Although several types of skin cancer have been identified, three types make up the vast majority of diagnoses: basal cell carcinoma, squamous cell carcinoma, and melanoma. Melanoma is the most dangerous of the skin cancers. It accounts for only 4% of all skin cancer diagnoses, but is responsible for more than 80% of skin cancer-related deaths (Miller 2006). The lifetime chance of a Caucasian developing an invasive melanoma of the skin is 1 in 35 among men and 1 in 54 among women (Siegel 2013). Melanoma readily invades local tissues and can metastasize (spread) to distant organs and establish new tumors. Advanced, metastatic melanoma is difficult to effectively treat (Miller 2006; Dunki-Jacobs 2013). While the incidence and mortality for most types of cancer in the United States have been steadily decreasing since the mid-1990s, the overall incidence of melanoma has increased in recent decades. Cutaneous (skin) melanoma currently ranks fifth for men and seventh for women in incidence of all new cancers diagnosed, with an estimated 76 690 diagnoses and 9480 deaths in 2013 (ACS 2013a; Siegel 2013). In the United States, statistical modeling indicates the incidence of new skin melanoma cases increased 2.6% per year between 2004 and 2014, while death rates remained stable (NCI 2014a). The cause of the increasing incidence of melanoma is controversial. A recent study that examined nine geographical areas in the United States suggests the increased incidence can be explained, to some degree, by an increase in screening and biopsies resulting in increased melanoma detection (Welch 2005). However, this conclusion is controversial, and disagreement exists as to identifying the possibility of fluctuation in melanoma database registrations (Lee 2009).
Non-melanoma skin cancers represent the majority of skin cancer diagnoses; about 80% of these are basal cell carcinomas and 20% are squamous cell carcinomas, with all other diagnoses representing only 1%. Non-melanoma skin cancers are usually not reported to cancer registries, so estimates of incidence are somewhat uncertain (Eisemann 2013). Although the incidence and prevalence of non-melanoma skin cancer is not being tracked by the National Cancer Institute (NCI), there are several studies that show that these cancers have become more frequent over the past 20 years (Firnhaber 2012). Surveys from the NCI in the early 1970s estimated 300 000 new cases per year (about 50% of all new cancers); other analyses from national Medicare data estimate the number of individuals in the United States with non-melanoma skin cancers in 2006 to be over 3.5 million (Rogers 2010). The average age of non-melanoma skin cancer diagnosis ranges from 59 to 65 years for men and 60 to 66 years for women (Kim 2012).
Squamous cell carcinomas have far greater capacity to metastasize in comparison with basal cell carcinoma of the skin (which may be locally invasive) and account for most deaths from non-melanoma skin cancers; they are also more likely to recur following removal compared to basal cell carcinomas. Most occur on the head, neck, and upper extremities, indicating their incidence is strongly correlated with sun-exposed skin. Basal cell carcinomas very rarely metastasize, but the risk of metastasis does appear to increase with tumor size (Kim 2012).
While genetics can influence the rate at which skin cancers can occur (Carless 2008; Dunki-Jacobs 2013), the predominant catalyst for skin carcinogenesis is exposure to ultraviolet (UV) radiation. Sun exposure leads to a measurable increase in risk of non-melanoma cancers (Leiter 2008; Fartasch 2012; Sánchez 2013) and has been implicated in melanoma progression (Erb 2008; Whiteman 2006). Not surprisingly, skin cancer incidence is more prevalent in individuals with light skin; the incidence of non-melanoma skin cancers in African Americans is 3.4 per 100 000 persons compared to 232.6 per 100 000 persons for white populations. White populations tend to have skin cancer rates 6-14 times higher than Hispanic populations living in the same areas. Lower skin cancer rates are attributed to the increased production of melanin, the UV-protective skin pigment, in darker-skinned individuals. In these individuals, squamous cell carcinoma is the more common cancer, often appearing in non-sun-exposed areas (eg, the lower extremities) and associated with trauma or chronic inflammation rather than exposure to UV radiation (Kim 2012).
Although skin cancer-related mortality figures at first appear sobering, skin cancers are quite treatable if identified early. Cure rates approach 99% for non-melanoma skin cancers (although recurrence rates can be high) and approximately 87% for melanoma (Kim 2012; Dunki-Jacobs 2013). Survival rates drop precipitously as skin cancers reach advanced stages; the percentages of patients who survive >5 years after being diagnosed with metastatic squamous cell carcinoma or melanoma are 26.8% and 15%, respectively (Kim 2012; ACS 2013a).