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Monday, 07 March 2011 17:38

Malignant Melanoma

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Malignant melanoma is rarer than non-melanocytic skin cancer. Apart from exposure to solar radiation, no other environmental factors show a consistent association with malignant melanoma of the skin. Associations with occupation, diet and hormonal factors are not firmly established (Koh et al. 1993).

Malignant melanoma is an aggressive skin cancer (ICD-9 172.0 to 173.9; ICD-10: C43). It arises from pigment-producing cells of the skin, usually in an existing naevus. The tumour is usually a few millimetres to several centimetres thick, brown or black in colour, that has grown in size, changed colour and may bleed or ulcerate (Balch et al. 1993).

Indicators of poor prognosis of malignant melanoma of the skin include nodular subtype, tumour thickness, multiple primary tumours, metastases, ulceration, bleeding, long tumour duration, body site and, for some tumour sites, male sex. A history of malignant melanoma of the skin increases the risk for a secondary melanoma. Five-year post-diagnosis survival rates in high incidence areas are 80 to 85%, but in low incidence areas the survival is poorer (Ellwood and Koh 1994; Stidham et al. 1994).

There are four histologic types of malignant melanoma of the skin. Superficial spreading melanomas (SSM) represent 60 to 70% of all melanomas in Whites and less in non-Whites. SSMs tend to progress slowly and are more common in women than in men. Nodular melanomas (NM) account for 15 to 30% of malignant melanomas of the skin. They are invasive, grow rapidly and are more frequent in men. Four to 10% of malignant melanomas of the skin are lentigo malignant melanomas (LMM) or Hutchinson’s melanotic freckles. LMMs grow slowly, occur frequently in the face of old persons and rarely metastasize. Acral lentiginous melanomas (ALM) represent 35 to 60% of all malignant melanomas of the skin in non-Whites and 2 to 8% in Whites. They occur frequently on the sole of the foot (Bijan 1993).

For the treatment of malignant melanomas of the skin, surgery, radiation therapy, chemotherapy and biologic therapy (interferon alpha or interleukin-2) may be applied singly or in combination.

During the 1980s, the reported age-standardized annual incidence rates of malignant melanoma of the skin varied per 100,000 from 0.1 in males in Khon Kaen, Thailand to around 30.9 in males and 28.5 in females in Queensland, Australia (IARC 1992b). Malignant melanomas of the skin represent less than 1% of all cancers in most populations. An annual increase of about 5% in melanoma incidence has been observed in most white populations from the early 1960s to about 1972. Melanoma mortality has increased in the last decades in most populations, but less rapidly than incidence, probably due to early diagnoses and awareness of the disease (IARC 1985b, 1992b). More recent data show different rates of change, some of them suggesting even downward trends.

Malignant melanomas of the skin are among the ten most frequent cancers in incidence statistics in Australia, Europe and North America, representing a lifetime risk of 1 to 5%. White-skinned populations are more susceptible than non-White populations. Melanoma risk in white-skinned populations increases with proximity to the equator.

The gender distribution of melanomas of the skin varies widely between populations (IARC 1992a). Women have lower incidence rates than men in most populations. There are gender differences in patterns of body distribution of the lesions: trunk and face dominate in men, extremities in women.

Malignant melanomas of the skin are more common in higher than in lower socio-economic groups (IARC 1992b).

Familial melanomas are uncommon, but have been well documented. with between 4% and 10% of patients describing a history of melanoma among their first degree relatives.

Solar UV-B irradiation is probably the major cause for the widespread increase in the incidence of melanomas of the skin (IARC 1993). It is not clear whether depletion of the stratospheric ozone layer and the consequent increase in UV irradiance has caused the increase in the incidence of malignant melanoma (IARC 1993, Kricker et al. 1993). The effect of UV irradiation depends on some characteristics, such as I or II phenotype and blue eyes. A role for UV radiation emanating from fluorescent lamps is suspected, but not conclusively established (Beral et al. 1982).

It has been estimated that reduction in recreational sun exposure and use of sun-screens could reduce the incidence of malignant melanomas in high risk populations by 40% (IARC 1990). Among outdoor workers, the application of sunscreens having a protective UV-B factor rating of at least 15 and UV-A sunscreen and the use of appropriate clothing are practical protective measures. Although a risk from outdoor occupations is plausible, given the increased exposure to solar radiation, results of studies on regular outdoor occupational exposure are inconsistent. This is probably explained by the epidemiological findings suggesting that it is not regular exposures but rather intermittent high doses of solar radiation that are associated with excess melanoma risk (IARC 1992b).

Therapeutic immunosuppression may result in increased risk of malignant melanoma of the skin. An increased risk with the use of oral contraceptives has been reported, but it seems unlikely to increase the risk of malignant melanoma of the skin (Hannaford et al. 1991). Melanomas can be produced by oestrogen in hamsters. There is no evidence of such an effect in humans.

In White adults, the majority of primary intraocular malignant tumours are melanomas, usually arising from uveal melanocytes. The estimated rates for these cancers do not show the geographic variations and increasing time trends observed for melanomas of the skin. The incidence and mortality of ocular melanomas are very low in Black and Asiatic populations (IARC 1990, Sahel et al. 1993) The causes of ocular melanoma are unknown (Higginson et al. 1992).

In epidemiological studies, excess risk for malignant melanoma has been observed in administrators and managers, airline pilots, chemical processing workers, clerks, electrical power workers, miners, physical scientists, policemen and guards, refinery workers and gasoline exposed workers, salesmen and warehouse clerks. Excess melanoma risks have been reported in industries such as cellulose fibre production, chemical products, clothing industry, electrical and electronics products, metal industry, non-metallic mineral products, petrochemical industry, printing industry and telecommunications. Many of these findings are, however, solitary and have not been replicated in other studies. A series of meta-analyses of cancer risks in farmers (Blair et al. 1992; Nelemans et al. 1993) indicated a slight, but significant excess (aggregated risk ratio of 1.15) of malignant melanoma of the skin in 11 epidemi-ological studies.

In a multi-site case-control study of occupational cancer in Montreal, Canada (Siemiatycki et al. 1991), the following occupational exposures were associated with a significant excess of malignant melanoma of the skin: chlorine, propane engine emissions, plastics pyrolysis products, fabric dust, wool fibres, acrylic fibres, synthetic adhesives, “other” paints, varnishes, chlorinated alkenes, trichloroethylene and bleaches. It was estimated that the population attributable risk due to occupational exposures based on the significant associations in the data of the same study was 11.1%.



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