Dermatofibroma

Meet the patient

A 32-year-old nurse points to a firm brown bump on her shin that has been there, unchanged, for three years. It itches when her sock rubs it. You squeeze it sideways between thumb and forefinger and the centre dimples inward; the lesion will not move independently of the skin above it, but glides freely over the fat beneath.[1]

Two questions settle this lesion at the bedside and everything below exists to answer them: is this the common benign dermatofibroma, or its rare malignant look-alike DFSP? and does it actually need anything done to it? The answer to both, in a classic lesion, is reassuring.[1][2]

One lesion, one sign, one stain — the three things that earn marks

The dimple sign is the bedside pathognomonic clue, and demonstrating it is a viva staple. Compress the lesion laterally — sideways, between thumb and index finger, never vertically — and the tethered epidermis is pulled inward so the centre invaginates. A melanocytic naevus, by contrast, is soft and protrudes or everts on the same manoeuvre; a neurofibroma gives the buttonhole sign, the whole lesion slipping through a dermal defect on firm pressure.[1]

Dermoscopy confirms the classic lesion. Two features sit together: a central white scar-like patch — a stellate structureless white area marking the collagenous core — ringed by a peripheral delicate ('busy') pigment network of basal-layer hyperpigmentation overlying the tumour. The central white patch is found in roughly 80 percent of classic dermatofibromas and is the single most specific dermoscopic feature.[1]

One immunohistochemistry panel closes the diagnosis when the lesion is atypical. The dermatofibroma is Factor XIIIa positive and CD34 negative; dermatofibrosarcoma protuberans is CD34 positive and Factor XIIIa negative. That inversion — the single most examined axis in dermatopathology — is why any lesion that is large, plaque-like, on the trunk, or recurrent after excision must reach the lab with a CD34 and Factor XIIIa request on the form.[1]

Etymology for viva gold: the lesion has worn four names — sclerosing haemangioma (its vascular-looking histology), histiocytoma (its histiocytic cells), dermatofibroma (the clinical term), and benign fibrous histiocytoma (the modern pathological term). The name-swapping survived because the reactive-versus-neoplastic argument ran for decades; modern pathology settles on benign fibrous histiocytoma.[1]

The classic lesion, read at the bedside

A firm dermal papule on the lower leg of a young woman is a dermatofibroma until something says otherwise. The default lesion is 3 to 10 mm, round or oval, skin-coloured through light brown to reddish-brown or purple, fixed to the overlying epidermis but freely mobile over the subcutaneous fat — never fixed to fascia or bone.[1][2]

Most are asymptomatic. When symptoms come, they are mild tenderness or itch, often provoked by the waistband or sock line. The natural history is stability: the lesion grows slowly to its final size over months, then sits unchanged for years or decades. A minority involute, leaving a depressed hyperpigmented scar. Rapid growth or change is atypical and mandates biopsy.[1]

[1] [2]

In about a fifth of cases the patient recalls a preceding local injury — an insect bite, a thorn prick, folliculitis, a ruptured follicle, minor trauma — the basis for the traditional reactive reading of the lesion. The cell of origin is the Factor XIIIa-positive dermal dendrocyte, which explains the defining immunoprofile and the firm, collagen-rich consistency that distinguishes it from a soft melanocytic naevus.[1]

Reactive proliferation or neoplasm — the modern answer is both

Whether dermatofibroma is reactive or neoplastic has been argued for decades, and the synthesis is that it depends on the variant. The classic lesion looks reactive: it follows a dermal injury, deposits abundant type I and III collagen, and does not transform. But clonality studies show non-random X-chromosome inactivation and cytogenetic abnormalities in a subset, and recurrent gene fusions such as MYADM::PRKCG now define the cellular variant.[1]

The practical consequence is that classic lesions are reactive and behave; cellular and atypical variants carry clonal, neoplastic features — and these are the subtypes that recur locally and, very rarely, metastasise. That distinction is the whole reason the variants matter clinically.[1]

