Module 08: Nonmelanocytic Lesions -- Dermatofibroma

A dermatofibroma (also known as benign fibrous histiocytoma) is a benign proliferation of fibrohistiocytic cells that may occur anywhere on the skin surface but has a predilection for the extremities of adults. These tumors are among the most commonly encountered benign skin lesions in dermatology.

• Presentation: Single or multiple, round to ovoid, firm papules, plaques, or nodules
• Size: Usually less than 1 cm in diameter
• Color: Variable -- light to dark brown, purple, or red
• Dimple (Fitzpatrick) sign: Characteristic dimpling of the tumor upon lateral compression; this clinical sign is a hallmark of dermatofibroma and aids bedside diagnosis
• Histologic variants: Multiple clinical and histologic variants exist, each producing different dermoscopic features
• Key differential: Must be differentiated from melanoma, basal cell carcinoma, squamous cell carcinoma, and melanocytic nevi

Although most dermatofibromas can be reliably identified as benign tumors after clinical and dermoscopic examination, some dermatofibromas cannot be distinguished from melanoma and therefore warrant histological evaluation for diagnostic confirmation. Understanding the full spectrum of dermoscopic patterns is essential for confident diagnosis and appropriate triage.

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The dermoscopic appearance of dermatofibroma varies significantly depending on the light source and technique employed. Awareness of these differences is critical for accurate interpretation.

Polarized versus Nonpolarized Dermoscopy

Feature	Nonpolarized Dermoscopy	Polarized Dermoscopy
Shiny white structures	Not visualized	Visualized (shiny white blotches and streaks)
Blood vessels	Less apparent or absent	Accentuated appearance
Superficial epidermal features	Accentuated (ridges, fissures, comedo-like openings, milia-like cysts, mammillated surface)	Less conspicuous
Central white patch	Present	Appears brighter, may have a pink hue

Contact versus Noncontact Dermoscopy

• Contact dermoscopy: Vasculature findings are less apparent or not visualized at all
• Noncontact dermoscopy: Vessels are more conspicuous, vascular blush and dotted vessels become prominent

The physical and optical properties underlying these differences are detailed in Module 01 (see also textbook Chapter 2).

The classically described presentation using nonpolarized dermoscopy consists of:

1. Central structureless white patch (previously termed "scar-like area")
2. Delicate peripheral pigment network surrounding the central patch

This is the most common pattern overall (Pattern 2 in Table 6a.1, prevalence 35%).

Histopathologic Correlates

Dermoscopic Structure	Histopathologic Correlate
Pigment network	Hyperpigmentation of basal layer keratinocytes situated along elongated rete ridges
Central structureless white patch	Attenuated epidermis overlying a fibrous histiocytic stroma in the papillary dermis
Ring-like globules	Widely spaced, flattened and broad hyperpigmented rete ridges surrounded by an attenuated epidermis

Check Your Understanding

What is the classic dermoscopic pattern of dermatofibroma under nonpolarized dermoscopy?

The classic pattern consists of a central structureless white patch (previously termed 'scar-like area') surrounded by a delicate peripheral pigment network. This combination is present in approximately 70% of dermatofibromas.

The pigment network in dermatofibroma has specific qualitative characteristics:

• Composition: Thin, light to dark brown lines
• Quality: Characteristically fine or lacey
• Periphery: Fades gradually into the surrounding normal skin
• Center-ward transition: Toward the center of the lesion, the network may cease to look reticulated and instead may resemble "ring-like" globular or clod-like structures

Prevalence Data (Zaballos et al., n = 412)

• Pigment network present in 72% of dermatofibromas
• Majority displayed a typical delicate pigment network
• 3% had an atypical or broadened pigment network
• Location of pigment network:
  • Periphery of lesion: 71%
  • Throughout the lesion: 27%
  • Irregularly distributed: 3%

Important: When the pigment network is atypical or irregularly distributed, excision with histopathologic confirmation is recommended.

Structureless Pigmentation

• Present in 25% of dermatofibromas
• Distribution:
  • Periphery: 55%
  • Throughout the entire lesion: 24%
  • Center of lesion: 24%
  • Irregularly and multifocally distributed: 3%
• Color: Most often brown; occasionally bluish or reddish-bluish colors observed

This is the second most common individual feature in dermatofibromas.

• Present in 57% of 412 dermatofibromas (Zaballos et al.)
• Most common presentation: single, well-demarcated patch located in the lesional center
• Less common variants:
  • Multiple white scar-like patches: 12%
  • Distributed throughout the lesion: 10%
  • Irregularly distributed: 3%

Key Takeaways

• The classic dermatofibroma pattern is a delicate peripheral pigment network surrounding a central white structureless patch, seen in 35% of cases.
• The pigment network in dermatofibroma is characteristically fine, lacey, and fades gradually into surrounding skin, present in 72% of cases.
• The central white patch has a histopathologic correlate of attenuated epidermis overlying fibrous stroma and is present in 57% of dermatofibromas.

