Acral lentiginous melanoma represents a distinct subtype of cutaneous melanoma that arises on non-hair-bearing surfaces of the hands, feet, palms, soles, and nail apparatus. This melanoma subtype accounts for approximately 5% to 10% of melanomas in Caucasian populations but represents 40% to 60% of melanomas in African, Asian, and Hispanic populations. Despite similar overall incidence rates, acral lentiginous melanomas present at substantially more advanced stages than other melanoma subtypes, resulting in significantly worse prognosis and lower survival rates.
Anatomical Distribution and Unique Features
The acral distribution pattern creates specific challenges for early detection and screening. Plantar and palmar lesions may remain undetected for extended periods because these surfaces receive less frequent self-examination and sun exposure is not the primary pathogenic mechanism. Subungual melanomas (arising beneath nail plates) present particular diagnostic difficulties as pigmentation may be attributed to benign conditions like trauma, fungal infection, or normal age-related changes. Approximately 15% to 20% of acral lentiginous melanomas occur as subungual lesions.
Unlike other melanoma subtypes that show clear association with cumulative solar ultraviolet exposure, acral lentiginous melanomas demonstrate relatively weak ultraviolet-exposure correlation. This distinction suggests alternative pathogenic mechanisms including chronic mechanical irritation, thermal injury, and potential viral factors. The reduced photodamage background makes histopathological recognition of acral melanomas more challenging, as adjacent tissue lacks the typical solar elastosis seen in sun-exposed melanomas.
Clinical Presentation and Diagnostic Challenges
Acral lentiginous melanomas typically present as darkly pigmented patches or plaques with irregular borders and color variegation. Lesions often measure 2 to 3 centimeters or larger at initial presentation, indicating delayed diagnosis relative to other melanoma subtypes. Plantar lesions may appear as dark streaks or patches within the plantar creases, potentially mimicking traumatic hematomas or benign pigmented macules. Palmar presentations show similar features, often with prominent color variation ranging from dark brown to black with intervening tan or red areas.
Subungual melanomas present unique diagnostic challenges. Early lesions appear as brown or black bands within the nail plate, vertical striping affecting a portion of the nail width, or longitudinal melanonychia. These findings overlap significantly with benign nail melanosis, which represents normal age-related pigmentation accumulation in older individuals. The presence of pigmentation extending into the nail fold (Hutchinson sign) increases melanoma suspicion. Additionally, subungual melanomas often involve nail bed involvement, creating nail dystrophy, onycholysis (nail separation), or nail plate thickening.
Breslow Thickness and Prognostic Implications
Acral lentiginous melanomas carry substantially worse prognosis than other subtypes when matched for Breslow thickness, suggesting intrinsic biological aggressiveness or advanced stage at diagnosis. The mean Breslow thickness at diagnosis averages 2.8 to 3.5 millimeters for acral lesions compared to 1.5 to 2.0 millimeters for superficial spreading melanomas. This thickness difference reflects delayed recognition and diagnosis. Even when Breslow thickness is controlled for statistically, acral lentiginous melanomas demonstrate higher rates of lymph node involvement and distant metastasis.
Five-year survival rates for acral lentiginous melanoma vary dramatically by stage at diagnosis. Stage I lesions (Breslow thickness less than 2.0 millimeters without ulceration) show five-year survival rates of approximately 80% to 85%. Stage II lesions (Breslow thickness 2.0 to 4.0 millimeters or less than 2.0 millimeters with ulceration) demonstrate five-year survival declining to 60% to 70%. Stage III disease with lymph node metastasis shows five-year survival of only 40% to 50%. Stage IV disease with distant metastasis carries five-year survival of 15% to 20%.
TNM Staging and Lymph Node Involvement
TNM staging of acral melanomas incorporates Breslow thickness as the primary prognostic variable for localized disease. Ulceration significantly worsens prognosis; ulcerated melanomas show increased melanoma-specific death risk of 1.4 to 2.0 times compared to non-ulcerated lesions of equivalent thickness. Mitotic rate, defined as number of mitotic figures per square millimeter in the most mitotically active area, further refines risk stratification. Lesions with mitotic rate greater than 10 per square millimeter show substantially elevated recurrence risk.
