Author: Dr Amanda Oakley, Dermatologist, Hamilton, New Zealand, 2011.
Acral lentiginous melanoma is a type of melanoma arising on the palms or soles. It is a form of melanoma characterised by its site of origin: palm, sole, or beneath the nail (subungual melanoma). It is more common on feet than on hands. It can arise de novo in normal-appearing skin, or it can develop within an existing melanocytic naevus (mole).
Acral lentiginous melanoma starts as a slowly-enlarging flat patch of discoloured skin. At first, the malignant cells remain within the tissue of origin, the epidermis — the in situ phase of melanoma, which can persist for months or years.
Acral lentiginous melanoma becomes invasive when the melanoma cells cross the basement membrane of the epidermis, and malignant cells enter the dermis. A rapidly-growing nodular melanoma can also arise within acral lentiginous melanoma and proliferate more deeply within the skin.
About 1 to 3% of melanomas in Australia and New Zealand are acral lentiginous melanoma. Melanoma is a potentially serious skin cancer that arises from pigment cells (melanocytes). Although acral lentiginous melanoma is rare in Caucasians and people with darker skin types, it is the most common subtype in people with darker skins.
Acral lentiginous melanoma is relatively rare compared to other types of melanoma. There is no connection with the colour of skin (skin phototype), and it occurs at equal rates in white, brown or black skin. Acral lentiginous melanoma accounts for 29-72% of melanoma in dark-skinned individuals but less than 1% of melanoma in fair-skinned people, as they are prone to more common sun-induced types of melanoma such as superficial spreading melanoma and lentigo maligna melanoma.
Acral lentiginous melanoma is equally common in males and females. The majority arise in people over the age of 40.
The cause or causes of acral lentiginous melanoma are unknown. It is not related to sun exposure.
Acral lentiginous melanoma on palm, soles or fingers and toes starts as an enlarging patch of discoloured skin. It is often thought at first to be a stain. It can also be amelanotic (non-pigmented, usually red). Like other flat forms of melanoma, it can be recognised by the ABCDE rule: Asymmetry, Border irregularity, Colour variation, large Diameter and Evolving.
The characteristics of acral lentiginous melanoma include:
When acral lentiginous melanoma arises in the nail region, it is known as melanoma of the nail unit. If it starts in the matrix (nail growth area), it is called subungual melanoma. It may present as diffuse discolouration or irregular pigmented longitudinal band(s) on the nail plate. Advanced melanoma destroys the nail plate altogether. It can be amelanotic.
Acral lentiginous melanoma is due to the development of malignant pigment cells (melanocytes) along the basal layer of the epidermis. These cells may arise from an existing melanocytic naevus or more often from previously normal-appearing skin. What triggers the melanocytes to become malignant is unknown, but it involves genetic mutations.
Dermoscopy, or the use of a dermatoscope, by a dermatologist or other doctor trained in its use, can be very helpful in distinguishing acral lentiginous melanoma from other skin lesions, particularly:
The most frequently observed dermoscopic features of acral lentiginous melanoma are:
If the skin lesion is suspicious of acral lentiginous melanoma, it is best cut out (excision biopsy). A partial biopsy is best avoided, except in unusually large lesions. An incisional or punch biopsy could miss a focus of melanoma arising in a pre-existing naevus. However, sometimes the lesion is very large, and before performing significant surgery, a biopsy is arranged to confirm the diagnosis. Biopsy of a lesion suspicious of acral lentiginous melanoma should remove a long ellipse of skin, or there should be several biopsies taken from multiple sites, as a single site could miss a malignant focus.
The pathological diagnosis of melanoma can be challenging. Histological features of acral lentiginous melanoma include an asymmetrical proliferation of melanocytes at the dermoepidermal junction.
The pathologist's report should include a macroscopic description of the specimen and melanoma (the naked eye view) and microscopic description.
The report may also include comments about the cell type and its growth pattern, invasion of blood vessels or nerves, inflammatory response, regression, and whether there is an associated naevus (original mole).
The Breslow thickness is reported for invasive melanomas. It is measured vertically in millimetres from the top of the granular layer (or base of superficial ulceration) to the deepest point of tumour involvement. It is a strong predictor of outcome; the thicker the melanoma, the more likely it is to metastasise (spread).