The variants — each carries its own trap

The aneurysmal variant is the one most often biopsied, because it mimics melanoma. Blood-filled cavernous spaces and haemosiderin turn the lesion dark blue-black, sometimes tender; the classic central white patch is often absent on dermoscopy, replaced by a polymorphous mix of brown, red and black structureless areas. The instinct to biopsy is correct — nodular melanoma must be excluded — but the histology will show an otherwise typical dermatofibroma, S100 negative and Factor XIIIa positive.[3]

The cellular variant is the one that recurs. Densely cellular, it may push into the subcutis in a honeycomb pattern that raises the spectre of DFSP until CD34 staining resolves it (dermatofibroma stays CD34 negative). It carries a 20 to 30 percent local recurrence rate after excision, and the MYADM::PRKCG fusion confirms its neoplastic basis. Complete excision with clear margins and follow-up is required.[1]

The remaining variants are viva currency. Atrophic dermatofibroma loses substance and presents as a depressed, wrinkled plaque in older adults — easily confused with morphea or an atrophic scar, and more hypocellular on histology in older patients.[4] Epithelioid dermatofibroma is a dome-shaped, vascular-looking nodule of epithelioid cells, and a subset harbour ALK rearrangements.[5] Atypical (pseudosarcomatous) fibrous histiocytoma sits in the borderline category with low-grade malignant potential: scattered atypical cells, occasional mitoses, complete excision mandatory.[1]

Eruptive or multiple dermatofibromas — dozens of lesions appearing over weeks to months — are uncommon and change the work-up entirely. This is no longer a lesion to observe; it is a clue to an underlying trigger, and the search is for HIV and immunosuppression, systemic lupus erythematosus, myelodysplasia or leukaemia, and atopy.[1]

Face-off — dermatofibroma versus its closest mimics

The single must-not-miss mimic is dermatofibrosarcoma protuberans (DFSP), because misdiagnosing a low-grade sarcoma as a benign dermatofibroma leads to under-excision and undertreatment. The discriminator is immunohistochemistry — CD34 and Factor XIIIa — requested on any lesion that is over 2 cm, plaque-like, on the trunk, growing, or recurrent.[1]

[1]

The discriminator line: CD34 negative plus a positive dimple sign plus the lower leg is a dermatofibroma; CD34 positive plus a plaque on the trunk is DFSP; everything else is a softer call settled by a second bedside manoeuvre.[1]

The three bedside brown-papule manoeuvres are worth holding together. Lateral compression of a dermatofibroma gives central dimpling (the dimple sign); firm compression of a neurofibroma lets the lesion buttonhole through a dermal defect; and compression of a naevus makes it protrude or evert. Three manoeuvres, three diagnoses — a compact viva answer.[1]

Investigations — when to biopsy, and how to do it right

For a classic lesion with a positive dimple sign and characteristic dermoscopy, no investigation is required — the diagnosis is clinical and biopsy is avoided. A biopsy of a classic dermatofibroma leaves a scar on the shin that is often more conspicuous than the lesion itself. Reserve biopsy for the atypical lesion where the histology will change management: over 2 cm, growing, painful, plaque-like, on the trunk, recurrent, or with atypical colour.[1]

When you do biopsy, biopsy to the dermis — a punch or excisional biopsy, never a superficial shave. A shave may show only acanthotic epidermis with basal-layer hyperpigmentation and be misread as a basal cell carcinoma, one of the well-described histological mimics of BCC.[6] Send the specimen for CD34 and Factor XIIIa so the DF-versus-DFSP question is settled on the same block.[1]

The defining histological features, in the order a pathologist reads them: spindle cells in a storiform (cartwheel) pattern in the dermis with no significant atypia or mitoses; epidermal acanthosis and pseudo-epitheliomatous hyperplasia with basal-layer hyperpigmentation; fenestrated collagen at the deep and lateral margins, where tumour cells entrap and weave between thickened collagen bundles; and often a grenz zone of normal papillary dermis between epidermis and tumour. The immunoprofile — Factor XIIIa positive, CD34 negative, stromelysin-3 positive, HMGA1 and podoplanin positive — is the working diagnostic matrix.[1][6]

