Epidemiology
Generally, an individual's risk for developing melanoma depends on two groups of factors: intrinsic and environmental. "Intrinsic" factors are generally an individual's family history and inherited genotype, while the most relevant environmental factor is sun exposure.
Epidemiologic studies suggest that exposure to ultraviolet radiation (UVA and UVB) is one of the major contributors to the development of melanoma. UV radiation causes damage to the DNA of cells, typically thymine dimerization, which when unrepaired can create mutations in the cell's genes. When the cell divides, these mutations are propagated to new generations of cells. If the mutations occur in protooncogenes or tumor suppressor genes, the rate of mitosis in the mutation-bearing cells can become uncontrolled, leading to the formation of a tumor. Data from patients suggest that aberrant levels of Activating Transcription Factor in the nucleus of melanoma cells are associated with increased metastatic activity of melanoma cells; studies from mice on skin cancer tend to confirm a role for Activating Transcription Factor-2 in cancer progression. Occasional extreme sun exposure (resulting in "sunburn") is causally related to melanoma. Melanoma is most common on the back in men and on legs in women (areas of intermittent sun exposure). The risk appears to be strongly influenced by socio-economic conditions rather than indoor versus outdoor occupations; it is more common in professional and administrative workers than unskilled workers. Other factors are mutations in or total loss of tumor suppressor genes. Use of sunbeds (with deeply penetrating UVA rays) has been linked to the development of skin cancers, including melanoma.
Possible significant elements in determining risk include the intensity and duration of sun exposure, the age at which sun exposure occurs, and the degree of skin pigmentation. Exposure during childhood is a more important risk factor than exposure in adulthood. This is seen in migration studies in Australia where people tend to retain the risk profile of their country of birth if they migrate to Australia as an adult. Individuals with blistering or peeling sunburns (especially in the first twenty years of life) have a significantly greater risk for melanoma. This does not mean that sunburn is the cause of melanoma. Instead it is merely statistically correlated. The cause is the exaggerated UV-exposure. It has been shown that sunscreen — while preventing the sunburn — does not protect mice, injected with melanoma cells a day after UV exposure, from developing melanoma.
Fair and red-headed people, persons with multiple atypical nevi or dysplastic nevi and persons born with giant congenital melanocytic nevi are at increased risk.
A family history of melanoma greatly increases a person's risk because mutations in CDKN2A, CDK4 and several other genes have been found in melanoma-prone families. Patients with a history of one melanoma are at increased risk of developing a second primary tumour.
The incidence of melanoma has increased in the recent years, but it is not clear to what extent changes in behavior, in the environment, or in early detection are involved.
To understand how sunscreen can reduce sunburn and at the same time cause melanoma it is necessary to distinguish between direct DNA damage and indirect DNA damage. Genetic analysis has shown that 92% of all melanoma are caused by the indirect DNA damage. Although some people believe that dark-skinned people such as those of African descent cannot get sunburns, they are in fact susceptible, and should use sunscreen accordingly, as sunscreen has been proven to protect against other cancers such as squamous cell carcinoma and basal cell carcinoma.
Read more about this topic: Melanoma