The Clark level indicates the anatomic plane of invasion.
Level 1 — In situ melanoma
Level 2 — Melanoma has invaded the papillary dermis
Level 3 — Melanoma has filled the papillary dermis
Level 4 — Melanoma has invaded the reticular dermis
Level 5— Melanoma has invaded subcutaneous tissue
The deeper the Clark level, the higher the risk of metastasis (secondary spread). It is useful in predicting outcome in thin tumours and less useful for thicker ones in comparison to the value of the Breslow thickness.
The initial treatment of primary melanoma is to cut it out; the lesion should be completely excised with a 2–3 mm margin of healthy tissue. Further treatment depends mainly on the Breslow thickness of the lesion.
After initial excision biopsy; the radial excision margins, measured clinically from the edge of the melanoma, recommended in The Australian and New Zealand Guidelines for the Management of Melanoma (2008) are shown in the table below. A flap or graft may be needed to close the wound. In the case of acral lentiginous and subungual melanoma, this may include partial amputation of a digit. Occasionally, the pathologist will report incomplete excision of the melanoma, despite wide margins requiring further surgery or radiotherapy to ensure the tumour has been completely removed.
Melanoma in situ — excision margin 5 mm
Melanoma < 1.0 mm — excision margin 1 cm
Melanoma 1.0–2.0 mm — excision margin 1–2 cm
Melanoma 2.0–4.0 mm — excision margin 1–2 cm
Melanoma > 4.0 mm — excision margin 2 cm
Melanoma staging means finding out if the melanoma has spread from its original site in the skin. Most melanoma specialists refer to the American Joint Committee on Cancer (AJCC) cutaneous melanoma staging guidelines (2009). In essence, the stages are:
Stage 0 — In situ melanoma
Stage 1 — Thin melanoma <2 mm in thickness
Stage 2 — Thick melanoma > 2 mm in thickness
Stage 3 — Melanoma spread to involve local lymph nodes
Stage 4 — Distant metastases have been detected
If the local lymph nodes are enlarged due to metastatic melanoma, they should be removed entirely. This requires a surgical procedure, usually under general anaesthetic. If they are not enlarged, they may be tested to see if there is any microscopic spread of melanoma. The test is known as a sentinel node biopsy.
In New Zealand, many surgeons recommend sentinel node biopsy for melanomas thicker than 1 mm, especially in younger persons. However, although the biopsy may help in staging cancer, it does not offer any survival advantage. The necessity for sentinel node biopsy is controversial at present.
Lymph nodes containing metastatic melanoma often increase in size quickly. An involved node is usually non-tender and firm to hard in consistency. If this occurs between planned follow-up visits, let your doctor know promptly.
If the melanoma is widespread, other forms of treatment may be necessary but are not always successful in eradicating cancer. Immunotherapy, biologics such as ipilimumab, and BRAF inhibitors such as vemurafenib and dabrafenib are showing promise.
The primary purpose of follow-up is to detect recurrences early.
The Australian and New Zealand Guidelines for the Management of Melanoma (2008) make the following recommendations for follow-up for patients with invasive melanoma.
The follow-up appointments may be undertaken by the patient's general practitioner or specialist, or they may be shared.
Follow-up appointments may include:
In those with more advanced primary disease, follow-up may include:
Tests are not typically worthwhile for stage 1/2 melanoma patients unless there are signs or symptoms of disease recurrence or metastasis. And no tests are thought necessary for healthy patients who have remained well for five years or longer after removal of their melanoma, whatever stage.
Acral lentiginous melanoma in situ is not dangerous; it only becomes potentially life-threatening if an invasive melanoma develops within it. The risk of spread from invasive melanoma depends on several factors, but the main one is the measured thickness of the melanoma at the time it was surgically removed.
The Melanoma Guidelines report that metastases are rare for melanomas <0.75mm and the risk for tumours 0.75–1 mm thick is about 5%. The risk steadily increases with thickness so that melanomas >4 mm; result in a 10-year survival of around 50%, according to the American Joint Committee on Cancer (AJCC) statistics.
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