Psychiatric disorder as a risk factor for cancer: different analytic strategies produce different findings

Reported associations between psychiatric disorders and cancer incidence are inconsistent, with cancer rates in psychiatric patients that are variously higher than, similar to, or lower than the general population. Understanding these associations is complicated by difficulties in establishing the timing of onset of psychiatric disorders and cancer, and by the possibility of reverse causality. Some studies have dealt with this problem by excluding patients with cancers predating their psychiatric illness; others have not considered the issue.

We examined associations between psychiatric hospitalization and cancer incidence in a cohort of 1,165,039 Swedish men, and we explored the impact of different analytic strategies on these associations using real and simulated data.

Relative to men without psychiatric hospitalization, we observed consistent increases in smoking-related cancers in those with psychiatric hospitalizations, regardless of analytic approach (eg, hazard ratio = 1.73 [95% confidence interval = 1.52-1.96]). However, associations with cancers unrelated to smoking were highly dependent on analytic strategy. In analyses based on the full cohort, we observed no association or a modest increase in cancer incidence in those with psychiatric hospitalizations (1.14 [1.07-1.22]). In contrast, when men whose cancer predated their psychiatric hospitalizations were excluded, future cancer incidence was lower in psychiatric patients (0.72 [0.67-0.78]). Results from simulated data suggest that even modest exclusions of this type can lead to strong artifactual associations.

Psychiatric disorder-cancer incidence associations are complex and influenced by analytic strategy. A better understanding of the temporal relationship between psychiatric disorder and cancer incidence is required.