Dermatofibroma — Viva

Q1: The lesion and the bedside sign (2 min)

Examiner: A patient shows you a firm brown papule on her shin. Talk me through what you would do at the bedside.

Candidate: I would inspect and palpate the lesion — confirming a firm dermal papule, light brown to reddish-brown, tethered to the overlying skin but mobile over the subcutis. I would then elicit the dimple sign (Fitzpatrick sign): compressing the lesion laterally between thumb and finger produces central dimpling because the epidermis is tethered to the underlying dermal tumour. A positive dimple sign is highly suggestive of dermatofibroma and distinguishes it from a naevus, which everts on compression.^[1]

Q2: Confirming the diagnosis (3 min)

Examiner: How else would you confirm it?

Candidate: With dermoscopy — the classic pattern is a central white scar-like patch with a peripheral delicate ('busy') pigment network. The central white patch is a frequent and characteristic dermoscopic feature. For a classic lesion with a positive dimple sign and this dermoscopy, the diagnosis is clinical — no biopsy is needed.^[1]

Q3: The must-not-miss differential (3 min)

Examiner: What is the single most important condition to exclude, and how?

Candidate: Dermatofibrosarcoma protuberans (DFSP) — a low-grade malignant sarcoma. DFSP is CD34 positive and Factor XIIIa negative, and presents as a slow-growing plaque on the trunk that becomes protuberant and infiltrates the subcutis. By contrast dermatofibroma is CD34 negative and Factor XIIIa positive, shows the dimple sign, favours the lower leg, and is benign. Any lesion that is large, plaque-like, truncal, growing, or recurrent should be biopsied with CD34 and Factor XIIIa staining to settle the distinction.^[1]

Q4: Histology and immunohistochemistry (2 min)

Examiner: Describe the histology of a dermatofibroma.

Candidate: Spindle cells in a storiform (cartwheel) pattern in the dermis, with epidermal acanthosis and basal-layer hyperpigmentation, and fenestrated collagen at the deep and lateral margins — tumour cells weaving between thickened collagen bundles. Immunohistochemistry is Factor XIIIa positive, CD34 negative, stromelysin-3 positive. A practical pitfall: a superficial shave may show only acanthotic epidermis and be misread as a basal cell carcinoma, so biopsy to the dermis (punch or excisional).

^[3]

Q5: Variants and their behaviour (3 min)

Examiner: Tell me about the variants that behave differently.

Candidate: The cellular variant is densely cellular, may extend into the subcutis, and has a higher local recurrence rate after excision. The atypical (pseudosarcomatous / borderline) variant shows atypical cells and occasional mitoses and requires complete excision. The aneurysmal / haemosiderotic variant is dark and vascular and mimics melanoma, so it is often biopsied. These variants are the ones that recur and, very rarely, metastasise; classic cutaneous dermatofibroma does not metastasise.^[1][2]

^[2]

Q6: Management and the safety-net (2 min)

Examiner: How would you manage this patient?

Candidate: The default is reassurance and observation — the lesion is benign and stable. If it is symptomatic or a cosmetic concern, I offer surgical excision to the subcutis, with histology. I avoid cryotherapy and intralesional steroid (they do not remove the dermal component) and I do not biopsy a classic lesion. I give a written safety-net: return if it grows, changes colour, ulcerates, or becomes painful. For an atypical lesion I biopsy with CD34 and Factor XIIIa, and if DFSP is confirmed I refer to the sarcoma pathway.^[1]

References3ShowHide

1. [1]Wan L, Park A, Almatroud L, et al. Dermatofibroma: Reappraisal and Updated Review. Clinical, cosmetic and investigational dermatology, 2025.PMID 40785832
2. [2]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
3. [3]Stanoszek LM, Wang GY, Harms PW. Histologic Mimics of Basal Cell Carcinoma Arch Pathol Lab Med, 2017.PMID 29072946