosarcoma protuberans (DFSP), schwannoma, neurofibroma, and lipoma.

06Dermatofibroma in darker skin (Fitzpatrick IV-VI) shows several distinctive features. Pigment is more intense, the peripheral network appears thicker and darker, and the central white patch may be partly obscured by background hyperpigmentation. Postinflammatory hyperpigmentation often surrounds the lesion. The dimple sign and palpable firmness remain reliable across phototypes. Clinically, hypertrophic and keloidal DFs are more common in darker skin and on the chest and back.

07Polypoid and pedunculated DFs occur on the lower extremities, perianal region, and rarely the trunk. They differ from classic DF in shape (raised, sometimes stalked) but retain the dermoscopic vocabulary of central white plus peripheral network when the lesion is photographed obliquely. Lentiginous border (a darkly pigmented periphery resembling a solar lentigo or junctional nevus) is a recognized variant pattern, and the lesion can simulate a melanocytic neoplasm at first glance. Thick, dark, slightly irregular network may shade into atypical network and trigger excision when the central scar-like patch is incomplete.

08Differential diagnosis is the central skill. Atypical fibroxanthoma (AFX) presents on chronically sun-damaged scalp or face of older patients as a rapidly growing red nodule, often ulcerated, with polymorphic vessels and white shiny structures; the absence of the DF prototype features and the actinic context separate the two. Basal cell carcinoma in pigmented form may show a white shiny patch but adds arborizing vessels, blue-grey ovoid nests, leaf-like areas, or spoke-wheel structures. Melanoma rarely mimics DF, but lesions with white scar-like areas plus atypical network, multiple colors, or atypical vessels deserve excision; the rule is that any DF-like lesion that does not fully fit the prototype should be biopsied. ROS-1 positive epithelioid fibroblastic tumors and other emerging fusion-driven entities are increasingly recognized in atypical cases and may require molecular workup.