D. Michal Freedman
Some previous epidemiologic studies have suggested that having Parkinson’s disease decreases your risk of developing cancer, and vice versa. If true, this finding could provide insight into underlying biologic mechanisms for the two diseases.
How we set out to answer the question
In a study published in the International Journal of Epidemiology we used data from Medicare, a U.S. federal health insurance programme for those aged ≥65 years, linked to U.S. cancer registry data to examine the relationship between Parkinson’s disease and cancer. Because we used Medicare data, we were limited to people aged 65 years and older. However, as the Medicare database is very large, we were able to examine relationships in whites and non-whites, in men and women, and in different age groups (all above 65 years).
We hypothesized that previous studies may have found a lower risk of cancer after Parkinson’s disease because cancer screening or medical work-ups were less frequent in people with a debilitating condition such as Parkinson’s than in people without such a condition. We first looked at whether people with cancer were less likely to have had a prior Parkinson’s diagnosis than those without cancer. We also examined whether patients with cancer were less likely to be diagnosed with Parkinson’s, compared to those without cancer. Finally, we analyzed whether people who had another serious medical condition, trauma from car accidents, were less likely to be diagnosed with cancer, and vice versa. As there is no plausible biological association between trauma from car accidents and cancer, if we found an association between injuries from car accidents and lower likelihood of cancer, it could suggest a bias in diagnosing cancers after serious medical diagnoses rather than a true biologic relationship between Parkinson’s disease and cancer.
What we found
Cancer patients were just as likely to be later diagnosed with Parkinson’s or have trauma from a car accident as those who were not cancer patients. However, both the group with Parkinson’s disease and the group who had been in serious car accidents had about a 20% lower risk of being later diagnosed with cancer compared to those without Parkinson’s or a history of trauma from car accidents.
In our study, having cancer was not associated with later developing Parkinson’s disease; but we did find a lower risk of being diagnosed with cancer after Parkinson’s disease. However, this risk was essentially the same as that for being diagnosed with cancer after trauma from a car accident. This suggests a possible bias in diagnosing cancer less frequently after a serious medical condition. This is consistent with research showing that individuals with a disability are less likely to get screened for cancer than those who are not disabled. Additional studies are needed to explore this possibility. Such studies might, for example, compare people with and without Parkinson’s disease who have the same frequency of screenings for a particular cancer, such as mammograms for breast cancer, to examine whether incidence for that cancer differed between the two groups.
Freedman DM, Wu J, Chen H, Engels EA, et al. Associations between cancer and Parkinson’s disease in U.S. elderly adults. Int J. Epidemiol. 2016. DOI: 10.1093/ije/dyw016 [free to access until 21 June 2016].
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D. Michal Freedman (email@example.com) received her M.P.H. and Ph.D. from the Johns Hopkins University Bloomberg School of Public Health where she specialized in cancer epidemiology. She is an epidemiologist with the Division of Cancer Epidemiology and Genetics, National Cancer Institute, U.S. National Institutes of Health. One of her research interests is the relationship between cancer and neurodegenerative diseases. She is the lead author of the paper ‘Associations between cancer and Parkinson’s disease in U.S. elderly adults’ in the International Journal of Epidemiology. You can follow the NCI Division of Cancer Epidemiology and Genetics on Twitter @NCIEpiTraining.