More KCs were observed than MSCs in this study population. The majority of KC lesions were in the skin of head and neck in males and skin of trunk in females and the majority of MSC were on the skin of the lower limbs. HIV infection, living in a rural area, and smoking was positively associated with KC. Living in a rural area was also positively associated with MSC.
Findings from this study are consistent with the world cancer report statistics that KCs account for the majority of skin cancers reported with the age at diagnosis in the mid-60’s (1, 28). Generally, males are diagnosed with KCs more often than females (14, 17, 28). However, in the current study more females were diagnosed with KCs compared to males (may be due to having more females in the JCS than males) and females were diagnosed at a younger age (37% at ages ≤ 40 years). The differences in reported numbers between the two sexes may be a result of health-seeking behavioural differences between the two sexes (14). Females are reportedly more likely than males to seek medical help over a suspicious looking lesion on the skin (14).
The skin cancer subtype that was reported more frequently in this study population was SCC compared to MSC, consistent with literature that the Black population is mostly affected by SCC (29–31). However, the distribution of histological subtypes of KC in the SA general population (all ethnicities) shows that BCC is the most abundant subtype, followed by SCC (32, 33). This pattern is due to the overwhelming numbers of BCC diagnosed in the White population in SA. However, when population group specific analysis was conducted, SCC was the leading skin cancer in the Black population, followed by BCC (23).
The differences in KC histological subtypes distribution suggest different risk factors among different population groups. A risk factor for SCC in the Black population is hypothesized to be immunosuppression resulting from HIV infection (18). SA has one of the world’s largest HIV epidemics and the Black population is the most affected population (18). A rise of SCC incidence in the Black population of SA was observed after the beginning of the HIV epidemic (18). Shortly after the introduction of anti-retroviral treatment, a significant decline of SCC incidence was seen in SA and this is consistent with the theory that relates SCC to immunosuppression resulting from HIV infection (18). Being HIV positive however showed negative association with MSC on univariable analysis, this may suggest different risk factors for KCs and MSCs.
Skin cancer is known to develop on areas of the skin that are exposed to sunlight as a result of UVR exposure (3, 13). This phenomenon is explained by a lack of melanin in non-Black populations which predisposes to skin injury from UVR exposure, increasing the risk of skin cancer (6). Interestingly, the Black SA population in this study showed that the skin of the head and neck, and trunk were more susceptible to KCs. The body area (trunk) that is normally covered by clothing and not normally exposed to the sun was susceptible to KC especially in females. These findings show that other than the main risk factor (solar UVR exposure), there may be additional risk factors associated with skin cancer in the Black population. The risk factor analysis results showed that being male resulted in double the odds of having skin cancer for both KC and MSC, as supported by other studies in other countries (14, 17). Sex differences exist in many physiological conditions and can be explained by various theories (14). Given sun exposure as a primary risk factor for KC susceptibility, the high incidence seen in males can be explained by the nature of jobs usually done by men (outdoor jobs) and by the behaviour during childhood (spending more time outdoors) which explains most of exposure to risk (8, 34). The Black SA economy relies on agricultural and mining activities. These activities increase exposure to risk factors and cause skin injuries and scar tissue, a precursor of skin cancer development (18).
In this study, a positive association of KC with smoking (current), and a positive HIV status was seen (17, 35, 36). Current smokers, and persons with a history of smoking have increased odds of being diagnosed with SCC (17). Our findings are consistent with literature on the association between smoking and KC. Univariable analysis showed that using wood for cooking increases the odds of KC, but multivariable analysis showed the association was not significant. In China, wood impregnators (the people who introduce chemical substances into wood in order to improve its characteristics and impart new properties) have been shown to likely develop skin cancer (37). Upon burning, wood compounds can be absorbed by the skin and cause inflammation, increasing the risk for cancer (38). Univariable analyses showed that wood, coal, paraffin and gas (as either a cooking fuel or a fuel for heating) increased the odds of MSC, however, in multivariable analyses the association diminished. Literature has shown that industrial workers with paraffin exposure are at a higher risk of developing skin cancer (39–41).
The strength of the current study is that it addresses the existing gap in knowledge on the skin cancer risk factors in the Black population. Even though the study participants were recruited from one study site (limiting the generalizability of findings to SA Black population), the Johannesburg population is essentially a mixture of all SA provinces. Johannesburg is a central hub of economic activity in SA (42). People from all SA provinces migrate to Johannesburg for economic advancement, and for medical treatment; hence, this sample is a good representation of the Black SA population. Our findings were consistent with existing literature on the association between KC and smoking, and KC and HIV. New associations were also found; using coal as a cooking and a heating fuel, although not significant in the adjusted model (thereby warranting further investigation). Although risk factor information collected depended on self-reported data from participants, the diagnoses of skin cancers were definitive as all cases had a laboratory report confirming a diagnosis.
The limitations of this study are that UVR was not adjusted for in the analysis, as we did not have a measure of sun exposure (a well-established skin cancer risk factor). However, we did not anticipate that this would cause a problem in the analysis as both controls and cases were assumed to be exposed to the same degree of ultraviolet light as they live in the same country, hence exposure to the same intensity of UVR (same latitude and longitude). The study did not collect all risk factor information relating to skin cancer for example; family history of skin cancer, trauma to a site of skin cancer, information on albinism, among other factors, were not captured since the original study’s aim was to collect major risk factors for all cancers. The conception of the original study did not focus on skin cancer risk factors. The selection of participants in the current study may have underestimated ratio of cases. We had very few BCC cases, as most of the BCC cases are referred to and excised in dermatology departments rather than radiation or medical oncology (where the participants of this study were recruited), thus there is a bias in the parent study in terms of selection of cases.