Ring-like globules are a distinctive feature of dermatofibroma:

• Characteristically round or oval structures
• The periphery is relatively darker than the center
• Correspond to widely spaced, flattened, and broad hyperpigmented rete ridges
• Typically found toward the center of the lesion, at the transition zone between the pigment network and the central white patch

The "White Network" Phenomenon

• Early descriptions noted a "white network" in dermatofibromas
• This is created when ring-like globules or small islands of pigmented rete ridges are surrounded by collagen (i.e., white structureless scar-like patch, shiny white structures)
• Diagnostic pitfall: This white network appearance can sometimes be difficult to differentiate from a negative network found in melanocytic tumors

Visible exclusively under polarized dermoscopy:

• Shiny white streaks and blotches are frequently observed within the central white patch
• Under polarized light, the central white patch appears brighter, may acquire a pink hue, and reveals these shiny structures
• Correspond to dense collagen fibers in the papillary dermis

Check Your Understanding

Why are shiny white structures (crystalline structures) visible in dermatofibroma only under polarized dermoscopy?

Shiny white structures are caused by birefringence of collagen fibers. Birefringence is a property where certain structures rotate the plane of polarized light. Since this optical phenomenon requires polarized light to be detected, these structures are completely invisible under nonpolarized dermoscopy.

Clinical Scenario

A 33-year-old woman presents with a 6 mm firm brown papule on the right lower leg that has been stable for 2 years. Lateral compression produces a dimpling of the overlying skin. Dermoscopy with nonpolarized light reveals a central structureless white patch surrounded by a delicate, lacey pigment network that fades gradually into the surrounding normal skin. At the transition zone between the network and the central white area, you observe ring-like globular structures.

What is your diagnosis and key dermoscopic findings?

Dermatofibroma (classic pattern)

This is the most common dermoscopic pattern of dermatofibroma (Pattern 2, prevalence 35%). The central structureless white patch corresponds to attenuated epidermis overlying a fibrous histiocytic stroma in the papillary dermis. The delicate peripheral pigment network corresponds to hyperpigmentation of basal keratinocytes along elongated rete ridges. The ring-like globules at the transition zone represent widely spaced, flattened, broad hyperpigmented rete ridges. The positive dimple (Fitzpatrick) sign on lateral compression provides strong clinical confirmation. The gradual fading of the network into normal skin is characteristic of dermatofibroma.

Dermatofibromas can display a variety of vascular patterns, none of which are specific to the diagnosis:

• Vessel morphologies: Dotted, comma, hairpin, glomerular, and linear vessels
• Polymorphous vessels: Multiple vessel morphologies may be present in the same lesion
• Vascular blush: Structureless erythema may be the only vascular finding
• Best visualization: Noncontact polarized dermoscopy
• Less prevalent features: Scale, ulceration/erosion, fissures, ridges, and mammillated surface

Dermoscopic patterns vary by body site:

Location	Most Common Pattern
Chest, back, arms, legs	Delicate peripheral network with central structureless white patch
Face and hands	Delicate network throughout the lesion
Trunk (Brancaccio et al. series)	Structureless pigmentation
Extremities (Brancaccio et al. series)	Central white structureless patch surrounded by delicate network

A prospective study of 100 dermatofibromas in Fitzpatrick skin type IV--V (Kelati et al.) found:

• The classic pattern (peripheral network with central white structureless area) was the most common subtype
• However, this pattern was significantly less prevalent compared with studies examining lighter-skinned individuals
• In skin type VI: lesions show peripheral delicate network, a gray central structureless patch, and numerous darkly pigmented ring-like globules
• In skin type I (red hair): lesions may present as nonpigmented with dotted vessels and shiny white streaks, mimicking Spitz nevus or amelanotic melanoma

Key Takeaways

• Ring-like globules at the transition zone between network and central patch are distinctive for dermatofibroma and should not be confused with the negative network of melanoma.
• Shiny white structures (visible only under polarized dermoscopy) appear within the central white patch and correspond to dense collagen fibers.
• Dermoscopic patterns vary significantly with skin phototype: darker skin shows gray central patches and darkly pigmented globules, while very fair skin may show nonpigmented patterns mimicking amelanotic melanoma.

Ten global patterns have been identified, grouped into those with and without a peripheral delicate pigment network (see Reference Table in Section 5 below for full details).

