Trends and geographic variation in skin cancer incidence in Songkhla, Southern Thailand, 1989–2020: A Joinpoint regression and age-period-cohort analysis

Background The incidence trends of skin cancer are increasing across the world. However, data from Southeast Asian countries, including Thailand, are limited. Songkhla, a province in southern Thailand, has a predominant occupation and religion across different geographic areas. These differences may result in varying levels of UV exposure, potentially influencing the incidence of skin cancer. This study aimed to assess the trends in skin cancer incidence in Songkhla according to age, calendar period, birth cohort, and geographical area. Methods The study included patients who were incidently diagnosed with squamous cell carcinoma, basal cell carcinoma, and melanoma of the skin between 1989 and 2020, as recorded in the Songkhla population-based cancer registry. Geographic areas were classified into four categories according to predominant occupation and religion: farming and fishing area, city area, Muslim predominated area, and gardening area. Age-standardized incidence rates were calculated, and trend analyses were performed using Joinpoint regression and age-period-cohort (APC) analysis. Results The incidence of skin cancer in men remained unchanged before 2011; a similar pattern was observed in women before 2015. Thereafter, a significant decline was noted in both sexes, with annual percentage change rates of −9.53% in men and −11.51% in women. Based on geographic area, the highest incidence was observed in the city area (up to 7.14 in men, 4.62 in women), whereas the lowest was recorded in the Muslim predominated area (up to 3.93 in men, 3.32 in women). The APC analysis indicated that the incidence increased with age. The highest risk was observed in the cohort born in 1917, with a gradual decline in subsequent cohorts. No significant period effect was identified. Conclusions The incidence of skin cancer in Songkhla, Thailand, has declined over recent years. Variations in incidence are associated with differences in geographic region, occupation, and religious composition.