Module 24: Acrolentiginous Melanoma

Acrolentiginous melanoma (ALM) is the melanoma subtype that arises on the glabrous (hairless, non-sun-exposed) skin of the palms, soles, and subungual regions. Although ALM is the least common melanoma subtype overall, it represents a disproportionately high percentage of melanomas in patients with darker skin phototypes (Fitzpatrick types IV-VI) and in Asian populations. The plantar surface (sole of the foot) is the most common site.

The fundamental principle of acral dermoscopy rests on a single anatomic observation:

• In acral nevi, melanocytes reside at the crista limitans, which corresponds to the skin furrows (sulcus superficialis). Melanin is transferred preferentially to keratinocytes in the furrows, producing the parallel furrow pattern (benign).
• In acral melanoma, malignant melanocytes proliferate around the crista intermedia, likely because melanocytic stem cells reside in this environment. The result is pigmentation located on the ridges (crista superficialis), producing the parallel ridge pattern (malignant).

This ridge-versus-furrow distinction is the single most important concept in acral dermoscopy and forms the basis of all acral dermoscopic algorithms.

The openings of eccrine sweat glands are located on the ridges. On dermoscopy, these appear as regularly spaced whitish dots within pigmented bands. Their position helps the observer confirm whether pigmentation is on the ridges (malignant pattern) or in the furrows (benign pattern).

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The skin on palms and soles has a unique anatomy due to the presence of alternating furrows and ridges, which creates the individualized dermatoglyphic patterns (fingerprints). Understanding the particular histologic anatomy of palms and soles is essential for correctly interpreting the significance of dermoscopic colors, structures, and patterns of lesions in these locations.

Key Anatomic Structures

Structure	Location	Clinical Significance
Sulcus superficialis (furrow)	Surface depression between ridges	Corresponds to crista limitans; site of pigment deposition in nevi
Crista superficialis (ridge)	Elevated surface marking	Site of eccrine duct openings; site of pigment deposition in melanoma
Crista limitans	Deep dermal ridge beneath the furrow	Nevomelanocytic nests preferentially transfer melanin here
Crista intermedia	Deep dermal ridge beneath the surface ridge	Melanocytic stem cell niche; favored by malignant melanocytes
Eccrine ducts	Open on the ridges	Appear as whitish dots; landmark for identifying ridges

In melanocytic nevi, nevomelanocytic nests may be found at the tips of rete ridges associated with both the crista intermedia and crista limitans; however, melanin is preferentially transferred to keratinocytes from nests at the crista limitans, resulting in the parallel furrow pattern. In early melanoma, malignant melanocytes favor growth in the environment surrounding the crista intermedia (potentially because melanocytic stem cells reside there), resulting in pigmentation on the ridges.

The dermoscopic patterns of benign lesions on volar skin are described in detail in Module 31 (Chapter 11b).

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Six malignant patterns have been described for melanocytic lesions on volar skin:

4.2.1 Parallel Ridge Pattern (PRP)

The parallel ridge pattern is the most common and most specific dermoscopic finding in early (including in situ) acrolentiginous melanoma. It consists of linear pigmentation located predominantly on the ridges.

Diagnostic performance:

• Specificity: 99%
• Sensitivity: 86%
• Positive predictive value: 84%

Key characteristics:

• Band-like pigmentation on the surface ridges of the skin markings
• Whitish dots (eccrine duct openings) regularly distributed within the pigmented bands confirm ridge localization
• Typically found in the macular (flat) portions of volar melanomas
• Present in both in situ and invasive melanomas

In situ vs. invasive distinction:

• Melanoma in situ: Usually displays only tan to brown lines on the ridges without other colors or visible structures
• Invasive melanoma: May display pigmentation on the ridges, but it will rarely be the only feature seen and will rarely manifest as only tan to brown ridges; additional features such as blue-gray color, irregular dots, blue-white veil, and vascular structures are commonly present

