Acral Lentiginous Melanoma: Key Facts About This Rare Skin Cancer

Acral lentiginous melanoma (ALM) is a rare and often aggressive form of melanoma that occurs on the palms of the hands, soles of the feet, and under the nails. Unlike other melanomas, it is not linked to sun exposure, making it the most common melanoma in people with dark skin. Understanding its unique characteristics is vital for early detection and treatment.

Acral melanoma accounts for less than 5% of all melanomas in white populations but up to 70% in people of color. It can appear as a dark, irregular spot on the sole of the foot or palm, often mistaken for a bruise or fungal infection. Because of its hidden locations, ALM is often diagnosed at a later stage, emphasizing the need for awareness.

What Is Acral Lentiginous Melanoma?

This rare cancer is a subtype of melanoma that develops from melanocytes in the basal layer of the epidermis. The term "acral" refers to the extremities (hands and feet), and "lentiginous" describes its growth pattern—spreading horizontally before becoming invasive. It primarily affects the palms, soles, and nail beds, particularly subungual melanoma.

Unlike other melanomas that are strongly linked to UV radiation, its exact cause is unknown. Genetic mutations, such as in the BRAF gene, are less common compared to other melanomas, suggesting different biological pathways. This makes it a unique challenge for researchers and clinicians.

Symptoms of Acral Melanoma on Palms and Soles

Early signs include a new or changing pigmented lesion on the hands or soles. It may appear as a brown or black streak, patch, or irregularly shaped spot. The ABCDE rule applies, but ALMs often present with additional features.

• A dark spot or streak on the palm or sole that is growing or changing shape.
• A pigmented lesion that extends from the nail into the cuticle (Hutchinson's sign) – common in subungual melanoma.
• Ulceration, bleeding, or a lump under the nail or on the foot.
• Pain or tenderness in the affected area, sometimes misdiagnosed as a wart or callus.

ALM can also appear between toes or fingers – a site often overlooked. Any new pigmented lesion on the extremities, especially in people with dark skin, should be evaluated by a dermatologist.

Causes and Risk Factors for Acral Lentiginous Melanoma

While the exact cause is unknown, certain factors increase risk. ALM is more common in older adults, but can occur at any age. People with darker skin types (Fitzpatrick types IV–VI) are at higher risk, particularly African Americans, Asians, and Hispanics. Genetic mutations, such as in the NF1 or KIT genes, are sometimes implicated, but UV exposure is not a known risk factor.

Other possible risk factors include a history of trauma to the area, chronic inflammation, or immune suppression. However, no definite link has been established. The presence of a pre-existing mole or lesion on the palms or soles is also a risk factor.

Diagnosis and Staging of Acral Melanoma

Diagnosis begins with a thorough skin examination. Dermoscopy can help distinguish acral melanoma from benign lesions, but a biopsy is essential. The sample is examined under a microscope for atypical melanocytes and invasion depth (Breslow thickness). Staging involves imaging (CT, PET) to check for metastasis to lymph nodes or organs.

ALM has a higher rate of misdiagnosis due to its subtle appearance. Studies show that delays in diagnosis are common, leading to thicker tumors and worse outcomes. Public awareness and routine skin checks are critical.

Treatment Options for Acral Lentiginous Melanoma

Treatment depends on the stage. For early-stage ALM, surgical excision with wide margins is the standard. For subungual melanoma, amputation of the digit may be necessary to ensure clear margins. Sentinel lymph node biopsy is performed to check for microscopic spread.

Advanced ALM may require immunotherapy, targeted therapy (if BRAF or KIT mutations are present), or chemotherapy. Clinical trials are exploring new approaches, including intralesional therapies and novel immunotherapies. Because ALM is less responsive to some treatments, multidisciplinary care is important.

Prognosis and Prevention

Prognosis is directly related to thickness at diagnosis. Early detection leads to high survival rates, while advanced disease has a poorer prognosis. The five-year survival for localized acral melanoma is about 80–90%, but drops to 20–30% for metastatic disease. Prevention focuses on regular self-exams and professional skin checks, especially for high-risk individuals.

Because sun exposure is not a factor, sunscreen is not preventative for ALM. Instead, know the signs: any new or changing pigmented lesion on the palms, soles, toes, or between toes warrants immediate evaluation. If you have risk factors, consider monthly self-exams and annual skin checks.

ALM is a challenging cancer, but with awareness, early diagnosis is possible. Share this information to help others recognize the signs on their own skin.