Management — observe, unless there is a reason not to

The default is observation, and the lesion is never treated 'just in case'. The first step at the initial consultation is reassurance: a common benign growth, no cancer risk, no treatment needed. Document and photograph the lesion, give written safety-net advice (return if it grows beyond about 2 cm, changes colour, ulcerates, or becomes painful), and the patient is discharged. The only same-day action is a biopsy when any feature suggests DFSP, atypical fibrous histiocytoma, or melanoma.[1][2]

Intervention is reserved for specific indications. Surgical excision is indicated when the lesion is symptomatic (pain or irritation from clothing), a cosmetic concern, or diagnostically uncertain — excise with a narrow 2 to 5 mm margin, down to and including the subcutis so the entire dermal tumour is removed en bloc, and always send the specimen for histology with a clinical note stating the pre-operative diagnosis and any atypical feature. A superficial shave that leaves the deep dermal base behind is the commonest cause of recurrence.[1]

The classic trap: cryotherapy, intralesional steroid, and shave excision all fail because none removes the dermal component. Cryotherapy flattens the surface, leaves hypopigmentation, and the lesion recurs; steroid injection is painful and not curative; shave alone leaves the base behind. If surgery is undertaken at all, it is a complete excision to the subcutis.[1]

Cellular or atypical (borderline) fibrous histiocytoma changes the plan: complete excision with clear margins and close follow-up for local recurrence — the cellular variant recurs in roughly 20 to 30 percent. If DFSP is confirmed on histology, the patient enters the sarcoma pathway (Mohs or wide local excision; imatinib for unresectable or metastatic disease targeting the COL1A1-PDGFB fusion).[1]

How patients with a 'dermatofibroma' come to harm — the preventable list

• A DFSP mislabelled as a dermatofibroma and under-excised, recurring locally for years before the sarcoma is recognised — the classic preventable error.[1]
• A shave biopsy showing only acanthotic epidermis, misread as basal cell carcinoma, sending the patient down the wrong pathway.[6]
• An aneurysmal dermatofibroma called nodular melanoma on clinical grounds, over-treated — or, worse, a real nodular melanoma called an aneurysmal dermatofibroma and watched.[3]
• Dozens of eruptive lesions dismissed as 'just dermatofibromas' while an underlying HIV, lupus or myelodysplasia goes undiagnosed.[1]
• A scar on the shin worse than the original bump, because a classic lesion was biopsied or excised unnecessarily.[1]

Special populations

Eruptive or multiple dermatofibromas are the scenario that changes the work-up. Investigate for HIV and other immunosuppression, systemic lupus erythematosus (ANA, double-stranded DNA), myelodysplastic syndromes and leukaemia (full blood count with film), and atopy. Treat the lesions individually — most are observed — but the underlying condition is the priority.[1]

Pregnancy may enlarge existing lesions or generate new ones, consistent with hormonal influence; management is conservative, and cosmetic excision is deferred to the postpartum period. Children and adolescents are less often affected than adults, but eruptive disease in adolescents is recognised and managed as in adults — a solitary lesion in a young child lowers the threshold for biopsy, chiefly to exclude a Spitz naevus. Older adults more often show the atrophic variant, and cellular lesions are more hypocellular and harder to interpret on histology.[1][4]

Prognosis and disposition

The prognosis of classic dermatofibroma is excellent: benign, stable for years to decades, may involute, and does not undergo malignant transformation. No long-term follow-up is needed for a classic lesion, whether observed or completely excised. The cellular and atypical variants carry a low risk of local recurrence after incomplete excision and very rarely metastasise; these warrant complete excision and a period of surveillance.[1]