Important exceptions -- conditions that can mimic the PRP:

1. Ethnic pigmentation (darker skin phototypes) -- benign volar lesions may display a parallel ridge pattern
2. Peutz-Jeghers syndrome -- mucocutaneous pigmentation syndrome
3. Laugier-Hunziker syndrome -- benign acquired mucocutaneous hyperpigmentation; recognized by the clinical presentation of multiple small brownish macules on multiple fingers and lips
4. Chemotherapy-induced volar pigmentation -- drug-related hyperpigmentation
5. Exogenous dye -- external pigment application
6. Subcorneal blood (hemorrhage) -- blood tracks along the least resistant tissue planes (possibly along eccrine glands), accumulating on the ridges and creating a parallel ridge pattern or "pebbles on the ridges" pattern. Subcorneal blood has a reddish-black color composed of droplets with smooth margins. The scraping test (paring down the stratum corneum with a scalpel) can confirm subcorneal hemorrhage by removing the blood and revealing normal underlying skin
7. Congenital nevi -- can occasionally display a ridge pattern

4.2.2 Diffuse Pigmentation with Variable Shades of Brown and Serrated Pattern

The diffuse pigmentation with variable shades of brown is the second most common pattern seen in ALM. It consists of structureless diffuse pigmentation involving both the ridges and furrows, displaying various shades of brown color. This pattern can be seen in both in situ and invasive ALMs.

The serrated pattern is another feature occasionally encountered in ALM. It consists of projections (similar to streaks) at the edge of the tumor, producing a serrated or jagged peripheral margin.

4.2.3 Atypical Fibrillar Pattern

The fibrillar pattern on volar skin results from mechanical frictional forces (described in detail in Module 31, Chapter 11b). The distinction between benign and atypical (malignant) fibrillar patterns is critical:

Location considerations:

• Any lesion on the palms displaying a fibrillar pattern should be considered atypical unless extenuating circumstances account for constant frictional forces (since constant frictional pressure is required to form the fibrillar pattern)

Features of atypical fibrillar pattern (type B) on plantar skin:

• Increased variability in color of fibrillar lines
• Increased variability in thickness of fibrillar lines
• Disorganized distribution (lack of uniform alignment)
• Gray-colored fibrillar lines -- should always be considered atypical

The atypical fibrillar pattern may represent an artifactual expression of the parallel ridge pattern on pressure-loaded areas of the sole, and oblique-view dermoscopy can help differentiate.

4.2.4 Milky-Red Pattern

The milky-red pattern is defined by the presence of homogeneous reddish/whitish colors. This pattern is mainly seen in thicker melanomas that have a nodular component with prominent vascularization. Its presence suggests a more advanced (invasive) lesion.

4.2.5 Multicomponent Pattern

The multicomponent pattern consists of multiple patterns present focally within the lesion, including focal presence of a parallel ridge pattern, furrow pattern, or fibrillar pattern. The focal ridge pattern tends to be found at the periphery of the melanoma.

Additional features of the multicomponent pattern in invasive melanomas:

• Structureless diffuse pigmented areas with multiple colors (brown, red, blue-white, black)
• Irregular dots and globules
• Streaks (serrated edge)
• Irregular blotches
• Blue-white veil
• Regression structures
• Atypical vessels

Advanced invasive ALMs may present with ulceration, symmetry of overall architecture (despite being malignant), and an amelanotic central nodule with clearly visible atypical vessels surrounded by a pigmented periphery showing atypical dots, globules, and parallel ridge pattern.

4.2.6 ALM with Focal Benign Patterns

This is a critically important concept. Melanoma on volar skin may display patterns associated with nevi, including the parallel furrow pattern and lattice-like pattern. However, these benign patterns will be present only focally within the melanoma, with the rest of the lesion displaying malignant patterns (pigmentation on the ridges, diffuse pigmentation with variable shades of colors, atypical fibrillar pattern).

Clinical rule: Malignant patterns trump benign patterns.

