Primary cutaneous melanoma is the most common subtype of malignant melanoma, a malignant neoplasm that arises from melanocytes. Melanocytes predominantly occur in the basal layer of the epidermis but do occur elsewhere in the body. Primary cutaneous melanoma is by far the most common type of primary melanoma, although it may occur in other tissues, e.g. primary uveal malignant melanoma.
In general, malignant melanoma is one of the less common types of skin cancer. However, it causes the majority (75%) of skin-cancer-related deaths because of its tendency to metastasise - see metastatic melanoma. In general, radiology is involved in the assessment of metastatic disease.
It is worth noting that although ubiquitous in medical literature, prefacing melanoma with the term ‘malignant’ is not only unnecessary (all true melanomas are malignant) but also potentially confusion; the absence of the term ‘malignant’ in a pathology report in no way implies a ‘non-malignant’ lesion. Even melanoma that does not yet demonstrated histological invasion (in-situ melanoma) is still considered a malignant neoplasms, albeit with a very low to absent rate of metastatic spread.
Common risk factors for malignant melanoma include:
- exposure to high levels of ultraviolet radiation
- genetic predisposition
- more than 50 nevi
Immunosuppression, sun sensitivity and freckles may also confer a predisposition to the development of malignant melanoma.
Since sun-exposure is a risk-factor, there is significant variable incidence across the globe. Incidence is highest in countries with high sun-exposure and a fair-skinned immigrant population, e.g. Australia.
Melanocytes are cells that produce melanin, the pigment responsible for skin colour. They arise from the neural crest in the embryo and migrate throughout the body, the vast majority ending up at the dermo-epidermal junction of the skin, where they provide melanin for the epidermal keratinocytes. The concentration of melanocytes in the base of the epidermis does not differ significantly between ethnic groups, but in fact it is the secretion of melanin that results in ethnic variability of skin colour.
Melanocytes that undergo malignant change show varied architectural patterns of growth. This forms the basis of the histological sub-typing, with the four most important/common sub-types;
- lentigo maligna (in-situ, also known as the Hutchinson melanotic freckle) and lentigo maligna melanoma (invasive) - seen in sun damaged skin, especially the face of older patients
- superficial spreading melanoma
- acral lentiginous melanoma: involves hands and feet
- nodular melanoma: prominent ‘vertical’ growth pattern, with absent residual ‘radial’ growth component ie. ‘growing down, not around’
Multiple additional rarer subtypes are also described based on cell morphology or unusual growth patterns. These are usually distinguished for reasons of diagnostic difficulty (clinical or histological) but some are merely described because of fascinating morphology. These include, but are certainly not limited to;
- desmoplastic melanoma: melanocytes are spindled and relatively bland, sometimes making both initial diagnosis and demarcation of the lesions borders very difficult, the latter making complete excision difficult
- nevoid (minimal deviation) melanoma: ‘benign’ low power growth pattern
- amelanotic melanoma: As the term implies, these lesions are non-pigmented clinically and are often removed without a clinical suspicion of melanoma
- or the wildly named 'melanoma of the animal type'
Most subtypes appears to have minimal independent prognostic significance.
Although primary cutaneous melanoma is the most common location for melanoma, melanocytes are also found in smaller numbers throughout the body, especially mucosal surfaces of the gastrointestinal tract (oral cavity to anus), the genital tract, and the eye; melanoma arising in any of these sites is a rare but recognised event. See also malignant uveal melanoma, primary malignant melanoma of the prostate and melanoma of the urethra.
Prognostic staging for malignant melanoma is accomplished using either the Breslow thickness or Clark level 1, 2.
- <0.76 mm: 5-year survival rate: 95% to 100%
- 0.76-1.5 mm: 5-year survival rate: 80% to 96%
- 1.5-4 mm: 5-year survival rate: 60% to 75%
- >4 mm: 5-year survival rate: 37% to 50%
- level I: tumor cells are maintained above the basement membrane
- level II: tumor cells have infiltrated the papillary dermis
- level III: tumor cells extend between papillary and reticular dermis, but do not invade the reticular dermis
- level IV: tumor cells have infiltrated the reticular dermis
- level V: tumor cells extend into subcutaneous tissue
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