Dermatofibroma — SAQ

Stem

A 34-year-old woman presents with a 7 mm firm, reddish-brown papule on her right shin that has been present and stable for two years. She says it occasionally catches on her trousers and is mildly itchy. On examination the lesion is firm, tethered to the overlying skin but mobile over the subcutis, and on lateral compression between thumb and finger the centre of the lesion dimples inward. Dermoscopy shows a central white scar-like patch with a peripheral delicate brown network.

Questions

a) What is the most likely diagnosis, and which two clinical features in the stem support it? (2 marks)

b) Outline your assessment and state which investigation, if any, you would arrange. (3 marks)

c) Describe your stepwise management for this patient. (3 marks)

d) List three features that would prompt you to biopsy the lesion, and the differential diagnosis you would then be excluding. (2 marks)

Model answer

^[1]a) Diagnosis (2 marks). Dermatofibroma (benign fibrous histiocytoma). Supporting features: (i) a firm dermal papule on the lower leg of a young woman, tethered to overlying skin but mobile over subcutis; and (ii) the positive dimple sign (central dimpling on lateral compression), which is the bedside hallmark. The dermoscopic pattern (central white scar-like patch + peripheral delicate network) further supports the diagnosis.

^[2]b) Assessment and investigation (3 marks). The diagnosis is clinical — no investigation is required for a classic lesion with a positive dimple sign and characteristic dermoscopy. Confirm the firm dermal consistency, tethering to skin with mobility over fat, and elicit and document the dimple sign. Perform dermoscopy to show the central white scar-like patch and peripheral pigment network. Document with a scaled photograph and record site, size, and colour. No blood tests or imaging are indicated for a solitary classic lesion; biopsy is avoided because a biopsy scar on the shin may be more conspicuous than the lesion.

c) Management (3 marks).

1. Reassurance and observation — the lesion is benign, stable, and does not transform; explain this and provide written safety-net advice (return if it grows beyond about 2 cm, changes colour, ulcerates, or becomes painful).
2. Because the lesion is symptomatic (catches on clothing, itchy), offer surgical excision with a narrow margin (about 2 to 5 mm) to the subcutis, with histology, if the patient wishes the lesion removed.
3. Avoid cryotherapy and intralesional steroid (do not remove the dermal component; recurrence and hypopigmentation). Do not biopsy a classic lesion unless atypical features are present.

^[1]d) Features prompting biopsy (2 marks). Any of: lesion large, rapidly growing, plaque-like, on the trunk, painful, colour change/ulceration, or recurrence after excision. The principal differential to exclude is dermatofibrosarcoma protuberans (DFSP) — a CD34-positive low-grade sarcoma — and, for a dark vascular nodule, nodular melanoma; biopsy with CD34 and Factor XIIIa immunohistochemistry resolves this.

References2ShowHide

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]Felty CC, Linos K. Epithelioid Fibrous Histiocytoma: A Concise Review Am J Dermatopathol, 2019.PMID 30289773