When evaluating lesions on volar skin, it is imperative to:

1. Search the entire surface area of the lesion for malignant patterns
2. Not prematurely anchor the diagnosis based on the focal presence of a benign pattern
3. Look for malignant patterns first, then assess for benign patterns

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Check Your Understanding

What dermoscopic pattern on acral skin is most specific for melanoma?

The parallel ridge pattern, in which pigmentation follows the ridges (crista superficialis) of the dermatoglyphics, is most specific for acral melanoma. In contrast, benign acral nevi show a parallel furrow pattern where pigment follows the furrows (sulci). The distinction between ridge and furrow pigmentation is the most critical dermoscopic feature on acral skin.

Key Takeaways

• The parallel ridge pattern (pigment on the ridges (crista superficialis) of dermatoglyphics) is the hallmark of acral melanoma and has the highest sensitivity and specificity of any single dermoscopic feature for ALM.
• Parallel ridge pattern corresponds histopathologically to melanocyte proliferation around eccrine duct openings (acrosyringia) on the crista intermedia.
• Any lesion on palms or soles showing parallel ridge pattern should be biopsied regardless of size, clinical appearance, or patient demographics.

Clinical Scenario

A 68-year-old woman presents with an 8 mm pigmented macule on her right sole that she noticed 6 months ago. Dermoscopy reveals linear brown pigmentation on the ridges (parallel ridge pattern) with whitish dots (eccrine duct openings) distributed within the pigmented bands, along with focal areas of irregular blue-gray pigmentation.

What is the diagnosis, and what is the significance of the blue-gray areas?

Acrolentiginous Melanoma (ALM)

The parallel ridge pattern (pigment on the ridges confirmed by eccrine duct openings within the pigmented bands) is the hallmark of acral melanoma with 86% sensitivity and 99% specificity. Any volar lesion showing this pattern requires biopsy. The focal blue-gray areas suggest dermal invasion beyond the in situ stage, as melanoma in situ typically shows only tan-to-brown ridge pigmentation. This lesion likely has an invasive component requiring urgent excisional biopsy and appropriate surgical margins.

Amelanotic ALM presents a particular diagnostic challenge because it lacks the typical pigmented patterns. Key dermoscopic clues to amelanotic ALM include:

• Atypical vessels -- particularly corkscrew vessels, which are characteristic
• Remnants of pigment -- even small amounts of residual pigment can guide diagnosis
• Hemorrhage and ulceration
• Hyperkeratosis

These vascular and structural clues are often the only features available to suggest the diagnosis in the absence of melanin-based patterns.

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Most acquired nevi on volar skin are under 7 mm in greatest diameter. Therefore:

• Acquired melanocytic lesions on volar skin that are greater than 7 mm in greatest diameter should raise suspicion for melanoma, independently of the dermoscopic features displayed by the lesion.
• This size criterion serves as an important safety net when dermoscopic patterns are ambiguous or nontypical.

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Check Your Understanding

What is the parallel furrow pattern, and what does it indicate?

The parallel furrow pattern consists of pigmented lines following the sulci (furrows) of acral dermatoglyphics. It is the most common dermoscopic pattern of benign acral melanocytic nevi, found in approximately 45% of cases. Variants include the lattice-like pattern, fibrillar pattern, and crista dotted pattern, all of which are considered benign.

Key Takeaways

• The fibrillar pattern shows oblique pigmented lines crossing both furrows and ridges, caused by mechanical shearing forces on the thick stratum corneum of weight-bearing areas.
• Fibrillar pattern can occur in both benign nevi and melanoma; applying counter-pressure or changing the angle of dermoscope application can reveal whether pigment originates from furrows (benign) or ridges (malignant).
• Irregular diffuse pigmentation (structureless brown-black areas) on acral skin without recognizable parallel patterns should be considered suspicious for melanoma.

