Dermatofibroma
Summary
A Dermatofibroma (Benign Fibrous Histiocytoma) is a very common, harmless skin nodule. It represents a reactive proliferation of fibroblasts and histiocytes within the dermis, often triggered by minor trauma such as an insect bite, thorn prick, or ingrown hair. They are typically firm, hyperpigmented, and persistent. The classic diagnostic feature is the Dimple Sign (Fitzpatrick's Sign). They require no treatment unless diagnostic uncertainty exists. [1,2]
Key Facts
- Nature: It is NOT a true tumour (neoplasm) but a reactive inflammatory/fibrotic process.
- Location: Predominantly on the lower legs of women.
- Texture: Feels deeper than it looks. "Like a lentil or button sewn into the skin".
- Evolution: They grow slowly to reach a stable size (usually less than 1cm) and persist indefinitely.
Clinical Pearls
The Dimple Sign (Fitzpatrick's Sign): This is virtually pathognomonic. If you pinch the skin on either side of the lesion, it retracts inwards (dimples). This is because the scar tissue tethers the epidermis to the underlying subcutaneous fat. Other lumps (moles, lipomas, cysts) will bulge outwards when pinched.
Shaving Cuts: Women often complain that the lesion bleeds after shaving legs. This is because the nodule is slightly elevated and firm, catching the razor. This is an annoyance, not a sign of malignancy.
Don't Cut Them Out: Patients often ask for removal. Warn them: "I am swapping a small brown bump for a permanent white scar." Excision on the lower leg heals poorly and the scar often stretches.
Demographics
- Prevalence: Very common (approx 3% of dermatology visits).
- Sex: Female > Male (4:1).
- Age: Young to middle-aged adults (20-40s). Rare in children.
Aetiology
- Reactive: Fibrotic response to trauma (arthropod bite, folliculitis, puncture wound).
- Immunosuppression: Multiple eruptive dermatofibromas (>15) can be a sign of Lupus, HIV, or immunosuppressive therapy.
Histology
- Proliferation of spindle-shaped fibroblasts and histiocytes in the dermis.
- "Collagen trapping": Collagen bundles at the periphery are trapped by the proliferation.
- Overlying epidermis is hyperplastic and hyperpigmented (reactive melanosis).
Symptoms
Signs
- Inspection: Observe colour and borders.
- Palpation: Assess firmness.
- Dimple Test: Squeeze lateral margins. Positive = Depression.
- Dermoscopy:
- Central White Scar-like Patch: (Fibrosis).
- Pigment Network: Delicate, ring-like network at periphery.
Diagnosis
- Clinical: History + Exam + Dimple Sign is sufficient for 99% of cases.
- Dermoscopy: Very helpful to exclude melanoma.
Biopsy
- Indications:
- Atypical features (asymmetry, chaotic pigment).
- Rapid growth.
- Size > 2cm.
- Patient anxiety.
- Excision Biopsy: Preferred over punch biopsy to see deep margin.
Management Algorithm
SKIN NODULE ON LEGS
↓
CLINICAL EXAMINATION
(Firm? Dimple Sign? Insect bite hx?)
↓
TYPICAL FEATURES?
┌────────┴────────┐
YES NO
↓ ↓
IS PATIENT DERMOSCOPY
BOTHERED? (Exclude Melanoma)
┌─────┴─────┐ ↓
NO YES UNCERTAIN?
↓ ↓ ↓
REASSURE DISCUSS REFER /
(Leave it) REMOVAL BIOPSY
RISKS
1. Conservative (Gold Standard)
- Reassurance.
- Explain benign nature.
- Explanation that excision leaves a scar.
2. Surgical
- Excision: Elliptical excision.
- Cryotherapy: Liquid nitrogen can verify flatten the lesion and reduce colour, but rarely removes it entirely. Risk of hypopigmentation (white mark).
- Shave Biopsy: Not recommended as it leaves the deep part, leading to recurrence.
- Trauma: Bleeding from shaving.
- Recurrence: If incompletely excised.
- Benign: Does not metastasize.
- Persistence: Usually lifelong if not removed. Some regress spontaneously over decades.
Key Guidelines
| Guideline | Organisation | Key Recommendations |
|---|---|---|
| Benign Skin Lesions | BAD / PCDS | Use clinical criteria. Do not refer benign lesions unless diagnostic uncertainty. |
| Dermoscopy | IDS | "Central white patch" is the specific clue. |
Rare Mimics - Be Aware
Dermatofibrosarcoma Protuberans (DFSP)
- A rare, slow-growing soft tissue sarcoma.
- Looks like a large, multi-lobulated dermatofibroma.
- History: "A mole that kept growing for years".
- Requires wide local excision.
What is a Dermatofibroma?
It is a harmless overgrowth of scar tissue deeper in the skin. It often happens after a small injury like an insect bite, prickly pear caused splinters, or an ingrown hair, even if you don't remember it.
Why is it hard?
Because it is made of fibrous tissue (like gristle). That's why it feels like a small pebble or button under the skin.
Is it cancer?
No. It is completely benign. It will not spread to other parts of your body.
Should I have it removed?
We generally advise against it. Because it goes deep into the skin, cutting it out requires stitches and leaves a line-shaped scar. On the shin or arm, this scar can stretch and look more obvious than the original brown bump.
Primary Sources
- Primary Care Dermatology Society (PCDS). Dermatofibroma Clinical Guidance. 2021.
- Zaballos P, et al. Dermoscopy of dermatofibromas: a prospective morphological study of 412 cases. Arch Dermatol. 2008.
- Mentzel T, et al. Cutaneous fibrous histiocytoma: clinicopathologic analysis. 2005.
Common Exam Questions
- Diagnosis: "Skin lesion with positive dimple sign?"
- Answer: Dermatofibroma.
- Dermoscopy: "Central white patch with peripheral pigment?"
- Answer: Dermatofibroma.
- Mimics: "Large, growing nodule >2cm?"
- Answer: Suspect DFSP (Refer).
- Management: "Best treatment?"
- Answer: Reassurance (Don't excise).
Viva Points
- Multiple Dermatofibromas: If a patient presents with sudden eruption of many dermatofibromas, screen for Immunosuppression (SLE, HIV, Leukaemia).
- Fitzpatrick Sign: Demonstrate the pinch technique.
Medical Disclaimer: MedVellum content is for educational purposes and clinical reference. Clinical decisions should account for individual patient circumstances. Always consult appropriate specialists.