Superficial spreading melanoma

Author: Dr Amanda Oakley, Dermatologist, Hamilton, New Zealand, 2011.


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What is superficial spreading melanoma?

Superficial spreading melanoma is the most common type of melanoma, a potentially serious skin cancer that arises from pigment cells (melanocytes).

It is a form of melanoma in which the malignant cells tend to stay within the tissue of origin, the epidermis, in an ‘in-situ’ phase for a prolonged period (months to decades). At first, superficial spreading melanoma grows horizontally on the skin surface – this is known as the radial growth phase. The lesion presents as a slowly-enlarging flat area of discoloured skin.

An unknown proportion of superficial spreading melanoma become invasive, that is, the melanoma cells cross the basement membrane of the epidermis and malignant cells enter the dermis. A rapidly-growing nodular melanoma can arise within superficial spreading melanoma and start to proliferate more deeply within the skin.

Management of melanoma is evolving. For up to date recommendations, refer to Australian Cancer Council Clinical practice guidelines for the diagnosis and management of melanoma.

Who gets superficial spreading melanoma?

According to New Zealand Cancer Registry data, 2256 invasive melanomas were diagnosed in 2008; 48% were in males. At least 40% were reported to be superficial spreading melanoma. There were 371 deaths from all types of melanoma in 2008 (69% were male). 

Superficial spreading melanoma accounts for two-thirds of cases of melanoma in Australia and New Zealand. It nearly always arises in white skinned individuals. Although more common in very fair skin (skin phototype 1 and 2), it may also occur in those who tan quite easily (phototype 3). It is rare in brown or black skin (phototype 4-6).

Superficial spreading melanoma is equally common in males and females. Only 15% of melanomas arise before the age of 40, and it is rare under the age of 20 (<1%).

The main risk factors for superficial spreading melanoma are:

  • Increasing age (see above)
  • Previous invasive melanoma or melanoma in situ
  • Previous nonmelanoma skin cancer
  • Many melanocytic naevi (moles)
  • Multiple (>5) atypical naevi (funny-looking moles or moles that are histologically dysplastic)
  • Strong family history of melanoma with 2 or more first-degree relatives affected
  • Fair skin that burns easily.

Less strong factors include blue or green eyes, red or blond hair, indoor occupation with outdoor recreation and signs of sun damage.

What does superficial spreading melanoma look like?

Superficial spreading melanoma tends to occur at sites of intermittent, intense sun exposure, especially on the trunk in males (40%) and the legs in females (also 40%).

Superficial spreading melanoma presents as a slowly growing or changing flat patch of discoloured skin. At first, it often resembles a mole or freckle / lentigo. It becomes more distinctive in time, often growing over months to years or even decades before it is recognised. 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 superficial spreading melanoma include:

  • Larger size than most moles: > 6 mm and often 1–2 centimetres in diameter at diagnosis
  • Irregular shape
  • Variable pigmentation: colours may include light brown, dark brown, black, blue, grey, pink and red
  • There may be skip areas that are skin coloured, or white scars due to regression
  • Smooth surface at first, later becoming thicker with an irregular surface that may be dry or warty.

Deep invasive melanoma often has the following features:

  • Thickening of part of the lesion
  • Increasing number of colours, especially blue or black
  • Ulceration or bleeding
  • Itching or stinging.
Superficial spreading melanoma

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What is the cause of superficial spreading melanoma?

Superficial spreading melanoma is due to the development of malignant pigment cells (melanocytes) along the basal layer of the epidermis. The majority arise in previously normal-appearing skin. About 25% arise within an existing melanocytic naevus (mole), which can be a common or normal naevus, an atypical or dysplastic naevus or a congenital naevus.

What triggers the melanocytes to become malignant is not fully known. Specific gene mutations such as BRAFV600E have been detected in many superficial spreading melanomas and these mutations may change as the disease advances.

Damage by ultraviolet radiation results in a degree of immune tolerance, allowing abnormal cells to grow unchecked. This can occur from exposure to natural sunlight, particularly if sunburn has occurred, and artificial sources of ultraviolet radiation from sun beds / solaria.

What tests should be done?

It is essential to diagnose superficial spreading melanoma accurately. Clinical diagnosis is aided by dermoscopy and skin biopsy (usually excision biopsy). Those with melanoma that is more than 0.8 mm thick may be advised to have lymph node biopsy, imaging studies and blood tests.

Dermoscopy

Dermoscopy, or the use of a dermatoscope, by a dermatologist or other doctor trained in its use can be very helpful in distinguishing superficial spreading melanoma from other skin lesions, such as

The most frequently observed dermoscopic features of superficial spreading melanoma are:

  • Asymmetrical structure and colours
  • Atypical, broadened pigment network
  • Blue-grey structures
  • Multiple colours (5–6).

