Spatio-temporal modelling of breast cancer incidence between 2000 and 2021 at sub-national levels in Iran: Bayesian disease mapping

While in Iran trends in breast cancer incidence are generally monitored at the national level, little is known about sub-national variation in these trends. This project aims to assess levels and trends (2000-2021) of relative risk of breast cancer incidence and mortality at sub-national levels in Iran and their relation to key socioeconomic dimensions, to understand the full extent of geographical and social inequalities in the country associated with breast cancer morbidity and mortality. Data from the national cancer registry system of the Iranian Ministry of Health have been used, which is gathered on cancer incidence at provincial and district levels by age and sex. Census and Household Expenditure and Income Survey (HEIS) datasets have been then used to extract related covariates. The relative risk of breast cancer incidence was estimated in women aged 30+ years for all 316 districts in Iran from 2000 to 2010 using a Bayesian spatio-temporal model. Then, I’ve propagated uncertainty from the spatio-temporal model into the prediction model for the years from 2011 to 2021. The national relative risk of breast cancer incidence in Iran increased from 0.21 (95% credible interval (CrI): 0.19, 0.22) in 2000 to 0.66 (0.63, 0.68) in 2010 and 1.23 (1.18, 1.28) in 2021. The relative risk of breast cancer incidence was highest in Yazd (1.96 [1.63, 2.33]), Shiraz (1.90 [1.72, 2.09]) and Shemiranat (1.90 [1.12, 2.91]) in 2010 and Tehran (3.99 [3.86, 4.33]), Bushehr (3.89 [3.07, 4.77]) and Abadan (3.67 [2.99, 4.39]) in 2021. In contrast, Savojbolagh, Saravan and Nikshahr were found to have the lowest relative risks in both 2010 (0.11 [0.05, 0.20], 0.17 [0.08, 0.30] and 0.20 [0.09, 0.36], respectively) and 2021 (0.19 [0.10, 0.33], 0.34 [0.18, 0.54] and 0.35 [0.17, 0.62], respectively). The relative risk of breast cancer incidence was 60% higher across districts in the highest YOS quintile (average years of schooling: 3.9) than those in the lowest YOS quintile (average years of schooling: 2.2; relative index of inequality: 1.6). Results show that the relative risk of breast cancer incidence has increased over time (2000-2021) at national and sub-national levels in Iran. Breast cancer is one of the few diseases with a positive education gradient, with higher relative risk of breast cancer incidence among higher- compared to lower-educated women, probably due to better awareness of diagnostic approaches and access to those. While social inequalities are a major barrier to reducing the prevalence and incidence of breast cancer, it is important to track the progress made at the district level based on the characteristics of specific policies aimed at reducing health inequalities. A scaling-up in the quality of healthcare services, national and sub-national policies addressing prevention and treatment, and more specialised training programs in women's health are needed.