Classic dermatofibroma is managed in primary care with safety-net advice. An atypical lesion requiring biopsy or excision is referred to secondary care (dermatology); if DFSP or another sarcoma is confirmed on histology, the patient enters the sarcoma multidisciplinary team pathway.[1]

The mantra, and the memory device

[1]

The mantra: firm brown papule on a shin, dimple on lateral compression, CD34 negative — leave it alone.[1]

Ward-round test — three stems, thirty seconds each

Stem 1 — the firm brown bump on a shin (answer)

A 34-year-old woman has a 7 mm firm reddish-brown papule on her shin, stable for two years, that itches when her sock rubs. On lateral compression between thumb and finger the centre dimples inward. What is the diagnosis, what confirms it, and what is the management? Model: This is a classic dermatofibroma (benign fibrous histiocytoma), established clinically by the firm dermal papule on the lower leg of a young woman with a positive dimple sign and confirmed by dermoscopy (central white scar-like patch with a peripheral delicate pigment network). The lesion is benign; management is reassurance and observation — no biopsy, no excision — with safety-net advice to return if it grows beyond about 2 cm, changes colour, ulcerates, or becomes painful.[1]

Stem 2 — the 'dermatofibroma' that came back (answer)

A 42-year-old man had a 'dermatofibroma' excised from his shoulder 18 months ago. A new firm nodule has appeared in the scar. What went wrong, and what do you do now? Model: A recurrent nodule at the site of a previous 'dermatofibroma' excision on the trunk is dermatofibrosarcoma protuberans (DFSP) until proven otherwise — the classic stem. The original lesion was almost certainly under-diagnosed and under-excised. Re-biopsy with CD34 and Factor XIIIa immunohistochemistry: DFSP is CD34 positive and Factor XIIIa negative; dermatofibroma is the inverse. If DFSP is confirmed, refer to the sarcoma MDT for Mohs micrographic surgery (the standard of care, around 1 percent recurrence) or wide local excision with 2 to 3 cm margins, with imatinib reserved for unresectable, recurrent or metastatic disease.[1]

Stem 3 — the dark blue-black nodule (answer)

A 28-year-old woman has a 9 mm dark blue-black, slightly tender nodule on her thigh that appeared over four months. Dermoscopy shows a polymorphous mix of brown, red and black structureless areas with no central white patch. What is the differential, and what do you do? Model: The differential is aneurysmal (haemosiderotic) dermatofibroma versus nodular melanoma — both can present as a dark blue-black nodule, and the dermoscopy is unreliable in the aneurysmal subtype because the classic central white patch is often absent. Biopsy is mandatory — a punch or excisional biopsy to the dermis, sent for histology with S100, Factor XIIIa and CD34. Aneurysmal dermatofibroma is S100 negative and Factor XIIIa positive; nodular melanoma is S100 positive. Do not observe a dark changing nodule on the assumption it is a benign variant — the cost of missing a melanoma dwarfs the cost of a biopsy scar.[3]

References

1. [1]Wan L, Park A, Almatroud L, et al. Dermatofibroma: Reappraisal and Updated Review Clin Cosmet Investig Dermatol, 2025.PMID 40785832
2. [2]Wilson JL. Benign Skin Tumors Prim Care, 2025.PMID 40835288
3. [3]Zaballos P, Álvarez-Salafranca M, Llambrich À, et al. Dermoscopy of haemosiderotic/aneurysmal dermatofibroma: A morphological study of 110 cases J Eur Acad Dermatol Venereol, 2023.PMID 36251407
4. [4]Jordan C, Hodges W, Russell L, et al. Dermatofibroma Hypocellularity Is Associated With Patient Age: A Retrospective Study of 307 Cases J Cutan Pathol, 2026.PMID 42252143
5. [5]Felty CC, Linos K. Epithelioid Fibrous Histiocytoma: A Concise Review Am J Dermatopathol, 2019.PMID 30289773
6. [6]Stanoszek LM, Wang GY, Harms PW. Histologic Mimics of Basal Cell Carcinoma Arch Pathol Lab Med, 2017.PMID 29072946