Cancer patients appear to have a lower risk of developing Parkinson’s disease, even when taking into account important risk factors and overall survival, a study has found.
The study, “Cancers Preceding Parkinson’s Disease after Adjustment for Bias in a Danish Population-Based Case-Control Study,” was published in Neuroepidimiology.
Previous studies have shown that cancer patients are less likely to develop Parkinson’s during their lifetime than the general population. However, it is unclear whether this could be caused by the negative association between Parkinson’s disease and smoking, when, for many cancers, smoking is a known risk factor, or simply by the fact that most cancer patients do not survive long enough to reach a stage in which they are more likely to develop Parkinson’s.
Researchers at the University of California Los Angeles carried out a large population-based case-control study in Denmark (PASIDA) to investigate if cancer correlated with a lower risk of Parkinson’s disease, even after normalizing important risk factors, such as smoking, physical activity, and survival bias.
The study involved a total of 1,813 patients with Parkinson’s disease and 1,887 age- and sex-matched individuals without Parkinson’s used as controls.
Demographic analysis showed the percentage of non-smokers was higher in the group of Parkinson’s patients (49.6%) than in controls (35.3%). Conversely, the incidence of cancer was lower among Parkinson’s patients (3.8%) than in controls (4.2%), a difference that was even more pronounced when comparing the incidence of smoking-related cancers (1.7% versus 2.2%).
Apart from skin and breast cancer, further analysis showed a negative correlation between Parkinson’s disease and all types of cancers, including those related and not related to smoking. Even after normalization for risk factors and cancer patients’ survival bias, the negative association between Parkinson’s disease and cancer remained the same.
“Since PD [Parkinson’s disease] cases stop smoking many years prior to PD diagnosis, one might say that ‘PD prevents smoking’ and thus reduces the risk of smoking-related cancers and mortality. [However,] in PASIDA adjustment for pack-years of smoking did not change the observed associations between cancer and PD, which suggested that smoking is unlikely to be a strong confounder between cancer and PD,” the researchers wrote.
“In conclusion, our study suggested a lower frequency of most cancers preceding PD diagnosis after adjustment for major lifestyle factors. Our bias analysis indicated that survival bias minimally impacts the observed associations,” they added.