American Melanoma Institute · 5 min read
How melanoma appears on the palms, soles, and under nails, why doctors often find it late, and how doctors biopsy and treat it.
Most melanoma education centers on sun and moles. Acral lentiginous melanoma (ALM) follows neither script. Acral refers to the hands and feet, and lentiginous describes the way the tumor cells first spread out in a flat layer along the base of the epidermis. ALM grows on the palms, the soles, and the nail unit, skin that is thick, hairless, and mostly shielded from the sun.
Ultraviolet light does not appear to be a meaningful risk factor for ALM. Researchers have proposed pressure and repeated injury to weight-bearing skin as contributors. The evidence remains limited, and more data is needed. Nothing a person did or failed to do is known to cause it.
ALM makes up only about 2 to 3% of melanomas. Because sun-related melanomas are uncommon in darker skin, ALM accounts for a much larger share there: 40 to 60% of melanomas in people of Asian and African American heritage, according to DermNet. The average age at diagnosis is in the early 60s.
Early ALM is a flat tan, brown, or black patch with an irregular edge and uneven color. It enlarges slowly, over months to years. Later it may thicken, form a lump, ulcerate, or bleed. Some tumors lose their pigment and appear pink or red, and these have been mistaken for warts, calluses, fungal infection, or a foot ulcer that won't heal.
The skin of palms and soles has fine parallel ridges, the same ones that form fingerprints. Under a dermatoscope, a lighted magnifier, early melanoma tends to place its pigment along the ridges themselves. Dermatologists call this the parallel ridge pattern.
Nail melanoma begins in the nail matrix, the growth center hidden under the skin at the base of the nail. As the nail grows out, pigment from the tumor gets carried forward and forms a brown or black stripe running from cuticle to tip. The thumb and the big toe are affected most often.
Doctors use an ABCDEF memory aid designed for nails:
The E in that list has its own name, the Hutchinson sign: brown or black pigment spreading from the nail onto the cuticle or the skin folds beside the nail. Dermatologists treat it as a strong warning. Other late signs include a nail that splits down the middle, lifts from its bed, or is destroyed by a lump growing beneath it. Estimates of how many nail melanomas have little or no pigment run from 25% to 50%, so a persistent sore or lump under one nail also needs an explanation.
Most dark bands in nails are benign, and several nails with similar thin, stable bands point toward a harmless cause. Blood under the nail after an injury is another common look-alike. A bruise moves toward the tip as the nail grows out, while a melanoma band stays anchored at the cuticle.
ALM carries a poorer outlook than most melanomas, and late diagnosis is a leading reason. Few people inspect the soles of their feet.
Misdiagnosis adds more delay. AIM at Melanoma, a patient foundation, notes that a large number of people with ALM are first told they have something else. Doctors may also hesitate to biopsy a nail, since the procedure is more involved than a skin biopsy and can leave a permanent ridge or split.
On a palm or sole, the doctor numbs the area and removes the whole spot or, for a large one, a deep sample from its darkest or thickest part.
For a nail band, the sample must come from the matrix, where the pigment originates. Clipping the nail or scraping its surface cannot make the diagnosis. The doctor numbs the entire finger or toe with injections at its base, lifts or removes part of the nail plate, and takes tissue from the matrix and nail bed.
Surgery is the main treatment. For ALM on a palm or sole, the surgeon performs a wide excision with a margin of normal skin set by the tumor's thickness, generally between 0.5 cm and 2 cm.
For nail melanoma, surgery removes the whole nail unit. Amputation of the fingertip or toe at the nearest joint was once routine. Many centers now offer removal of the nail unit alone with a skin graft for in situ and very thin tumors, reserving partial amputation for thicker ones. Staging, including sentinel lymph node biopsy when the tumor is thick enough, follows the same rules as for other skin melanomas.
Advanced ALM is treated with the same immunotherapy drugs used for other melanomas. AIM at Melanoma reports that response rates to checkpoint inhibitors are lower in ALM than in sun-related melanoma, although many patients still benefit. Doctors also test the tumor for mutations in genes such as BRAF and KIT, since a targeted drug may be an option when one is found.
