There has been a great increase in public awareness of melanoma, and any dark lesions of the skin are sometimes regarded with the same dread as Long John Silver’s “black spot” in
Treasure
Island a sign of imminent demise.
However, the vast majority of pigmented lesions are simply moles or harmless pigmented naevi.
The most important thing is to know which moles can be safely ignored and which should be removed.
Benign moles are described first, then malignant melanoma, followed by a discussion of the differences between these two.
Benign moles
Benign moles are naevi with a proliferation of melanocytes and a variable number of dermal naevus cells.
Some moles are congenital and are present from birth, but most develop in early childhood and adolescence.
The number of moles remains constant during adult life with a gradual decrease from the sixth decade onwards.
There is often an increase in both the number of moles and the degree of pigmentation during pregnancy.
Acquired melanocytic naevi
Acquired melanocytic naevi are the familiar moles and present in a number of different ways depending on the type of cells and the depth in the skin.
Junctional naevi are flat macules with melanocytes proliferating along the dermo-epidermal border.
Compound naevi have pigmented naevus cells at the dermoepidermal border and in the dermis, producing a raised brown lesion.
The dermal melanocytes may accumulate around the skin appendages and blood vessels and form a band of cells without melanin or more deeply penetrating strands of spindle cells.
Proliferating naevus cells may throw the overlying epidermis into folds, giving a papillary appearance.
In a purely intradermal naevus the junctional element is lost, with the deeper cells showing characteristics of neural tissue.
Other types of acquired pigmented naevi include the following.
Blue naevus is a collection of deeply pigmented melanocytes situated deep in the dermis, which accounts for the deep slate-blue colour.
Spitz naevus presents as a fleshy pink papule in children.
It is composed of large spindle cells and epitheloid cells with occasional giant cells, arranged in “nests”.
It is benign and the old name of juvenile melanoma should be abandoned.
Halo naevus consists of a melanocytic naevus with a surrounding halo of depigmentation associated with the presence of antibodies against melanocytes in some cases.
The whole naevus gradually fades in time.
Becker’s naevus is an area of increased pigmentation, often associated with increased hair growth, which is usually seen on the upper trunk or shoulders.
It is benign.
Freckles or ephelides are small pigmented macules, less than 0·5 cm in diameter, that occur in areas exposed to the sun in fair skinned people.
These macules fade during the winter months.
Congenital pigmented naevi
Congenital pigmented naevi are present at birth, generally over 1 cm in diameter, and vary from pale brown to black in colour.
They often become hairy and more protuberant, possibly with an increased risk of malignant change.
Larger lesions can cover a considerable area of the trunk and buttocks, such as the bathing trunk naevi, and their removal may present a considerable problem.
Dysplastic naevi
These show very early malignant change and may progress to malignant melanoma.
They are deeply pigmented often with an irregular margin.
In dysplastic naevus syndrome multiple pigmented naevi that occur predominantly on the trunk, becoming numerous during adolescence.
They vary in size many being over 0·5cm and tend to develop into malignant melanoma, particularly if there is a family history of this condition.
Melanoma
Melanoma is an invasive malignant tumour of melanocytes.
Most cases occur in white adults over the age of 30, with a predominance in women.
Incidence
The incidence of melanoma has doubled over the past 10 years in Australia (currently 40/100 000 population) and shown a similar increase in other countries.
In Europe twice as many women as men develop melanoma about 12/100 000 women and 6/100 000 men.
Prognosis
The prognosis is related to the thickness of the lesion, measured histologically in millimetres from the granular layer to the deepest level of invasion.
Lesions less than 0·76 mm thick have a 100% survival at five years, 0·76–1·5 mm thick an 80% survival at five years, and lesions over 3·5 mm less than 40% survival.
These figures are based on patients in whom the original lesion had been completely excised.
A recent study in Scotland has shown an overall five year survival of 71·6–77·6% for women and 58·7% for men.
Sun exposure
The highest incidence of melanoma occurs in countries with the most sunshine throughout the year.
However, skin type and the regularity of exposure to sun are also important.
The incidence is much greater in fair skinned people from higher latitudes who have concentrated exposure to sun during holidays than in those with darker complexions who have more regular exposure throughout the year.
Severe sunburn may also predispose to melanoma.
Genetic factors
Since melanin protects the skin from ultraviolet light it is not surprising that melanoma occurs most commonly in fair skinned people who show little tanning on exposure to sun, particularly those of Celtic origin.
Members of families with the dysplastic naevus syndrome are more likely to develop melanoma in their moles.
These patients have multiple naevi from a young age.
Pre-existing moles
It is rare for ordinary moles to become malignant but congenital naevi and multiple dysplastic naevi are more likely to develop into malignant melanoma.
Types of melanoma
There are four main types of melanoma.
Superficial spreading melanoma is the more common variety.
It is common on the back in men and on the legs in women.
As the name implies the melanoma cells spread superficially in the epidermis, becoming invasive after months or years.
The margin and the surface are irregular, with pigmentation varying from brown to black.
There may be surrounding inflammation and there is often clearing of the central portion.
The invasive phase is associated with the appearance of nodules and increased pigmentation.
The prognosis is correspondingly poor.
Lentigo maligna melanoma occurs characteristically in areas exposed to sun in elderly people.
Initially there is a slowly growing, irregular pigmented macule that is present for many years before a melanoma develops.
Nodular melanoma presents as a dark nodule from the start without a preceding in situ epidermal phase.
It is more common in men than women and is usually seen in people in their fifties and sixties.
Because it is a vertical invasive growth phase from the beginning there is a poor prognosis.
Acral melanoma occurs on the palm and soles and near or under the nails.
Benign pigmented naevi may also occur in these sites and it is important to recognise early dysplastic change by using the criteria set out below.
A very important indication that discoloration of the nail is due to melanoma is Hutchinson’s sign pigmentation of the nail fold adjacent to the nail.
It is important to distinguish talon noir, in which a black area appears on the sole or heel.
It is the result of trauma for example sustained while playing squash causing haemorrhage into the dermal papillae.
Paring the skin gently with a scalpel will reveal distinct blood filled papillae, to the relief of doctor and patient alike.
Other types of melanoma
As the melanoma cells become more dysplastic and less well differentiated they lose the capacity to produce melanin and form an amelanonitic melanoma.
Such non-pigmented nodules may be regarded as harmless but are in fact extremely dangerous.
Prognosis
This depends on the depth to which the melanoma has penetrated below the base of the epidermis lesions confined to the epidermis having better prognosis than those penetrating into the dermis.
The Clark classification describes the depth of penetration as follows:
Level I within the epidermis
Level II few melanoma cells within the dermal papillae
Level III many melanoma cells in the papillary dermis
Level IV invasion of the reticular dermis
Level V invasion of the subcutaneous tissues
The Breslow classification is based on measurements of tumour thickness from the granular layer overlying epidermis.
A depth of less than 1·5 mm is associated with a 90% five year survival, 1·5–3·5 mm with a 75% five year survival, and greater than 3·5 mm with only a 50% five year survival.
In deeper tumours “sentinel lymph node” biopsy may be carried out to assess whether lymphatic spread has occurred.
How to tell the difference
Benign moles show little change and remain static for years.
Any change may indicate that a mole is in fact a melanoma or that a mole is becoming active.
Size, shape, and colour are the main features and it is change in them that is most important.
Patients with moles should have these changes explained to them, in particular that they indicate activity of the cells, not necessarily malignant change.
