In conformance with the arguments expressed in Table 12.3, the incidence rates of squamous and basal cell carcinomas in whites increase significantly more dramatically than those of mela- noma with increasing proximity to the equator and with increasing measured ambient UV radia- tion (cf. Armstrong and Kricker, 1996). Migrant studies, primarily from Australia, have demonstrated that the incidence and mortality rates of basal cell carcinoma and melanoma are higher in people born in countries with high ambient solar radiation than in migrants to these countries from areas with lower ambient solar radiation. The evidence for squamous cell carci- noma is similar but not so extensive. Furthermore, basal and squamous cell carcinomas are much more frequently found in areas of the body that are most frequently exposed to sun, mainly the head and neck, and to a lesser extent the shoulders and back (Armstrong and Kricker, 1996). Actual quantitation of exposure to sunlight is somewhat more difficult, but Kricker et al. (1995) have been able to develop a dose-response curve relating total sun exposure to the risk of devel- opment of basal cell carcinoma in an Australian cohort (Figure 12.6). When lifetime exposure is considered as in this figure, the quadratic model utilized showed an initial rise in the risk of basal cell cancer with increasing exposure of the site, with a peak at about 35,000 hours of exposure, followed by a fall. The interesting exception was the head and neck, while the trunk showed the greatest odds ratio of 2.4. The apparent inhibition of carcinogenesis by increasing sun exposure is suggestive of a hormetic effect (see above) perhaps different from that depicted in Figure 12.5. A benign lesion, which in some cases may be a precursor of squamous cell carcinoma, is the
Figure 12.6 Quadratic curve and 95% confidence intervals for the continuous relationship between total hours of sun exposure and the development of basal cell carcinoma. In this model, the interaction between exposure and body site was not significant. Abscissa: total hours of sun exposure. Ordinate: odds ratio. (From Kricker et al., 1995, with permission of the authors and publisher.)
actinic or solar keratosis that also occurs predominantly if not almost exclusively on the head and neck, the back of the hands, and rarely the trunk.