Sentinel lymph node biopsy serves critical prognostic and staging functions in acral melanomas. The technique identifies occult lymph node metastases undetectable by physical examination or imaging. Patients with positive sentinel lymph nodes face increased recurrence risk and warrant consideration of additional treatment. The frequency of positive sentinel lymph node biopsies ranges from 15% to 25% even in melanomas of intermediate Breslow thickness (1.2 to 3.5 millimeters). Standard recommendations include sentinel lymph node biopsy for all acral melanomas with Breslow thickness greater than 1.0 millimeter.
Histopathological Features and Molecular Characteristics
Acral lentiginous melanomas demonstrate distinctive histopathological patterns including predominant single-cell infiltration of the basal epidermis (single-unit growth pattern), significant acanthosis (epidermal thickening), and variable dermal involvement. The lentiginous growth pattern indicates orderly arrangement of melanocytes along the dermal-epidermal junction, in contrast to the sheets of melanocytes seen in nodular melanomas. Junctional and superficial dermal components often coexist with nodular invasion, creating combination patterns.
Molecular analysis of acral melanomas reveals distinct mutation profiles compared to other subtypes. While cutaneous melanomas commonly harbor BRAF V600E mutations (frequency 50% to 70%), acral melanomas show lower BRAF mutation rates (20% to 30%) and higher frequencies of KIT mutations and NRAS mutations. These molecular differences have important implications for targeted therapy selection. KIT mutations, present in 15% to 30% of acral melanomas, potentially respond to tyrosine kinase inhibitors like imatinib. NRAS mutations, present in 15% to 20%, do not respond to standard BRAF/MEK inhibitors but may respond to MEK inhibitor monotherapy or emerging therapies.
Treatment Strategies and Immunotherapy Response
Surgical excision remains primary treatment for localized acral lentiginous melanoma. Wide local excision achieving 1 to 2 centimeter margins minimizes local recurrence risk. For plantar and palmar lesions, surgical planning requires careful consideration of functional anatomy and preservation of critical structures. Amputation may prove necessary for subungual melanomas or extensive lesions, though less aggressive approaches preserving functional anatomy are generally preferred when achieving adequate surgical margins.
Adjuvant therapy improves recurrence-free and overall survival in stage III acral melanomas with lymph node involvement. Immunotherapy using checkpoint inhibitors including nivolumab and ipilimumab shows substantial benefit. Nivolumab monotherapy at 3 milligrams per kilogram administered intravenously every two weeks for 12 weeks followed by maintenance therapy improves three-year recurrence-free survival to 65% compared to 55% with observation alone. Combination immunotherapy with nivolumab (1 milligram per kilogram) plus ipilimumab (3 milligrams per kilogram) further improves recurrence-free survival but increases toxicity rates.
For stage IV disease, first-line systemic therapy typically involves immunotherapy or targeted therapy depending on mutation status. Combination immunotherapy with nivolumab plus ipilimumab achieves response rates of 40% to 50% with median overall survival of 15 to 18 months. BRAF-mutated melanomas respond to BRAF plus MEK inhibitor combinations (dabrafenib 150 milligrams twice daily plus trametinib 2 milligrams daily) achieving response rates of 60% to 70% and median progression-free survival of 9 to 11 months. KIT-mutated acral melanomas may respond to imatinib (400 milligrams daily) or other kinase inhibitors.
FAQ
Should dark lines under fingernails always be biopsied?
Longitudinal pigmentation of the nail plate is common, particularly in older individuals and those with darker skin types. However, certain features increase melanoma suspicion: involvement of a single digit rather than multiple nails, Hutchinson sign (pigment extending to nail fold), progressive widening, nail dystrophy, or associated bleeding. When any of these concerning features are present, dermatological evaluation and likely biopsy are warranted.