Some volar lesions will not display any of the typical dermoscopy patterns described for nevi or melanomas. When a lesion cannot be classified into recognized benign (patterns 1-6) or malignant (patterns 10-12) patterns, it is classified as having a "nontypical pattern."

For such lesions, Mikoshiba et al. demonstrated that the following features should raise concern for melanoma:

• Asymmetry in the distribution of colors and structures
• Presence of more than two colors
• Blue-white structures
• Vessels
• Dots
• Streaks
• Abrupt edges

Context (age, size, and location) should not be ignored when trying to differentiate nevi from melanomas.

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Clinical Scenario

A 55-year-old man presents with a 5 mm pigmented lesion on his left palm. Dermoscopy reveals linear pigmentation confined to the furrows (parallel furrow pattern) with a lattice-like arrangement. There is no ridge pigmentation, no irregular blotch, and the pattern is symmetric in structure and color.

Is this benign or malignant, and how does the BRAAFF algorithm classify it?

Benign Acral Melanocytic Nevus

The parallel furrow pattern (pigment in the furrows, not on the ridges) is the most common benign dermoscopic pattern on volar skin. Using BRAAFF: irregular Blotch = 0, parallel Ridge pattern = 0, Asymmetry of structures = 0, Asymmetry of colors = 0, parallel Furrow pattern = -1, Fibrillar pattern = 0. Total score = -1 (negative). A score of 0 or below favors a benign lesion. However, if this lesion were >7 mm, closer monitoring would be warranted regardless of the benign pattern, as the 7 mm rule increases suspicion for acral melanocytic lesions above this size threshold.

flowchart TD
 A[Volar Melanocytic<br/>Lesion] --> B{Irregular<br/>Blotch?}
 B -->|Yes: +1| C{Parallel Ridge<br/>Pattern?}
 B -->|No: 0| C
 C -->|Yes: +3| D{Asymmetry of<br/>Structures?}
 C -->|No: 0| D
 D -->|Yes: +1| E{Asymmetry of<br/>Colors?}
 D -->|No: 0| E
 E -->|Yes: +1| F{Parallel Furrow<br/>Pattern?}
 E -->|No: 0| F
 F -->|Yes: -1| G{Fibrillar<br/>Pattern?}
 F -->|No: 0| G
 G -->|Yes: -1| H[Calculate<br/>BRAAFF Score]
 G -->|No: 0| H
 H -->|Score >= 1| I[Suspicious for<br/>Acral Melanoma --<br/>Biopsy]
 H -->|Score < 1| J[Low Suspicion --<br/>Monitor]

Lallas et al. created the BRAAFF algorithm to assist clinicians in analyzing volar lesions. This algorithm uses both positive and negative criteria, with each criterion weighted by its association strength with nevi or melanoma.

Code	Criterion	Type	Score
B	Irregular blotch	Positive (anywhere within the lesion)	+1
R	Parallel ridge pattern	Positive (anywhere within the lesion)	+3
A	Asymmetry of structures	Positive (anywhere within the lesion)	+1
A	Asymmetry of colors	Positive (anywhere within the lesion)	+1
F	Parallel furrow pattern	Negative (anywhere within the lesion)	-1
F	Fibrillar pattern	Negative (anywhere within the lesion)	-1

Interpretation: A total score of 1 or greater is required for the lesion to be considered suspicious for acral melanoma.

Key features of the BRAAFF algorithm:

• The parallel ridge pattern carries the highest individual weight (+3), reflecting its strong association with melanoma
• Benign patterns (parallel furrow, fibrillar) serve as negative criteria but cannot fully offset positive findings
• Both positive and negative criteria are assessed anywhere within the lesion
• The algorithm incorporates both pattern-based and symmetry-based criteria

Source: Lallas A, et al., Br J Dermatol, 173(4), 1041-1049, 2015.

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Check Your Understanding

How should you approach a pigmented acral lesion that shows neither a clear parallel ridge nor parallel furrow pattern?

