Dermatofibroma (benign fibrous histiocytoma) is a common benign cutaneous lesion consisting of fibroblast and histiocyte proliferation within the dermis. Most commonly seen on lower extremities (70%), it is more frequent in women aged 20-40 (F:M = 2:1). It may develop reactively after insect bites, trauma, or folliculitis, although in some cases true neoplastic clonal proliferation has been demonstrated. Clinically presents as a small (<2 cm), firm, dermal, skin-colored or brownish nodule with the pathognomonic 'dimple sign' — central depression with lateral compression. Ultrasonography is the diagnostic modality, showing a small, oval, hypoechoic solid nodule within the dermis; real-time demonstration of the dimple sign on dynamic US strengthens the diagnosis.
Age Range
15-60
Peak Age
35
Gender
Female predominant
Prevalence
Common
Dermatofibroma is a reactive or neoplastic proliferation of fibroblasts and histiocytes within the dermis. A triggering factor (insect bite, folliculitis, trauma) initiates an inflammatory cascade in the dermis — macrophages and fibroblasts become activated → collagen production increases and histiocyte infiltration develops. The lesion starts at the dermis-epidermis junction and expands within the dermis; reactive acanthosis and basal layer hyperpigmentation develop in the overlying epidermis. Histopathologically, spindle cells and collagen fibers show a characteristic 'storiform' (matted, cartwheel) pattern — this organized cellular architecture determines the homogeneous hypoechoic appearance on ultrasonography. The hypoechoic appearance on US results from the dense cellular structure (fibroblasts/histiocytes) creating distinctly different acoustic impedance from the normal ordered collagen network of the dermis — cellular areas have low impedance while surrounding collagen has high impedance, providing clear contrast. The anatomic basis of dimple sign is the lesion's tight integration into the dermis: dermatofibroma shows interdigitation with the dermal collagen skeleton and cannot move independently; when lateral compression is applied, the epidermis-dermis junction is stretched and the epidermis over the lesion is pulled downward → central depression forms. Minimal vascularity reflects that the lesion consists of mature fibrous tissue with low metabolic activity, indicating limited neoangiogenesis.
Central depression with lateral compression (dimple sign) indicates firm dermal attachment and is pathognomonic. Also observed clinically with finger compression. Real-time demonstration on dynamic US confirms the diagnosis — seen as inward collapse of the hyperechoic epidermal line over the lesion center. This finding is not seen in any other dermal lesion.
A small (<2 cm), oval, hypoechoic solid nodule within the dermis is seen with high-frequency (15-18 MHz) linear probe. Margins are generally well-defined but may be irregular. The long axis may be parallel or slightly oblique to the skin surface. The lesion is located in the lower half of the dermis and may show mild convex protrusion toward subcutaneous fat — 'mushroom sign'. Internal structure is homogeneously hypoechoic, reflecting the storiform cellular arrangement. Reactive thickening of the overlying epidermis (widened hyperechoic band) may be seen, indicating chronic irritation by the lesion.
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A small, oval, homogeneously hypoechoic solid nodule within the dermis is seen; consistent with dermatofibroma.
Minimal or no vascular signal within the dermatofibroma is seen on color and power Doppler. In some cases, minimal peripheral vascularity may be observed — reflecting the lesion's association with the normal capillary network of surrounding dermis. Mild perilesional increased vascularity may be seen in the setting of inflammation or trauma, but no significant internal vascularity exists. This avascular/hypovascular pattern supports benign nature and is an important criterion for differentiating from hypervascular malignant lesions (melanoma, dermatofibrosarcoma protuberans).
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No significant vascular signal detected within the lesion on color and power Doppler; consistent with a benign solid lesion.
During dynamic ultrasonography, the center of the lesion depresses when lateral compression is applied with the probe — 'dimple sign'. This reflects the firm attachment of dermatofibroma to the dermis and is the ultrasonographic correlate of the clinical dimple sign. The lesion does not slide laterally during compression and shows no sliding movement — it is fixed to the dermis. During the same maneuver, surrounding subcutaneous fat deforms while the lesion maintains its shape. This dynamic assessment highlights the importance of real-time US in diagnosing dermatofibroma and cannot be demonstrated on static imaging.
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Central depression of the lesion with lateral compression (dimple sign) is seen on dynamic evaluation; pathognomonic for dermatofibroma.
The epidermis overlying the dermatofibroma may be reactively thickened — the hyperechoic epidermal line appears significantly thicker (0.3-0.5 mm) than normal (0.1-0.2 mm). This reactive epidermal hyperplasia reflects chronic irritation by the lesion and stimulation of epidermal proliferation by growth factors produced by the dermatofibroma. Histologically, epidermal hyperpigmentation and basal layer hyperplasia accompany the finding. The thickened epidermis above the hypoechoic nodule of the dermatofibroma creates a 'sandwich' appearance: thick hyperechoic epidermis above + hypoechoic lesion in the middle + hyperechoic subcutaneous fat border below.
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The overlying epidermis appears reactively thickened; a finding consistent with dermatofibroma.