What is the survival difference between acral and superficial spreading melanoma?
When matched for Breslow thickness, acral lentiginous melanomas still show 10% to 15% worse five-year survival rates compared to other subtypes. This difference reflects higher rates of lymph node metastasis and more aggressive biological behavior. Much of the observed survival difference, however, reflects delayed diagnosis with thicker lesions at presentation.
Are acral melanomas more common in any particular population?
Yes, acral lentiginous melanomas represent a disproportionately large fraction of melanomas in African, Asian, and Hispanic populations, accounting for 40% to 60% of cases compared to 5% to 10% in Caucasians. This geographic and ethnic variation suggests different biological pathways and may reflect underrecognition of melanomas in heavily pigmented skin.
What are typical dosages for immunotherapy in acral melanoma?
Nivolumab adjuvant therapy is administered at 3 milligrams per kilogram intravenously every two weeks for up to 12 months. For metastatic disease, dosing changes to 240 milligrams every two weeks or 480 milligrams every four weeks. Ipilimumab is dosed at 3 milligrams per kilogram intravenously every three weeks for four doses when used as monotherapy, or 1 milligram per kilogram when combined with nivolumab.
References
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2. Chang JW, Sosa JM, Callender GG, et al. Acral lentiginous melanoma: prognosis and comparison of thin and thick lesions. Archives of Surgery. 2006;141(5):456-462. Analysis of surgical outcomes and prognostic factors specific to acral presentations.
3. Googe PB, Harris S, Hata TR, et al. Subungual melanoma: clinical and pathological study of 46 cases. Journal of the American Academy of Dermatology. 1999;40(4):539-545. Detailed review of subungual melanoma features, presentation patterns, and diagnostic approaches.
4. Coit DG, Thompson JA, Albertini MR, et al. Cutaneous melanoma, version 2.2019, NCCN clinical practice guidelines in oncology. Journal of the National Comprehensive Cancer Network. 2019;17(4):367-402. Evidence-based guidelines for sentinel lymph node biopsy, adjuvant therapy, and systemic treatment in advanced acral melanoma.
5. Curtin JA, Fridlyand J, Kagane T, et al. Distinct sets of genetic alterations in melanoma. New England Journal of Medicine. 2005;353(20):2135-2147. Molecular characterization demonstrating elevated KIT and NRAS mutation frequencies in acral melanomas.
6. Borgognone A, Maurya N, Lissoni A, et al. Immunotherapy in acral lentiginous melanoma: emerging challenges and future perspectives. Clinical & Experimental Metastasis. 2021;38(4):425-438. Contemporary review of immunotherapy responses in acral melanoma subtypes.
7. Wong SL, Balch CM, Hurley P, et al. Sentinel lymph node biopsy for melanoma: American Society of Clinical Oncology and Society of Surgical Oncology joint clinical practice guideline. Journal of Clinical Oncology. 2012;30(23):2912-2918. Recommendations for sentinel lymph node biopsy application across melanoma subtypes including acral presentations.
8. Kirkwood JM, Strawderman MH, Ernstoff MS, et al. Interferon alpha-2b adjuvant therapy of high-risk resected cutaneous melanoma: the Eastern Cooperative Oncology Group Trial EST 1684. Journal of Clinical Oncology. 1996;14(1):7-17. Landmark adjuvant therapy trial establishing benefit of systemic immunotherapy in high-risk melanoma.
9. Larkin J, Chiarion-Sileni V, Gonzalez R, et al. Combined nivolumab and ipilimumab or monotherapy in untreated melanoma. New England Journal of Medicine. 2015;373(1):23-34. Pivotal trial demonstrating superior response rates with combination immunotherapy in advanced melanoma.
10. Hauschild A, Dummer R, Schadendorf D, et al. Dabrafenib plus trametinib versus dabrafenib alone in cutaneous melanoma with BRAF mutation status. Lancet Oncology. 2015;14(12):e438-e448. Data regarding targeted therapy outcomes in BRAF-mutated melanomas including acral presentations.