Synonym: fibrous histiocytoma

Dermatofibroma are common and benign skin tumours but frequently cause concern upon discovery.

Aetiology

Traditionally, dermatofibroma were attributed to a reactive reaction to trauma such as insect bites. However, the precise aetiology is unclear. They appear to be clonal proliferative growths of dermal dendritic histiocyte cells.

Epidemiology

• They are more frequent in women than in men (ratio of 4:1).
• They can occur at any age, most commonly in young adulthood.

Presentation

• Dermatofibromas are usually single nodules that develop on an extremity, most commonly the lower legs.¹ They are freely moving, firm to hard nodules of 0.5-1.0 cm diameter, sometimes compared to the feel of a small lentil under the skin surface.
• The skin's surface is generally smooth, occasionally scaly. The overlying skin may be tethered (the 'dimple' sign). The skin colour varies from skin-coloured to pink/red to cream/white to brown.
  
  
• Lesions can occur at any skin site and individuals may have several lesions (up to 15).
• Multiple variants tend to occur where immunity is impaired (e.g. autoimmune disease, systemic lupus erythematosus, HIV, leukaemia).
• The nodule is usually asymptomatic but can be itchy or tender.
• After initial growth, they tend to remain static in size.

Diagnosis

• Diagnosis is usually straightforward provided you palpate the lesion, as few other skin lesions are as firm.
• The pinch test is helpful (but not definitive): squeezing the lesion from the sides results in dimpling of overlying skin (see image above).
• With a dermatoscope, dermatofibromas typically show a pigment network and central white patch but there is considerable variation.²
• Excision biopsy is useful where diagnostic uncertainty remains after examination.

Differential diagnosis

Includes:³

• Atypical mole.
• Basal cell carcinoma.
• Keloid or hypertrophic scar.
• Keratoacanthoma.
• Malignant melanoma.
• Metastatic carcinoma of the skin.
• Spitz' naevus.
• Blue naevus.
• Squamous cell carcinoma.

Deep penetrating dermatofibroma may be difficult to distinguish, even histologically, from rare malignant fibrohistiocytic tumours, e.g. dermatofibrosarcoma protuberans.⁴

Primary care management

• Reassure - generally no treatment is required.
• Remove where cosmetically disliked, symptomatic or there is diagnostic uncertainty. Note that there is a significant local recurrence rate (in about a quarter of patients).
• Removal by elliptical excision or punch biopsy usually provides the most satisfactory results: shave excision or cryotherapy risk incomplete excision and recurrence.
• About 2% of GP subcutis excisions yield unexpected or rare malignancies so, even where excision is for cosmetic or symptomatic reasons, it is nonetheless worth sending specimens to histology.⁵

Prognosis

• Lesions are almost invariably benign - there are extremely rare case reports of metastasising cellular dermatofibromas, although histological distinction from other tumours can prove difficult.⁶ Follow up lesions that are histologically atypical or undergo recurrence.
• Most are static and persist indefinitely although, uncommonly, they spontaneously regress.
• They may become repeatedly irritated by shaving, for example.

When to refer

Normally for diagnostic assistance, to differentiate from other potentially harmful pigmented lesions.

Document references

1. Dermatofibroma, Primary Care Dermatology Society; images of typical and atypical dermatofibroma
2. Zaballos P, Puig S, Llambrich A, et al; Dermoscopy of dermatofibromas: a prospective morphological study of 412 cases. Arch Dermatol. 2008 Jan;144(1):75-83. [abstract]
3. Pierson JC et al, Dermatofibroma, Medscape, Apr 2010
4. Hanly AJ, Jorda M, Elgart GW, et al; High proliferative activity excludes dermatofibroma: report of the utility of MIB-1 in the differential diagnosis of selected fibrohistiocytic tumors. Arch Pathol Lab Med. 2006 Jun;130(6):831-4. [abstract]
5. Buis PA, Verweij W, van Diest PJ; Value of histopathologic analysis of subcutis excisions by general practitioners. BMC Fam Pract. 2007 Jan 26;8:5. [abstract]
6. Bandarchi B, Ma L, Marginean C, et al; D2-40, a novel immunohistochemical marker in differentiating dermatofibroma from Mod Pathol. 2010 Mar;23(3):434-8. Epub 2010 Jan 8. [abstract]

Internet and further reading

• Dermatofibroma, DermNet NZ, July 2011