A 70-year-old White man presents for evaluation of a recent foot injury. The patient states that he hit his foot at home and noticed blood on the floor; because of a previous injury, he was unable to view his sole. A large (several cm), irregularly shaped plaque is seen on the mid-anterior volar foot. The lesion is brown and black, with central red discoloration and overlying yellow-white scale.
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Acral lentiginous melanoma (ALM) is an uncommon type of cutaneous melanoma that occurs on the acral surfaces (palm, sole, or nail unit), with the sole being the most common location.1 In 1976 Reed first reported on ALM, describing it as the fourth distinct subtype of cutaneous melanoma, along with superficial spreading, nodular, and lentigo maligna melanomas.2 Arrington et al helped further characterize ALM, and first documented its poor overall prognosis a year later.3
ALM accounts for approximately 2% to 3% all cutaneous melanomas, and affects men and women equally.1 Although the overall incidence of ALM is low (2.0 per million patient-years), it represents the most common type of melanoma diagnosed in darker-skinned individuals, including those of African, Asian, and Hispanic descent.1 Data suggest that ALM is disproportionately represented in dark-skinned individuals because they develop fewer cutaneous melanomas overall, particularly the types linked to chronic sun exposure.1
ALM tends to present in those aged 50 to 60 years and older, which is in line with most cutaneous melanomas.1 Recent, larger studies have attributed the poor prognosis of ALM to delayed diagnosis resulting in more advanced disease stage at presentation.1,4 These studies have shown that the independent prognostic risk factors (ulceration, tumor thickness, and tumor spread) are consistent with those detailed in the American Joint Committee on Cancer (AJCC) staging system, supporting the speculation that the decreased overall survival associated with ALM likely is due to delayed diagnosis, not because the melanoma acts more aggressively.5
A delay in diagnosis may occur because the patient may not realize the lesion is there (they may be unable to examine the bottom of their foot, as was the case in this patient) or they may not recognize the lesion as concerning. In addition, it can be difficult to perform a biopsy on the acral surfaces. In 1 study, physicians (mostly nondermatologists) clinically misdiagnosed 20% of melanomas on the palms and soles, leading to a delay in diagnosis of 12 months.6
Chronic sun exposure does not seem to be a risk factor for the development of ALM, as is the case for other types of melanoma.7 In fact, most of the typical risk factors for cutaneous melanoma — sun exposure, fair skin, family or personal history of melanoma, and multiple melanocytic nevi — do not seem to apply to this subtype.7 It has been proposed that long-term mechanical trauma may be a risk factor because ALM most commonly is found on the foot, a highly traumatized site. Although the exact pathogenesis of ALM remains uncertain, whole-genome sequencing has revealed mutated genes including BRAF, NRAS, NF1, and KIT.8,9 The overall mutation burden, however, is less than that associated with other types of melanoma.8
Aside from the location, ALM presents similarly to other types of melanoma; pigmented lesions with asymmetry, color variation, and irregular borders that grow and change over time. Raised areas, ulceration, and bleeding are signs of more advanced lesions.7, However, ALM may demonstrate unique dermoscopic findings, including a parallel ridge pattern or irregular diffuse pigmentation.7 For this reason, dermoscopy has been used widely as an adjunctive tool for the diagnosis of ALM. It can be helpful particularly in identifying early lesions and in differentiating ALM from acral nevi, traumatic lesions, tinea nigra, warts, and calluses, all of which should be included in the differential diagnosis.7
Biopsy is an invaluable tool for diagnosing ALM; there should be a low threshold for performing biopsy if the clinical suspicion for ALM is high. Dermatopathologists use multiple architectural and cytological features to make the diagnosis. Biopsy of the acral surfaces presents its own challenges. In general, it is best to provide the dermatopathologist with a biopsy of the entire lesion if ALM (or any melanoma) is suspected. This affords the dermatopathologist the opportunity to evaluate the architectural as well as cytological features of the lesion.
A partial incisional biopsy may be acceptable if removal of the entire lesion is not feasible; for very large lesions, multiple biopsies may be obtained. When providing the dermatopathologist with a partial biopsy of a larger lesion, it is very important to communicate this information to them, along with clinical details about the lesion (history of change, size, dermoscopic features, etc).
Work up and treatment of ALM primarily is based on the AJCC tumor, node, metastasis staging system, which accounts for tumor characteristics, lymph node involvement, and presence or absence of distant metastases.5 The American Academy of Dermatology also has published guidelines for the treatment of primary cutaneous melanoma.10 Tumor thickness, expressed as the Breslow thickness (mL), is arguably the most important variable because it directly correlates with prognosis. Typically, an excision of the primary site is performed, and consultation with a surgeon familiar with working on the acral surfaces should be considered. Adjuvant treatments often are added for metastatic lesions.5,8
In this case, multiple punch biopsies were performed, with evaluation confirming the diagnosis of ALM. On further workup, the patient was found to have metastatic disease; he was started on adjuvant treatment but later succumbed to the disease.
Andrew Fischer, MD, is a board-certified dermatologist at Elite Dermatology in Houston, Texas.
1. Huang K, Fan J, Misra S. Acral lentiginous melanoma: incidence and survival in the United States, 2006-2015, an analysis of the SEER Registry. J Surg Res. 2020;251:329-339.
2. Reed RJ. New Concepts in Surgical Pathology of the Skin. Wiley; 1976;89-90.
3. Arrington JH 3rd, Reed RJ, Ichinose H, Krementz ET. Plantar lentiginous melanoma: a distinctive variant of human cutaneous malignant melanoma. Am J Surg Pathol. 1977;1(2):131-143.
4. Teramoto Y, Keim U, Gesierich A, et al. Acral lentiginous melanoma: a skin cancer with unfavourable prognostic features. A study of the German Central Malignant Melanoma Registry (CMMR) in 2050 patients. Br J Dermatol. 2018;178(2):443-451.
5. Gershenwald JE, Scolyer RA, Hess KR, et al. Melanoma of the skin. In: Amin MB, Edge S, Greene F, et al, eds. AJCC Cancer Staging Manual. 8th ed. Springer; 2017:563.
6. Metzger S, Ellwanger U, Stroebel W, Schiebel U, Rassner G, Fierlbeck G. Extent and consequences of physician delay in the diagnosis of acral melanoma. Melanoma Res. 1998;8(2):181-186.
7. Darmawan CC, Jo G, Montenegro SE, et al. Early detection of acral melanoma: a review of clinical, dermoscopic, histopathologic, and molecular characteristics. J Am Acad Dermatol. 2019;81(3):805-812.
8. Hayward NK, Wilmott JS, Waddell N, et al. Whole-genome landscapes of major melanoma subtypes. Nature. 2017;545(7653):175–180.
9. Doma V, Barbai T, Beleaua MA, Kovalszky I, Rásó E, Tímár J. KIT mutation incidence and pattern of melanoma in Central Europe. Pathol Oncol Res. 2020;26(1):17-22.
10. Swetter SM, Tsao H, Bichakjian CK, et al. Guidelines of care for the management of primary cutaneous melanoma. J Am Acad Dermatol. 2018;80(1):208-250.