Trends in hormone use and ovarian cancer incidence in US white and Australian women: implications for the future

To compare trends in ovarian cancer incidence in the USA and Australia in relation to changes in oral contraceptive pill (OCP) and menopausal hormone therapy (MHT) use. US cancer incidence data (1973–2013) were accessed via SEER*Stat; Australian data (1982–2012) were accessed from the Australian Institute of Health and Welfare Cancer Incidence and Mortality books. Age-period-cohort models were constructed to assess trends in ovarian cancer incidence by birth cohort and year of diagnosis. Ovarian cancer rates were increasing until the cohorts born around 1918 in the USA and 1923 in Australia who were the first to use the OCP. They then declined dramatically across subsequent cohorts such that rates for the 1968 cohort were about half those of women born 45 years earlier; however, there are early suggestions that this decline may not continue in more recent cohorts. In contrast, despite the large reduction in MHT use, there was no convincing evidence that ovarian cancer incidence rates in either country were lower after 2002 than would have been expected based on the declining trend from 1985. The major driver of ovarian cancer incidence rates appears to be the OCP. This means that when those women born since the late 1960s (who have used the OCP at high rates from an early age) reach their 60s and 70s, incidence rates are likely to stop falling and may even increase with changes in the prevalence of other factors such as tubal ligation and obesity. Forward predictions based on past trends may thus underestimate future rates and numbers of women likely to be affected.