On T2-weighted MRI, dermatofibroma shows low-to-intermediate signal — reflecting short T2 relaxation of collagen-rich fibrous tissue. Signal slightly different from surrounding dermis is observed, but the lesion may be difficult to evaluate on MRI due to its small size. It is isointense or mildly hypointense on T1-weighted images. Mild-moderate enhancement may be seen on contrast-enhanced sequences — reflecting increased capillary permeability in granulation tissue and histiocytic component within the lesion. In histiocytic variants, hemosiderin deposition may cause signal loss on T2* and SWI sequences.
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The lesion shows low-to-intermediate signal on T2-weighted images; consistent with fibrous tissue.
Dermatofibroma is located in the lower dermis and may show convex bulging ('mushroom sign' / 'iceberg pattern') toward subcutaneous fat. This protrusion results from the lesion pushing the dermis-subcutis interface downward as it expands within the dermis. There is no true invasion into subcutaneous fat — the lesion remains within dermal boundaries. This protrusion is most prominently seen on transverse section, and the convex relationship of dermatofibroma to the dermis-subcutis interface creates the typical appearance. This finding may be more prominent in the cellular dermatofibroma variant.
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The lesion is located in the lower dermis with convex protrusion toward subcutaneous fat.
On strain elastography, dermatofibroma appears markedly stiff (blue coding) compared to surrounding dermis — reflecting its fibrous tissue composition. Strain ratio is generally >3 (lesion/surrounding tissue). This stiffness is consistent with the histological composition of the lesion: dense collagen fibers and cellular elements create a structure resistant to elastic deformation. On shear wave elastography, elasticity values are typically 50-100 kPa (normal dermis 10-20 kPa). The elastography finding is not diagnostic alone but supports solid nature and aids differentiation from soft lesions (lipoma, cyst).
Report Sentence
The lesion appears markedly stiff compared to surrounding tissue on elastography; consistent with a fibrous solid lesion.
Criteria
Most common type (80%). Small, firm, brown, dermal nodule. Histologically storiform pattern spindle cells with collagen-dominant stroma.
Distinct Features
Homogeneously hypoechoic on US, well-defined, dimple sign positive, avascular. Low T2 signal on MRI (dense collagen). Low recurrence risk.
Criteria
Higher cellularity, larger size (usually >2 cm), may extend into subcutaneous fat. Histologically dense cellular proliferation forming fascicles.
Distinct Features
More prominently hypoechoic on US (low-impedance cellular tissue dominant), mildly increased vascularity, higher local recurrence risk (10-25%). May need differentiation from DFSP — DFSP is larger, irregular margins, tentacle-like extensions.
Criteria
Rare variant (2%) with hemorrhagic changes. Contains blood-filled pseudovascular spaces. Rapid growth and bluish skin discoloration.
Distinct Features
Mixed cystic-solid appearance on US, hemorrhagic areas show different echogenicity (acute hemorrhage hyperechoic, organized hemorrhage hypoechoic). Subacute hemorrhage hyperintense on MRI T1. Rapid growth may raise concern for melanoma/sarcoma — biopsy may be needed.
Criteria
Prominent hemosiderin deposition and vascular component. Histologically hemosiderin-laden macrophages and small vascular structures.
Distinct Features
Hemosiderin effect on MRI: marked signal loss on T2/GRE (blooming), low signal on T1. May be more heterogeneous on US than other variants. Brown-blue clinical appearance.
Distinguishing Feature
Schwannoma is located along nerve course with eccentric peripheral position, tail sign (entering/exiting nerve ends) and prominent posterior enhancement — myxoid component provides low attenuation. Dermatofibroma is within dermis, no nerve association, dimple sign positive, no posterior enhancement.
Distinguishing Feature
Epidermoid cyst is cystic — prominent posterior enhancement, laminated (onion-skin) pattern, subcutaneous location, avascular internal structure. Dermatofibroma is solid — no posterior enhancement, homogeneously hypoechoic, dermal location, dimple sign positive.
Distinguishing Feature
Pilomatrixoma shows prominent calcification + posterior acoustic shadowing and is common in children. Dermatofibroma has no calcification, is common in young women, and shows positive dimple sign. In pilomatrixoma, peripheral hypoechoic halo and internal calcified areas also create a 'target-like' appearance.
Distinguishing Feature
DFSP is a dermal-origin malignant tumor showing tentacle-like extensions, irregular margins, larger size (>3 cm), and increased vascularity. Dermatofibroma is small (<2 cm), well-defined, dimple sign positive, and avascular. DFSP has protuberant appearance and may show 'multiple nodule sign'.
Urgency
routineManagement
conservativeBiopsy
Not NeededFollow-up
no-follow-upDermatofibroma is completely benign with no malignancy potential. Diagnosis is confidently made with typical clinical (dimple sign) and US findings without need for biopsy. No treatment needed; excision may be performed for cosmetic reasons or patient discomfort (recurrence rate 2-3%, 10-25% in cellular type). Biopsy is needed to exclude dermatofibrosarcoma protuberans (DFSP) in those showing atypical features (rapid growth, >3 cm, significant invasion into subcutaneous fat, high vascularity). Association of multiple dermatofibromas with Gardner syndrome or lupus erythematosus has been reported.
Dermatofibromas are benign lesions and require no treatment. Biopsy is recommended for cosmetic indications or atypical clinical findings (rapid growth, >2 cm, deep extension). Differential diagnosis from dermatofibrosarcoma protuberans is important — DFSP is larger, deeper, and more aggressive. Multiple dermatofibromas may be associated with immunosuppression or SLE.