Epidemiology
Acral lentiginous melanoma (ALM) is one of the less common melanoma variants, affecting primarily the palms, soles, and subungual areas. People most commonly affected are of African American, Latino or Asian descent, though anyone may develop this type of melanoma. The incidence in the U.S. is 1.8 per million person-years. Delay in diagnosis results in a poorer prognosis in ALM than in other melanoma subtypes. Average age at diagnosis is also somewhat older than other melanoma variants, and this subtype is not typically associated with chronic ultraviolet light exposure or sunburns.
Clinical Presentation
Patients may present with an irregular pigmented lesion on the palm, sole, or volar aspect of a finger or toe (the great toe and the thumb being most often affected of the digits). Features that should be considered in differentiating melanoma from a benign nevus include the ABCDE's: Asymmetry, Border irregularity, Color variation, Diameter (larger lesions are more worrisome), and Evolution (a lesion that is changing or growing) (Figure 1). Because of the thick stratum corneum in these areas, diagnosis is often delayed, and lesions may appear warty or non-pigmented. This may lead to misdiagnosis of a verruca, callus, or chronic paronychia. In more advanced lesions, the patient may present with a non-specific ulcer, mimicking diabetic foot ulcer or a traumatic ulcer. In subungual lesions, there is often a pigmented streak beneath the nail plate, typically with pigmentation of the proximal or lateral nail fold (Figure 2). Most subungual lesions originate in the nail matrix. Dermoscopy can increase diagnostic accuracy for ALM when used correctly.
Diagnosis
Skin biopsy is imperative in making an accurate diagnosis, and the type of biopsy obtained can affect the accuracy of the interpretation. Ideally, excisional biopsy of the entire lesion is performed, but this is not often possible with large lesions. The most important factors in obtaining a biopsy are not to transect the base of the lesion and to obtain a representative sample. For flat lesions a deep shave biopsy or punch biopsy at least 4 mm in diameter may be a reasonable alternative to excision. Assessing the lateral borders of the lesion for architecture can aid in the diagnosis, and therefore, the larger the specimen, the more likely the pathologist is to correctly diagnose the lesion. For pigmented streaks involving the nail, biopsy of the matrix is essential. If the clinician is unsure whether the nail pigmentation represents hemorrhage or melanin pigment, a nail clipping for histologic evaluation is a reasonable first step.
Histopathology
On biopsy, the dermatopathologist will report whether the lesion is a true melanocytic neoplasm (nevus, melanoma in situ, or invasive melanoma) or whether the pigmentation is due to other causes, such as a lentigo, hemorrhage or a pigmented fungal infection (such as tinea nigra). Biopsy of ALM in situ demonstrates a proliferation of single atypical melanocytes that are arranged in a contiguous pattern along the basal cell layer, with poor lateral circumscription, and some nesting and / or pagetoid spread involving the upper layers of the epidermis (Figure 3). The histologic changes in ALM in situ may be deceptively bland, and diagnosis is very difficult with small biopsy specimens. Acral nevi are typically more nested, sharply circumscribed, and symmetrical, without nuclear atypia. Invasive melanoma will demonstrate tumor cells that involve the dermis or in more advanced lesions, even the subcutaneous tissue. Nail clipping of a melanocytic lesion will demonstrate melanin pigment granules within the nail plate, whereas pigmentation due to trauma or fungal infection will show extravasated red blood cells or fungal organisms, respectively.
The Biopsy Report and Prognosis
ALM in situ is nearly 100% curable with surgical excision. When there is an invasive component, prognosis is dependent upon a number of histologic characteristics, all of which are described in the pathology report. Immunostains may be helpful in further characterizing the lesion and / or determining melanocyte density on biopsy (Figure 4). Most dermatopathologists report invasive melanomas in a template format to include the following characteristics:
- Melanoma subtype (ALM, nodular, superficial spreading, lentigo maligna, desmoplastic)
- Clark level (scale of I to V, depending on the level of invasion)
- Breslow thickness (reported in millimeter thickness from the top of the skin to the deepest portion)
- Radial / Vertical growth phas
- Mitotic count (number of mitoses per millimeter squared)
- Tumor-infiltrating lymphocytes (density of inflammation associated with the tumor)
- Ulceration (presence or absence)
- Regression (presence or absence)
- Precursor lesion (such as a nevus or dysplastic nevus)
- Perineural / Vascular invasion (presence or absence)
- Satellitosis (tumor aggregates not connected to the main neoplasm)
The most important determinants of prognosis in invasive melanoma are the Breslow thickness, presence or absence of ulceration, and presence or absence of dermal mitoses. These three features are independent predictors of melanoma free survival and recurrence, and are also used in staging the patient.
Conclusions
ALM is a rare but often overlooked variant of melanoma. With proper physical examination and biopsy of suspicious lesions, diagnosis can be made early and prognosis improved. Podiatrists are instrumental in early diagnosis by performing a careful physical exam, including between the toes and all toenails. Appropriate biopsy of suspicious lesions will help the dermatopathologist make an accurate diagnosis, so that patients may have the best chance of cure.
References
1. Bradford PT et al: Acral lentiginous melanoma: Incidence and survival patterns in the United States, 1986-2005. Arch Dermatol 2009;145:427-34.
2. Bodman M: Acral lentiginous melanoma: What sets it apart. Podiatry Today 2015;29:May2016.
3. Hale C: Melanocytic tumor melanoma - Acral lentiginous melanoma. PathologyOutlines.com February 21, 2017.
