A 26-year-old African American man with no significant past medical history presents with a growth on his left foot that had been increasing in size for 1 year. He initially thought it was a wart but had concerns when it continued to grow and became painful. Examination reveals an exophytic, brown, and mildly tender nodule, 3cm in diameter, on his left medial heel and instep. The overlying skin is crusted and friable. His nails and hair show no abnormalities. There is no associated lymphadenopathy. The remainder of the examination is unremarkable.
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Melanoma is a mucocutaneous malignant tumor arising from melanocytes and the sixth leading cause of malignancy in the United States.1 It causes more than 75% of all skin cancer deaths. One subtype, acral lentiginous melanoma (ALM), accounts for less than 5% of all melanoma cases and presents on the palms, soles, and subungual sites. The mean age of diagnosis for this condition is in the 60s. It begins with lentiginous (radial) growth and evolves into dermal (vertical) invasion after months or years.2
Although the exact etiology remains unknown, melanoma is postulated to be a disease caused partially by ultraviolet (UV) radiation, with a higher incidence in whites and on sun-exposed sites. ALM, however, appears disproportionately in African Americans, Asians, and Hispanics, and no consistent results show a correlation with UV exposure.1-3 ALM exhibits increased expression of KIT, a tyrosine kinase receptor, and fewer BRAF kinase mutations than are seen in other cutaneous melanomas, indicating genetic risk factors.4
Compared with other subtypes, a higher proportion of ALMs are diagnosed at a later stage due to their unusual locations and visual similarity to benign lesions. This may explain the increased mortality associated with this cancer subtype.3
One-third1 to one-half3 of ALM cases are initially misdiagnosed as benign lesions, such as verrucae, ulcers, hematomas, or fungal infections. Some may present as chronic wounds or dysplastic nevi, and dermoscopy is helpful for differentiation.5,6 Like other melanomas, ALM can present classically as pigmented or mottled macules, or papules with irregular borders. Benign acral nevi share some features with ALM but are usually symmetric, light brown, and uniform, with clear borders. Some ALMs become amelanotic, with red-pink coloration and are easily missed, especially on plantar surfaces. Advanced tumors can form large, exophytic, friable nodules. Many lesions are associated with pain, bleeding, or itching.7
Melanonychia, a black-brown pigmentation in the nail plate, is an important characteristic seen in two-thirds of subungual melanomas but is also seen in benign conditions (eg, trauma, onychomycosis, paronychia). If the streak extends onto the nail fold, it is called Hutchinson sign. Important diagnoses to consider include Bowen disease and drug-induced nail pigmentation, which can be ruled out based on patient history, dermoscopy, or biopsy, if necessary.1,3,7
Early diagnosis is essential for survival. Educating patients about the “ABCDEs” of melanoma (asymmetrical shape, border, color, diameter, evolution) is as important as looking for changes in color, shape, or size of skin lesions on sun-exposed and protected skin. Another mnemonic, “EFG,” stands for elevated, firm, or growing, and accounts for amelanotic or nodular melanomas. Surrounding erythema or inflammation should also increase suspicion, as well as individual nevi that are distinct from surrounding ones.1
Dermoscopy aids the diagnosis of melanoma and involves a two-step algorithm. First, a lesion is considered melanotic in origin if it contains a pigment network, streaks, aggregated globules, homogeneous blue pigment, or parallel pattern (seen in acral lesions). Once the lesion is deemed melanocytic, it must be determined if it is benign nevi or melanoma.1,5 Accentuated pigmentation on the ridges in a parallel pattern is a finding specific to ALM.5 Other dermoscopic signs of ALM are irregular diffuse pigmentation or irregular lines within pigmented nail lesions. Amelanotic ALM exhibits remnants of pigmentation and heterogeneous vascular patterns. Recognition of these dermoscopic patterns can help identify early disease.6
Excisional biopsy is the gold standard for diagnosis. Incisional biopsies are acceptable with lesions in the facial or acral regions, those with low clinical suspicion, or those that are very large but may not show the full Breslow depth.7,8 Histologically, ALM possesses an asymmetric proliferation of single melanocytes at the dermal-epidermal junction and atypical melanocytes in the hyperplastic epidermis. Single, pleomorphic melanocytes with enlarged nuclei and dendritic processes predominate. In contrast, benign nevi are composed mainly of nested melanocytes. In late stages of disease, pagetoid migration is visible. Deep dermal mitoses and frequent skip areas within the tumor are also features of ALM.1,7 Prognostic markers include Breslow thickness, Clark level, ulceration, dermal mitoses, sentinel lymph node positivity, and signs of intravascular or neural involvement.2,4,7
Surgical excision of the primary neoplasm is the only curative procedure. The goals are to achieve histologically negative margins and prevent local recurrence. For tumors <1 mm thick, 1-cm surgical margins are sufficient. Tumors that are 1 to 2 mm thick require surgical margins of 1 to 2 cm. For tumors >2 mm, 2-cm margins are recommended with sentinel lymph node biopsy (SLNB) in all tumors >1 mm thick.4,8 Amputation of the toes or fingers may be indicated in cases of subungual ALM. Lesions >4 mm thick are difficult to manage and often require adjuvant high-dose interferon therapy. Other adjuvant therapies for patients at high risk of recurrence or metastases include radiation and tyrosine kinase inhibitors. Once diagnosis has been confirmed, a full staging workup is necessary.8
In our case, an incisional biopsy of the patient’s neoplasm confirmed the diagnosis of ALM >4 mm thick. The lesion was surgically removed with clean margins down to the muscle, necessitating a graft. SLNB results came back positive, and the patient was referred to oncology for further treatment.
Eman Bahrani, BA, is a medical student and Maura Holcomb, MD, is a dermatology resident at Baylor College of Medicine in Houston.
- Garbe C, Bauer J. Melanoma. In: Bolognia JL, Jorizzo JL, Schaffer JV, eds. Dermatology. 3rd ed. Philadelphia, PA: Elsevier Saunders; 2012:1885-1894.
- Bradford PT, Goldstein AM, McMaster ML, Tucker MA. Acral lentiginous melanoma: incidence and survival patterns in the United States, 1986-2005. Arch Dermatol. 2009;145(4):427-434.
- Stubblefield J, Kelly B. Melanoma in non-Caucasian populations. Surg Clin North Am. 2014;94(5):1115-1126.
- Bello DM, Chou JF, Panageas KS, et al. Prognosis of acral melanoma: a series of 281 patients. Ann Surg Oncol. 2013;20(11):3618-3625.
- Oh TS, Bae EJ, Ro KW, et al. Acral lentiginous melanoma developing during long-standing atypical melanosis: usefulness of dermoscopy for detection of early acral melanoma. Ann Dermatol. 2011;23(3):400-404.
- Phan A, Dalle S, Touzet S, et al. Dermoscopic features of acral lentiginous melanoma in a large series of 110 cases in a white population. Br J Dermatol. 2010;162(4):765-771.
- Piliang MP. Acral lentiginous melanoma. Clin Lab Med. 2011;31(2):281-288.
- Bichakjian CK, Halpern AC, Johnson TM, et al; American Academy of Dermatology. Guidelines of care for the management of primary cutaneous melanoma. J Am Acad Dermatol. 2011;65(5):1032-1047.