According to statistics provided by the International Agency for Research on Cancer (IARC), female breast cancer is the leading cancer in women worldwide today, with nearly 2.3 million cases estimated in 2020 [1]. Incidence rates are two times higher in North America (ASR=89.4) and Europe (74.3) relative to Asia (36.8), and Iran, with an estimated 17,000 breast cancers annually, has similar rates (35.8) to the region [2]. Iran together with several neighboring countries have reported increasing breast cancer incidence trends in recent years [3], [4].

Identifying factors related to the incidence trends of breast cancer may provide important evidence to health policy makers in designing effective preventative programs. Age-period-cohort (APC) models are well-known statistical analysis for assessing the effects of time-related factors including age, year of diagnosis (period) and year of birth (cohort) on cancer trends. Several studies have used APC modeling to investigate incidence trends in breast cancer in the region, with significant cohort effects identified in the increasing breast cancer incidence trends in China [5] and India [6].

Golestan province, located in Northern Iran is known for its high risk of esophageal and gastric cancers since the 1970s[7], [8], [9], [10]. The Golestan Population-based Cancer Registry (GPCR) reports that breast cancer is the most common malignancy in the Golestan population, with an ASR of 45.78 in 2020 [11]. As well as increasing trends in breast cancer incidence in Golestan [11], [12], previous GPCR reports have suggested differences in risk between the population’s two main ethnicities, the Fars and the Turkmens, with lower incidence rates of breast cancer in women with Turkmens ethnicity compared to non-Turkmens women (Fars and others ethnicities) [13]. In this study, we investigate the effects of age, period and cohort on time trends in the incidence rates of breast cancer in Golestan women between 2004 and 2018, examining the trends by ethnic region (Turkmens/non-Turkmens) and residential status (urban/rural).