Aneurysmal / Hemosiderotic Dermatofibroma

• Histology: Uncommon but distinct variant characterized by blood-filled spaces occupying up to one-half of the lesion and abundant hemosiderin in the surrounding stroma
• Relationship: Hemosiderotic dermatofibroma is considered an earlier stage in the development of the aneurysmal dermatofibroma
• Dermoscopic features:
  • Central bluish or reddish structureless area with shiny white streaks
  • Delicate peripheral network with variable degrees of vascular structures
  • Bluish to yellowish and red-brownish hues in the center
  • Scaly surface (correlates to acanthosis or hyperkeratosis)
  • Yellowish structureless areas at the periphery (correlate to hemosiderin or foamy histiocytes)
  • Prominent branched (arborizing-like) vessels
  • Violaceous-blue structureless areas
  • Prominent polymorphous vessels
• In darker skin (Fitzpatrick IV--V): A white ring around a central ulceration has been reported; the white ring may be explained by dermal fibrosis surrounding large blood-filled spaces
• Clinical significance: These dermoscopic features are not specific; melanoma cannot be confidently ruled out. Histopathologic evaluation of suspicious tumors is recommended.

Atrophic Dermatofibroma

• Clinical: Flat, atrophic, or indented papules or plaques
• Histopathology: Reduction in dermal thickness and reduction in elastic fibers
• Dermoscopy: Patchy delicate network with multiple white structureless areas

Dermatofibroma with Seborrheic Keratosis-Like Features

• Superimposed epidermal changes including acanthosis, hyperkeratosis, papillomatosis, and keratin-filled invaginations
• Dermoscopic features:
  • Ridges and fissures
  • Comedo-like openings
  • Milia-like cysts
  • Variable degrees of peripheral network, white structureless areas, and ring-like globules

Lipidized Dermatofibroma

• Histopathology: Scattered cells with foamy cytoplasm
• Dermoscopy: Yellowish structureless pigmentation in all cases
  • Of 13 cases studied: 5 had total yellowish structureless pattern, 5 had an atypical pattern, 3 had a central white patch with peripheral delicate network pattern
• May show yellow clods

Dermatofibroma with Sebaceous Induction

• Reported to display whitish or white-yellow lobules
• May feature overlying sebaceous hyperplasia

Check Your Understanding

How does an atypical dermatofibroma differ dermoscopically from the classic presentation, and why is this important?

Atypical dermatofibromas may show features that overlap with melanoma, including blue-black pigmentation, irregular streaks, and asymmetric patterns. This is important because these variants account for most dermoscopy misdiagnoses of dermatofibroma, and biopsy may be warranted when the classic central white patch and peripheral network pattern are absent.

The dimple (Fitzpatrick) sign -- dimpling with lateral compression -- is a clinical maneuver that aids in the diagnosis of dermatofibroma. Its relevance in dermoscopy includes:

• The sign provides clinical confidence when dermoscopic features are equivocal
• Particularly useful in nonpigmented dermatofibromas (skin type I, fair-skinned individuals) where dermoscopy may show only dotted vessels and shiny white streaks
• The presence of a dimple sign combined with additional dermatofibromas of the same morphology in the same patient strengthens the clinical diagnosis
• Does not have a direct dermoscopic structural correlate but is a complementary bedside maneuver

• Dermatofibromas may occur as single or multiple lesions
• When multiple lesions are present in the same patient, finding consistent morphology across lesions supports the diagnosis
• Multiple dermatofibromas with the same dermoscopic pattern in a patient can provide diagnostic confidence, particularly for atypical-appearing individual lesions

Clinical Scenario

A 28-year-old man with red hair and Fitzpatrick skin type I presents with a 5 mm pink, firm papule on the left forearm that he has had for several years. Dermoscopy reveals a nonpigmented lesion with dotted vessels and shiny white streaks under polarized light. No pigment network is visible. The lesion dimples on lateral pressure. The patient has two other similar-appearing papules on the extremities.

What is your diagnosis and key dermoscopic findings?

Dermatofibroma (nonpigmented variant in fair skin)

In Fitzpatrick skin type I individuals (especially those with red hair), dermatofibromas may present as completely nonpigmented lesions showing only dotted vessels and shiny white streaks -- a pattern that can mimic Spitz nevus or amelanotic melanoma. The positive dimple sign on lateral compression and the presence of multiple lesions with consistent morphology across the patient's body strongly support dermatofibroma. The shiny white streaks correspond to dense collagen fibers in the papillary dermis (visible only with polarized dermoscopy due to birefringence). When melanoma cannot be excluded in isolated nonpigmented papules, biopsy is warranted.

