Population-based cancer incidence and mortality rates and ratios among adults with intellectual disabilities in Scotland

Abstract

Structured AbstractObjectiveTo provide contemporary data on cancer mortality rates within the context of incidence in the population with intellectual disabilities.MethodsScotland’s 2011 Census was used to identify adults with intellectual disabilities and controls with records linked to the Scottish Cancer Registry and death certificate data (March 2011-December 2019). The control cohort without intellectual disabilities and/or autism were used for indirect standardisation and calculation of Crude Incident Rates/Crude Mortality Rates (CIR/CMR), and age-sex Standardized Incident Rate Ratios/ Standardized Mortality Ratios (SIR/SMR), with 95% Confidence Intervals (CI).ResultsAdults with intellectual disabilities were most likely diagnosed cancers of digestive, specifically colorectal (14.2%), lung (9.3%), breast (female 22.9%), body of the uterus (female 9.3%) and male genital organs (male 17.6%). Higher incident cancers included metastatic cancer of unknown primary origin (female SIR=1.70, male SIR=2.08), body of uterus (female SIR=1.63), ovarian (female SIR=1.59), kidney (female SIR=1.85), and testicular (male SIR=2.49). SMRs were higher, regardless of a higher, similar, or lower incidence (female SMR=1.34, male SMR=1.07). Excess mortality risk was found for colorectal (male SMR=1.59), kidney (female SMR=2.85u), female genital organs (ovarian SMR=2.86u, body of uterus SMR=2.11), breast (female SMR=1.58), and metastatic cancer of unknown primary origin (female SMR=2.50u, male SMR=2.84).ConclusionsAdults with intellectual disabilities were more likely to die of cancer than the general population. Reasons for this may include later presentation/diagnosis (so poorer outcomes), poorer treatment/compliance, or both. Accessible public health approaches are important for people with intellectual disabilities, and healthcare professionals need to be aware of the different cancer experiences faced by this population.Summary boxStrengths and limitations of this studyOur key strength is the comprehensive coverage of Scotland’s entire adult population with intellectual disabilities, and inclusion of a representative general population comparison group.By using nationwide robust data linkage of high-quality electronic health records, we provide reliable data with minimal bias.Limitations include our inability to account for cancer incidence before the census date. However, prospective collection of data over nearly 9 years provided well-powered person-time for rate calculation, allowing for a meaningful interpretation of mortality rates in the context of incidence.Death certificate data imprecision is considered, but our dual-analysis (main-cause and all-cause analyses), mitigates differences and indeed have similar interpretations.