When the pattern is ambiguous, the multicomponent pattern (multiple unrelated dermoscopic structures) or the irregular diffuse pigmentation pattern should raise suspicion for melanoma. In such cases, consideration of the lesion's clinical features (size >7 mm, asymmetry, recent change) and comparison with the contralateral site are helpful. Biopsy is recommended for any acral lesion with an irregular or unclassifiable dermoscopic pattern.

Key Takeaways

• The multi-component pattern in ALM combines parallel ridge, irregular diffuse pigmentation, and regression areas, indicating advanced melanoma with multiple growth phases.
• Subungual melanoma extends onto acral skin as a periungual pigmentation (Hutchinson sign) and follows acral dermoscopic rules in the periungual area.
• Globulo-streak pattern at the periphery of an acral lesion indicates lateral expansion of melanocytic nests and should raise concern for melanoma even if the center appears benign.

While the primary focus of Chapter 8d is volar skin melanoma, ALM also involves the nail apparatus. The following features are relevant to subungual melanoma (covered in greater detail in Module 33: Nails, and Chapter 11b):

Dermoscopic Features of Nail Melanoma

Feature	Description	Significance
Longitudinal melanonychia	Pigmented band running length of the nail plate	Starting point for evaluation; may be benign or malignant
Irregular band width/spacing	Variable width, color, or spacing of longitudinal lines	Suggests malignancy; benign bands tend to be regular
Micro-Hutchinson sign	Periungual pigmentation visible only with dermoscopy	Highly suspicious for subungual melanoma
Hutchinson sign	Periungual pigmentation visible clinically (naked eye)	Classic sign of subungual melanoma
Destruction of nail apparatus	Partial or complete nail plate destruction	Indicates advanced invasive melanoma
Parallel ridge pattern on hyponychium	PRP visible on the exposed hyponychial skin	Corresponds to in situ tumor component in the nail region

When the nail apparatus is destroyed by invasive melanoma, examination of the hyponychium and surrounding volar skin may reveal the stereotypical parallel ridge pattern, with white eccrine ducts opening on top of the pigmented ridge, corresponding to areas of in situ tumor.

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4.8.1 Acral Melanocytic Nevus

• Displays benign patterns: parallel furrow, lattice-like, fibrillar, globular, homogeneous, or acral reticular
• Usually under 7 mm in greatest diameter
• Uniform brown coloration
• Symmetric structure and color distribution
• See Module 31 for complete description of benign acral patterns

4.8.2 Subcorneal Hematoma (Black Heel, Talon Noir)

• Pigmentation prominent on the ridges, somewhat mimicking the parallel ridge pattern
• Composed of droplets with smooth margins (not diffuse bands)
• Reddish-black color (not brown)
• Pattern described as "pebbles on the ridges" -- discrete droplets rather than continuous band-like pigmentation
• Often the result of ill-fitting sport shoes or trauma
• Scraping test: Paring down the stratum corneum with a scalpel removes the blood, and the underlying skin appears normal
• Rust-colored parallel ridge pattern after sports activity should prompt consideration of subcorneal hemorrhage

4.8.3 Laugier-Hunziker Syndrome

• Benign acquired mucocutaneous hyperpigmentation
• Can display a parallel ridge pattern mimicking melanoma
• Recognized by the clinical context: multiple small brownish macules on multiple fingers AND lips
• Systematic clinical examination prevents misdiagnosis as melanoma

4.8.4 Peutz-Jeghers Syndrome

• Hereditary mucocutaneous pigmentation syndrome
• Can display parallel ridge pattern on acral skin
• Distinguished by characteristic perioral and buccal pigmentation, intestinal polyposis, and family history

4.8.5 Ethnic Pigmentation

• Benign volar pigmentation in patients with darker skin phototypes
• Can display a parallel ridge pattern
• Context of multiple similar lesions, bilateral symmetry, and clinical history help distinguish from melanoma

