Anatomical Distributions of Basal Cell Carcinoma and Squamous Cell Carcinoma in a Population-Based Study in Queensland, Australia

Keratinocyte cancers (KCs), including basal cell carcinoma (BCC) and squamous cell carcinoma (SCC), are the most common cancers among fair-skinned populations worldwide. Although studies have indicated that the anatomical distribution of BCC and SCC differ, few have compared them directly in well-defined population samples.To describe and compare the anatomical distribution of BCC and SCC in a population-based sample in Queensland, Australia.This study was nested within the population-based QSkin Sun and Health Study in Queensland, Australia. Of 37 103 study participants linked to national medical insurance records, 3398 diagnosed with KCs from September 1, 2010, to September 30, 2012, were identified, and information about their KCs was extracted from pathology reports. Data were analyzed from January 1, 2013, to March 30, 2016.The relative tumor densities (RTDs) on defined body sites, calculated by dividing the proportion of tumors occurring at a specified site by the proportion of skin area of that site.A total of 5150 KCs with complete data were identified in 2374 study participants (1339 men [56.4%] and 1035 women [43.6%]; mean [SD] age, 59.7 [7.4] years). Of these, 3846 KCs (74.7%) were BCCs. Most BCCs were on the head and/or neck (1547 [40.2%]) and the trunk (1305 [33.9%]); most SCCs were on the head and/or neck (435 [33.4%]) and upper limbs (455 [34.9%]). The greatest differences in RTDs between BCC and SCC were on the hand (BCC:SCC ratio, 1:14) and the back and/or buttocks (BCC:SCC ratio, 8:1). Relative tumor densities of KCs were higher on the scalp and ear in men compared with women, and on the upper arm in women compared with men. The pattern of RTDs did not differ with age for BCC. Compared with younger adults (40-54 years), the RTDs in older adults (55-69 years) were 2-fold higher for SCC on the scalp (0.38 [95% CI, 0.00-0.81] vs 1.07 [95% CI, 0.75-1.38]) and the back and/or buttocks (0.05 [95% CI, 0.00-0.12] vs 0.12 [95% CI, 0.07-0.16]).The high RTDs on sun-exposed body sites for BCC and SCC are in keeping with sun exposure as the primary etiologic factor for both tumors. However, for BCC, the low RTD on the hand and high RTDs on less sun-exposed sites suggest a complex association between sun exposure and occurrence of BCC. Knowledge about the anatomical distribution of BCC and SCC may provide insight into their diagnoses and causes.