Acral lentiginous melanoma (ALM) is a rare subtype of melanoma that occurs on the palms, soles, and nail apparatus. Unlike more common types of melanoma, ALM occurs on sun-protected areas of the skin and has distinct clinical, histologic, and genetic features. Acral lentiginous melanoma accounts for a larger proportion of melanomas in individuals with skin of color and has a worse prognosis and recurrence rate than other forms of melanoma.
Population Trends in Skin of Color
Much of the literature on malignant melanoma historically has involved non-Hispanic white patients, but the incidence in lighter-skinned populations has been increasing steadily over the last few decades.1 Although ALM can occur in any race, it disproportionately affects skin of color populations; ALM accounts for only 0.8% to 1% of all melanomas in white populations, but it constitutes 4% to 58% of melanomas in ethnic populations and is the most common melanoma subtype among black Americans.2-5 Acral lentiginous melanoma also is associated with a worse prognosis compared to other subtypes, which may indicate a more aggressive biological nature6 but also may point toward socioeconomic and cultural barriers (eg, low income or education levels, lack of insurance, lower health literacy), leading to disparities in access to care and diagnosis at advanced stages.5
Similarly, the distribution of acral melanocytic nevi appears to demonstrate an association with ethnicity and skin pigmentation. Although skin of color patients have fewer nevi than non-Hispanic whites, the proportion of acral melanocytic nevi tends to be greater.6,7 Given its grim prognosis, accurately differentiating ALM from acral nevi is of utmost importance.
Diagnostic Challenges of Acral Lesions
Due to the unique nature of the surfaces of acral sites, melanocytic lesions on the palms, soles, and nail apparatus present many diagnostic challenges. It can be difficult to distinguish acral melanoma from benign lesions using the naked eye alone. Volar surfaces are characterized by the presence of dermatoglyphics, and pigment deposition along ridges and furrows create particular dermoscopic patterns exclusive to these sites.8 Thus, dermoscopy can be useful on acral surfaces, but the dermoscopic features are different from those on the rest of the body and must be learned separately.
In addition, nearly half of patients are unaware of their acral lesions.6 Acral surfaces may not always be examined by clinicians during total-body skin examinations, leading to further possibility of overlooking a lesion. Obtaining biopsies on glabrous skin or nails also is challenging because they can be more painful and hemostasis can be more difficult, especially in the nail. Acral melanomas also may be amelanotic, including those at subungual sites. Although the overall incidence of amelanotic ALM is low, approximately 20% to 28% of amelanotic melanomas in Asian patients are located on acral sites.9 Due to these challenges, acral lesions may be overlooked or misdiagnosed as warts,10 tinea pedis,11 or traumatic ulcers.12