4.8.6 Chemotherapy-Induced Pigmentation

• Drug-related volar hyperpigmentation
• Can manifest with a parallel ridge pattern
• History of chemotherapy is the key distinguishing factor

4.8.7 Congenital Nevi

• Can occasionally display a ridge pattern on volar skin
• History of presence since birth or early infancy
• Typically larger and with characteristic clinical features

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Key Takeaways

• Acral melanoma must be differentiated from acral nevi (parallel furrow), subcorneal hemorrhage (grows out with nail/skin, satellite globules of blood), and ethnic pigmentation (regular parallel furrow on soles).
• The crista dotted pattern (regular dots on ridges) is a benign variant of acral pigmentation commonly seen in children and should not be confused with parallel ridge pattern.
• Any new pigmented lesion on acral skin in adults over 50 should be evaluated with dermoscopy, as ALM incidence increases with age and early detection dramatically improves prognosis.

Pattern	Description	Dermoscopic Appearance	Significance
Parallel furrow pattern	Pigmentation following the furrows (sulcus superficialis)	Linear pigment bands in the furrows; ridges remain unpigmented	Benign -- most common nevus pattern on volar skin
Lattice-like pattern	Pigmentation following the furrows plus cross-hatching	Furrow pigmentation with linear bands crossing over the ridge from one furrow to the next (like rungs of a ladder)	Benign -- variant of parallel furrow pattern
Fibrillar/filamentous pattern	Fine parallel streaks crossing dermoglyphics tangentially	Parallel fine lines crossing the furrows and ridges at an oblique angle	Benign -- caused by mechanical friction on pressure-bearing areas; benign when uniform in color, thickness, and alignment
Globular pattern	Globules not associated with a parallel pattern	Round or oval brown structures without alignment to furrows or ridges	Benign
Homogeneous pattern	Light brown amorphous pigmentation	Diffuse, uniform light brown color without discernible structure	Benign -- only in lesions less than 7-10 mm
Acral reticular pattern	Pigment network independent of skin markings	Well-defined reticular lines not associated with dermatoglyphics	Benign

Pattern	Description	Dermoscopic Appearance	Significance
Parallel ridge pattern (PRP)	Linear pigmentation on the ridges (crista superficialis)	Band-like pigmentation on ridges with whitish eccrine duct dots within bands	Malignant -- hallmark of ALM; sensitivity 86%, specificity 99%, PPV 84%
Diffuse pigmentation with variable brown	Structureless pigmentation involving ridges and furrows	Diffuse pigmentation with multiple shades of brown obscuring dermatoglyphics	Malignant -- second most common ALM pattern
Serrated pattern	Streak-like projections at tumor edge	Jagged, irregular projections (similar to streaks) at the periphery	Malignant -- occasionally seen in ALM
Atypical fibrillar pattern (type B)	Disorganized fibrillar lines with color/thickness variability	Fibrillar lines with variable color, variable thickness, disorganized distribution; gray fibrillar lines	Malignant -- any fibrillar pattern on palms is suspicious; gray fibrils always atypical
Milky-red pattern	Homogeneous reddish/whitish coloration	Structureless reddish-white areas	Malignant -- indicates thicker, vascularized, invasive melanoma
Multicomponent pattern	Multiple patterns and structures focally present	Focal PRP (often peripheral), diffuse multi-colored pigmentation, irregular dots/globules, streaks, blotches, blue-white veil, regression, atypical vessels	Malignant -- indicates invasive melanoma with multiple dermoscopic features

When a volar lesion cannot be classified into recognized benign or malignant patterns, the following features raise concern for melanoma:

Feature	Significance
Asymmetry of color distribution	Suspicious
Asymmetry of structure distribution	Suspicious
More than two colors	Suspicious
Blue-white structures	Suspicious
Vessels	Suspicious
Dots	Suspicious
Streaks	Suspicious
Abrupt edges	Suspicious

Clinical context (age, size >7 mm, location) should always be integrated into the assessment.

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