Cancer in the Context of Severe Mental Illness in New Zealand: An Epidemiological Study

Abstract

Background Experience of severe mental illness is associated with poor physical health and premature death. This issue has received little research attention in New Zealand. This thesis explores the burden of cancer amongst people with severe mental illness in New Zealand.

Aims This thesis aims to answer four questions: 1. Is cancer an important cause of morbidity and mortality among adults living with severe mental illness in New Zealand? 2. Is cancer contributing to differences in health outcomes between people with severe mental illness and others in the population? 3. What are the factors that are contributing to any differences in cancer outcomes between people with severe mental illness and others in the population? 4. Does the relationship between mental illness and cancer vary by mental health diagnosis or cancer type?

Methods Recent contact with secondary mental health services was used as a proxy for experience of severe mental illness. Anonymised national data on secondary mental health service contacts for adults aged 18-64 (2001-2010) were linked to cancer registrations and mortality records (2006-2010). Annual cancer incidence and mortality rates among people in contact with mental health services in the five years prior to cancer diagnosis/death were estimated, and standardised for comparison with annual rates for the New Zealand population. People diagnosed with breast and colorectal cancers in 2006-2010 were identified. Cancer-specific survival was compared for recent mental health service users and nonusers using Cox regression. The contribution of cancer stage at diagnosis, deprivation and physical comorbidity to survival differences were explored for people with diagnoses of schizophrenia and bipolar disorder (Group A) and others in contact with mental health services (Group B).

Results Nearly two thousand (1876) cancers occurred over five years among people with a history of recent mental health service use. The standardised incidence of cancer was similar in this group and the general population [SIR 1.03 (95% CI 0.98-1.08)], while lung cancer was more common [SIR 1.98 (1.73-2.26)] and prostate cancer less common [SIR 0.66 (0.54-0.8)]. Mortality from all cancers combined was higher among people in recent contact with mental health services compared to the general population [SMR 2.21(2.07-2.37)]. Of 8762 and 4022 people with breast and colorectal cancer respectively, 440 (breast) and 190 (colorectal) had recent contact with mental health services. After adjusting for demographic confounders, the risk of death from breast cancer was increased for Group A [Hazard Ratio (HR) 2.55 (1.49–4.35)] and B [HR 1.62 (1.09–2.39)], and from colorectal cancer for Group A [HR 2.92 (1.75–4.87)], compared to others in the population. Later stage at diagnosis contributed to survival differences for Group A, and comorbidity contributed for both groups. Fully adjusted HR estimates were breast: Group A 1.65 (0.96–2.84), B 1.41 (0.95–2.09); colorectal: Group A 1.89 (1.12–3.17), B 1.25 (0.89–1.75)].

Conclusions Cancer diagnosis overall was equally common, and the risk of lung cancer was higher, among people with severe mental illness compared to the general population, while cancer mortality was more than doubled. Commonly used methods can, however, result in biased underestimation of cancer incidence. Survival disparities between people with mental illness and others in the population were evident for both breast and colorectal cancers, and related to the high burden of comorbid physical illness, and late stage at cancer diagnosis (for Group A only). Interventions to reduce tobacco use and improve cancer detection and care have the potential to improve physical health in those with experience of mental illness.