What is acral lentiginous melanoma?

Acral lentiginous melanoma (ALM) is a specific type of melanoma that appears on the palms of the hands, the soles of the feet, or under the nails. The word “acral” means “extremity” in Greek and refers to the occurrence of this type of melanoma on the extremities (hands and feet). The word “lentiginous” refers to the fact that the spot of melanoma is often much darker than the surrounding skin.

Though relatively uncommon in the general population, ALM is the most common type of melanoma in people with darker skin and those of Asian descent. However, it can appear on all skin types.

Because of the misconceptions that melanomas only occur in sun-exposed areas and that people of color are not at risk for melanoma, these melanomas can be discovered later than other types, after they have invaded deeper layers of skin or metastasized.

Acral lentiginous melanoma (ALM) can be difficult to diagnose and has a lower response rate to available therapies for cutaneous melanoma such as immune checkpoint inhibitors, but survival in the metastatic setting is second only to pure cutaneous melanoma.

Where ALM Can Develop

ALM most often develops on or around hands and feet, or in nail beds. Like superficial spreading melanoma, ALM grows on the surface of the skin or under the nail bed before becoming invasive.

ALM sometimes develops from an existing mole. It can also occur seemingly out of nowhere on healthy skin.

What ALM Looks Like

ALM is typically a dark spot of skin that is surrounded by skin that remains your normal skin color. There is often a clear border between the dark skin and the lighter skin around it. This contrast in color is one of the most noticeable characteristics of this type of melanoma. ALM may be hard to recognize at first if the patch of darkened skin is small and looks like little more than a stain or bruise.

Note that ALM may not always be dark-colored or even dark at all. Some spots may be reddish or orange in color — these are called amelanotic (or non-pigmented) melanoma. Read more about amelanotic melanoma.

Warning signs of ALM include:

  • a new streak in a nail that is not caused by an accident or bruise
  • a nail streak that has damaged the fingernail
  • a changing spot in or connected to a mole on the foot or hand
  • an irregularly-shaped growth on the foot or hand that is changing, growing or has an unusual color
  • an elevated, thickened patch growing on the sole of the foot or palm of the hand

ALM on the Palm or Sole

Most people first notice an oddly shaped black, gray, tan, or brown mark with irregular borders.

Acral lentiginous melanoma affects the feet and hands, as well as the skin beneath the nails. Image credit: Image credit: Will Blake, (2006, May 29)

ALM Under a Nail

The first sign may be a “nail streak,” a narrow, dark stripe under the nail. Not all nail streaks are melanoma; many dark-skinned people have nail streaks that are completely benign. ALM usually develops on the thumb or big toe; however, it can occur under any fingernail or toenail. As the ALM grows, your nail might begin to crack or break altogether.

Subungual (nailbed) melanoma is less common but represents about 33% of melanomas in dark-skinned people. Characteristics include a brown or black linear nail band(s) often wider than 5 mm with a blurred or irregular border with the thumb and big toe being the primary nail site. Approximately half of these tumors may have little or no color leading to a delay in diagnosis. The colored nail band variety can be confused with a fungal infection (especially in the toe) or trauma thus leading to misdiagnosis and/or delay in treatment.

Causes and risk factors

Anyone can develop ALM. Unlike most other forms of melanoma, ALM does not appear to be related to sun exposure. In some people, it may be due to a genetic risk factor.

Death rates from ALM are higher than those of other forms of melanoma. This may be because ALM can go unnoticed longer, allowing it to spread and become more aggressive before treatment begins. Early diagnosis and prompt treatment have a major impact on whether a person will survive. It’s important to note that a large number of people who have ALM are initially misdiagnosed with something else.

Prevention and Early Detection

As researchers do not yet know what causes ALM, they also do not know how to prevent it. The best opportunity for a favorable outcome is an early diagnosis. ALM is rare, but it can be deadly. Monitoring the skin for changes can be life-saving.

It is important to schedule annual skin examinations with your dermatologist and to make sure your doctor checks the palms of your hands, the soles of your feet, and your nail beds. If an unexplained lesion appears on your hand, foot, or nail, you should see your dermatologist as soon as possible so that s/he can take a biopsy of the area and decide whether the spot is cancerous. As with any form of melanoma, diagnosing it early is key.

Other Things to Know About Acral Lentiginous Melanoma

As an ALM tumor increases in size, it usually becomes more irregular in shape and color (although some ALM lesions can be lightly colored or colorless).

The surface of the ALM lesion may remain flat, even as the tumor invades deeply into the skin.

Thickening ALM on the sole of the foot can make walking painful and be mistaken for a plantar wart.

The surface of a spot of ALM may also start out smooth and become bumpier or rougher as it evolves. If a tumor begins to grow from the cancerous skin cells, the skin will become more bulbous, discolored, and rough to the touch.

Less advanced cancers and thinner tumors have better survival rates. Raised tumors tend to be more aggressive.

Men are more likely than women to have thick, large tumors at diagnosis.