Biopsy

If the skin lesion is suspicious of superficial spreading 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.

The pathological diagnosis of melanoma can be very difficult. Histological features of superficial spreading melanoma in situ include the presence of buckshot (pagetoid) scatter of atypical melanocytes within the epidermis. These cells may be enlarged with unusual nuclei. Dermal invasion results in melanoma cells within the dermis or deeper into subcutaneous fat.

Extra tests using immunohistochemical stains may be necessary.

Pathology report

The pathologist's report should include a macroscopic description of the specimen and melanoma (the naked eye view), and a microscopic description. The following features should be reported if there is invasive melanoma.

  • Diagnosis of primary melanoma
  • Breslow thickness to the nearest 0.1 mm
  • Clark level of invasion
  • Margins of excision i.e. the normal tissue around the tumour
  • Mitotic rate – a measure of how fast the cells are proliferating
  • Whether or not there is ulceration

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 associated in-situ disease and/or associated naevus (original mole).

What is Breslow thickness?

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).

What is the Clark level of invasion?

The Clark level indicates the anatomic plane of invasion.

LevelCharacteristics
Level 1 In situ melanoma
Level 2 Melanoma has invaded papillary dermis
Level 3 Melanoma has filled papillary dermis
Level 4 Melanoma has invaded reticular dermis
Level 5 Melanoma has invaded subcutaneous tissue

The deeper the Clark level, the greater 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.

What is the treatment for superficial spreading melanoma?

The initial treatment of a primary melanoma is to cut it out; the lesion should be completely excised with a 2 mm margin of normal 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 The Australian and New Zealand Guidelines for the Management of Melanoma (2008) are shown in the table below. This may necessitate flap or graft to close the wound. Occasionally, the pathologist will report incomplete excision of the melanoma, despite wide margins. This means further surgery or radiotherapy will be recommended to ensure the tumour has been completely removed.

ThicknessExcision margin
Melanoma in situ 5mm
Melanoma < 1.0mm 1cm
Melanoma 1.0–2.0mm 1–2cm
Melanoma 2.0–4.0mm 1–2cm
Melanoma > 4.0mm 2cm

Staging

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:

StageCharacteristics
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

Should the lymph nodes be removed?

If the local lymph nodes are enlarged due to metastatic melanoma, they should be completely removed. 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 the 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.

What happens at follow-up?

The main purpose of follow-up is to detect recurrences early but it also offers an opportunity to diagnose a new primary melanoma at the first possible opportunity. A second invasive melanoma occurs in 5–10% patients with melanoma; an unrelated melanoma in situ affects in more than 20% of melanoma patients.

The Australian and New Zealand Guidelines for the Management of Melanoma (2008) make the following recommendations for follow-up for patients with invasive melanoma.

  • Self skin examination
  • Regular routine skin checks by patient's preferred health professional
  • Follow-up intervals are preferably six-monthly for five years for patients with stage 1 disease, three-monthly or four-monthly for five years for patients with stage 2 or 3 disease, and yearly thereafter for all patients.
  • Individual patient’s needs should be considered before appropriate follow-up is offered
  • Provide education and support to help patient adjust to their illness

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:

  • A check of the scar where the primary melanoma was removed
  • A feel for the regional lymph nodes
  • A general skin examination
  • A full physical examination
  • In those with many moles or atypical moles, baseline whole body imaging and sequential macro and dermoscopic images of melanocytic lesions of concern (mole mapping)

In those with more advanced primary disease, follow-up may include:

  • Blood tests, including LDH
  • Imaging: ultrasound, X-ray, CT, MRI and/or PET scan.

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 5 years or longer after removal of their melanoma, whatever stage.

What is the outlook?

Superficial spreading melanoma in situ is not dangerous; it only becomes potentially life threatening if an invasive melanoma develops within it. The rates of melanoma in situ are not reported by cancer registries. The risk of spread and ultimate death 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 < 0.75 mm and the risk for tumours 0.75–1 mm thick is about 5%. The risk steadily increases with thickness so that melanomas > 4 mm have a risk of metastasis of about 40%.

New Zealand statistics gathered by the Cancer Registry between 1994 and 2004 revealed 15,839 invasive melanomas. Of these, 52% were under 0.75 mm in thickness, 22% were between 0.76 and 1.49 mm, 15% were between 1.5 and 3 mm in thickness and 11% were more than 3 mm in thickness. Thicker tumours were slightly more likely to be diagnosed in males, and more likely in older people than younger ones.

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