Acral lentiginous melanoma (ALM) is a type of skin cancer that develops on the palms, soles, fingers, toes, and under the nails. Unlike the more common forms of melanoma, it is not driven by sun exposure and tends to appear in areas most people rarely check. It accounts for less than 1% of melanomas in non-Hispanic White individuals but represents up to 19% of melanomas in Black, Asian, American Indian/Alaska Native, and Hispanic populations, making it the most significant melanoma subtype in people with darker skin.
Where It Develops and Why
ALM grows from melanocytes, the pigment-producing cells in the outermost layer of skin. What sets it apart from other melanomas is its location: the hairless skin of the palms, soles, and the nail units of fingers and toes. When it develops under a nail, the thumb and great toe account for 92% of cases.
The genetics behind ALM look nothing like the sun-related melanomas most people are familiar with. Sun-driven melanomas carry high numbers of DNA mutations with a clear ultraviolet radiation signature. ALM has a significantly lower mutation burden and lacks those UV fingerprints. Roughly half of acral melanomas have mutations in the BRAF, RAS, or NF1 genes that commonly drive other skin cancers. The other half belong to a “triple-wild-type” group with a diverse mix of less common genetic alterations. Mutations or amplifications in the KIT gene appear in 3% to 36% of cases and seem to specifically encourage the flat, spreading growth pattern typical of early ALM. Because UV radiation is not a major factor, aggressive sun protection alone does little to prevent this cancer.
What It Looks Like
In its earliest stages, ALM can be remarkably easy to miss. It often starts as a faint brown or gray patch with blurry, poorly defined edges that blend into the surrounding skin rather than forming a distinct border. Over time the spot may darken unevenly, developing areas of tan, brown, and black. The lesion is typically flat during its initial growth phase, which can last years before it begins to thicken or develop a raised, nodular area.
Under the nail, ALM usually shows up as a dark streak running lengthwise from the cuticle to the tip. A key warning sign is Hutchinson’s sign: pigment that extends from the nail bed into the surrounding skin of the cuticle or nail fold. It’s worth noting that a “pseudo-Hutchinson sign” also exists, where dark pigment beneath the nail simply shows through a translucent cuticle without actually involving the surrounding skin. Dermoscopy, a magnified examination of the skin’s surface, can help distinguish between the two.
On the palms and soles, dermatologists look for a pattern called “parallel ridges,” where pigment follows the ridges of the skin’s surface rather than the furrows. This pattern is strongly associated with acral melanocytic malignancy and helps differentiate ALM from benign moles in the same location.
Why It’s Often Misdiagnosed
ALM is frequently mistaken for something harmless. In a systematic review of misdiagnosed cases, the initial wrong diagnosis was a non-healing ulcer or traumatic injury 37.5% of the time, a benign growth like a wart or callus 29.2% of the time, and an infection such as a fungal nail condition 20.8% of the time. Lesions on the sole of the foot get blamed on repeated friction. Dark streaks under the nail get attributed to bruises (subungual hematomas) from stubbing a toe or wearing tight shoes.
The result is that ALM is often advanced at the time of diagnosis. People delay seeking care because the spot is on the bottom of a foot or hidden under a toenail. Even when they do show it to a clinician, the location and subtle early appearance can lead to a wait-and-see approach rather than an immediate biopsy. This delay is one of the main reasons ALM carries higher morbidity and mortality compared to melanomas caught earlier on more visible skin.
How It’s Staged
Like all melanomas, ALM is staged using the AJCC system, which relies heavily on tumor thickness measured in millimeters (called Breslow depth). The thresholds are straightforward:
- T1: 1.0 mm or thinner
- T2: 1.0 to 2.0 mm
- T3: 2.0 to 4.0 mm
- T4: thicker than 4.0 mm
Within each T category, the presence or absence of ulceration (a breakdown of the skin surface over the tumor) determines whether the substage is “a” or “b,” with ulcerated tumors carrying a worse prognosis at every thickness level. The overall stage also factors in whether cancer has reached nearby lymph nodes or spread to distant organs.
Treatment
Surgery is the primary treatment. The goal is to remove the melanoma along with a margin of healthy tissue around it. Recommended margins depend on the tumor’s thickness:
- Melanoma in situ (not yet invasive): 0.5 to 1 cm margin
- Up to 1.0 mm thick: 1 cm margin
- 1.0 to 2.0 mm thick: 1 to 2 cm margin
- Thicker than 2.0 mm: 2 cm margin
On the hands and feet, achieving those margins can be challenging. The palms and soles have limited loose skin for closure, and nail unit melanomas may require partial or complete amputation of the affected finger or toe. For in-situ disease, some surgeons use a staged approach similar to Mohs surgery, examining the margins under a microscope before closing the wound, because ALM can extend well beyond its visible borders in the thick skin of the sole.
For melanomas that have spread to lymph nodes or beyond, immunotherapy and targeted therapy play a role. The presence of KIT mutations in a subset of ALM cases is particularly relevant because these tumors may respond to treatments that specifically block KIT signaling, an option not typically used in BRAF-driven melanomas.
Survival and Prognosis
Overall five-year survival for ALM is around 56%, which is lower than the survival rates for melanoma in general. That gap is largely due to later-stage diagnosis rather than the biology of the tumor itself being more aggressive at the same thickness. In a large cohort study, more than half of patients had died within a median follow-up of 28 months, reflecting how many cases are diagnosed after the cancer has already invaded deeply or spread.
When caught early, while still in situ or thin, ALM is highly curable with surgery alone. The critical variable is thickness at diagnosis. Every millimeter of depth changes the outlook substantially, which is why periodic self-checks of the palms, soles, nail beds, and spaces between the toes matter. A new dark streak under a nail that wasn’t caused by obvious injury, or a slowly expanding patch on the sole that doesn’t heal, warrants a biopsy rather than observation.