UC Irvine

Disease burden increases with age. Cancer and dementia are two age-related chronic diseases that most commonly occur in older adults. They are also amongst the top 10 leading causes of death and disability in this age group. Despite the increased risk of comorbidity at older ages, it is unlikely for cancer and dementia to present in the same person at the same time. Surprisingly, evidence suggests that rates of dementia are lower in older adults with a history of cancer and vice versa. However, whether this pattern of inverse association holds true for the oldest-old, remains unclear. The oldest-old represent the fastest growing age group of the world’s population yet have occupied only a small percentage of the population included in the published literature. The objective of this dissertation is to examine the relationship between cancer and dementia in the oldest-old using data from the 90+ Study at the University of California, Irvine.

An initial cross-sectional study was performed as a preliminary analysis using data from the 90+ study. A logistic regression model of 1525 oldest-old participants with a) no history of cancer or b) a history of cancer, were assessed for having dementia at baseline enrollment into the 90+ study. Results showed a 36% reduced odds of dementia in participants with a prior history of cancer at baseline. Building off these preliminary analyses, two studies were performed to further evaluate the overall research question about the relationship between cancer and dementia in the oldest-old. Both research questions utilized data from the 90+ study. Specifically, the study population consisted of 761 older adults who were dementia free at baseline enrollment into the 90+ Study between 2003 to 2018.

The first research objective was to measure the risk of dementia in the oldest-old with a prior history of cancer compared to the oldest-old who remained cancer free. Using cox proportional hazards we estimated the cause-specific hazard ratio to address questions of etiology between the two diseases. The second research objective was to examine cognitive performance over time in participants with a history of cancer compared to those who remained cancer free. A linear mixed model for repeated measures was used to capture multiple longitudinal continuous outcomes in each of the groups.

Self-reported history of cancer was associated with a reduced rate of all-cause dementia in people aged 90 years and older as a function of the cause-specific hazard ratio. It was not associated with AD dementia. Although, the magnitude and direction of the association was similar to previous studies who found an inverse association. Additionally, we examined the role of type of cancer on the association between cancer and dementia, but the influence could not be determined in any definitive manner. In regard to cognitive performance, participants with a history of cancer showed slower rates of decline compared to participants who remained cancer free in global cognition and verbal fluency cognitive domains. No association in any other cognitive domain was found, largely due to missing information.

This is the largest study to date on cancer and dementia in a well-characterized cohort of the oldest-old. The results suggest that a history of cancer is associated with slower rates of cognitive decline over time. This, coupled with the magnitude and direction of the association between cancer and dementia suggests that a history of cancer does influence cognitive performance over time in the oldest-old. Furthermore, the inverse association suggests that a history of cancer may delay or protect from dementia in this particular age group. This research into how age-related diseases influence the progression or prognosis of other age-related diseases is important when examining disease burden in the oldest age groups at highest risk for them. This research has the potential to influence healthcare screening practices in the oldest-old and how health care providers approach long-term and palliative care in patients with a history of cancer or diagnosed with dementia. Knowing whether a person in this age group is at greater or lesser risk of dementia as a result of their cancer history may influence how often they need to be screened for dementia and possibly whether or not signs of cognitive decline are attributed to dementia before ruling out other potential causes.