Dermatofibromas must be differentiated from several important entities:

Melanoma

• Dermatofibromas with atypical pigment network, irregularly distributed pigment, or blue-red structureless areas may closely mimic melanoma
• The "white network" of dermatofibroma can be confused with the negative (inverse) network of melanoma
• Aneurysmal/hemosiderotic variants with blue-violet structureless pigmentation can be mistaken for melanoma
• Management: When melanoma cannot be confidently excluded, histopathologic evaluation is mandatory

Spitz Nevus

• Nonpigmented dermatofibromas with dotted vessels and shiny white streaks (especially in fair-skinned, red-haired individuals) enter the differential with Spitz nevus
• The dimple sign and presence of multiple similar lesions help distinguish dermatofibroma

Basal Cell Carcinoma (BCC)

• Dermatofibromas may display arborizing-like vessels, particularly the hemosiderotic variant
• The delicate pigment network of dermatofibroma is not a typical feature of BCC
• BCC-specific structures (leaf-like areas, spoke-wheel structures, blue-gray ovoid nests) are absent in dermatofibroma

Dermatofibrosarcoma Protuberans (DFSP)

• Both are fibrohistiocytic tumors, but DFSP is a malignant neoplasm requiring wide excision
• DFSP tends to be larger, present as a plaque, and lacks the dimple sign
• Dermoscopic overlap can occur; histopathology is the definitive discriminator

Amelanotic Melanoma

• Nonpigmented dermatofibromas with a central white patch, shiny white streaks, and polymorphous vessels may resemble amelanotic or hypomelanotic melanoma
• The overall pattern and dimple sign help differentiate

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Key Takeaways

• Dermatofibromas with atypical pigment network, blue-red structureless areas, or arborizing-like vessels can closely mimic melanoma or BCC and require histopathologic confirmation.
• The dimple (Fitzpatrick) sign combined with consistent morphology across multiple lesions strengthens the clinical diagnosis of dermatofibroma.
• Ten classified dermoscopic patterns exist, grouped by presence or absence of peripheral pigment network, with the atypical pattern (6%) being the most diagnostically challenging.

Pattern	Description	Prevalence (%)
1	Delicate pigment network throughout	15
2	Peripheral delicate pigment network and central white structureless patch	35
3	Peripheral delicate pigment network and central white structureless patch with ring-like globules	9
4	Peripheral delicate pigment network and central homogeneous pigmentation	5
5	Central white structureless patch with ring-like globules	2
6	Structureless pigmentation throughout	5
7a	White structureless patch throughout	6
7b	Multiple focal white structureless patches	6
8	Peripheral structureless pigmentation and central white structureless patch	7
9	Peripheral structureless pigmentation and central white structureless patch with ring-like globules	5
10	Atypical (cannot be classified into patterns 1--9)	6

Source: Adapted from Zaballos, P., et al., Arch Dermatol, 144(1), 75--83, 2008. Percentages do not sum to 100% due to rounding.

Grouping:

• Patterns with peripheral delicate pigment network: 1, 2, 3, 4
• Patterns without peripheral delicate pigment network: 5, 6, 7a, 7b, 8, 9, 10

Feature	Frequency (%)	Notes
Pigment network (any)	72	Typical delicate in most; atypical/broadened in 3%
Central structureless white patch	57	Most often single, well-demarcated, central
Structureless pigmentation	25	Peripheral (55%), throughout (24%), central (24%), irregular (3%)
Ring-like globules	Present in Patterns 3, 5, 9	Round/oval, periphery darker than center
Shiny white structures (polarized only)	Common	Blotches and streaks within the white patch
Vascular structures (any)	Variable	Dotted, comma, hairpin, glomerular, linear; polymorphous; vascular blush
Scale	Less prevalent	--
Ulceration/erosion	Less prevalent	More common in aneurysmal variant
Fissures and ridges	Less prevalent	Associated with SK-like variant
Mammillated surface	Less prevalent	Composed of ring-like globules

Variant	Key Dermoscopic Features
Classic	Peripheral delicate network, central white structureless patch, ring-like globules, shiny white structures (polarized)
Aneurysmal / Hemosiderotic	Central bluish/reddish structureless area, shiny white streaks, peripheral network, polymorphous/branched vessels, yellowish peripheral areas, violaceous-blue structureless zones; in dark skin: white ring around central ulceration
Atrophic	Patchy delicate network, multiple white structureless areas
Seborrheic keratosis-like	Ridges and fissures, comedo-like openings, milia-like cysts, variable peripheral network and ring-like globules
Lipidized	Yellowish structureless pigmentation, yellow clods; may have central white patch with peripheral network
Sebaceous induction	Whitish or white-yellow lobules

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