Is There an Increased Risk of Parkinson's Disease in Patients with Breast Cancer?
No, breast cancer does not increase the risk of developing Parkinson's disease; rather, patients with Parkinson's disease have a modestly increased risk of developing breast cancer (SIR = 1.17,95% CI 1.02-1.34). The directionality of risk flows from Parkinson's disease to breast cancer, not the reverse.
Evidence for the Direction of Association
The most robust evidence comes from a large Danish population-based cohort study of 20,000 patients with Parkinson's disease followed through 2008, which demonstrated that women with Parkinson's disease had a 17% increased risk of developing breast cancer compared to the general population 1. This finding has been confirmed across multiple epidemiological studies examining cancer patterns in Parkinson's disease patients 2.
No Evidence for Reverse Causation
There is no credible biological mechanism or epidemiological evidence suggesting that having breast cancer increases the subsequent risk of developing Parkinson's disease 3, 4. A comprehensive meta-analysis of 16 studies specifically examining the link between Parkinson's disease and breast/prostate cancers found no association when examining whether cancer diagnosis preceded Parkinson's disease onset 3.
Understanding the Biological Relationship
The increased breast cancer risk in Parkinson's disease patients likely reflects shared underlying risk factors rather than a causal relationship 1:
- Both conditions may share common genetic susceptibility pathways
- Hormonal factors (particularly estrogen exposure) influence both breast cancer risk and potentially neuroprotective mechanisms in Parkinson's disease 5
- The association appears bidirectional only in the sense that both conditions may arise from overlapping etiological factors, not that one causes the other
Clinical Implications for Breast Cancer Patients
Breast cancer patients should not be counseled about increased Parkinson's disease risk, as this association does not exist in the evidence 3, 4. Standard Parkinson's disease risk factors apply to breast cancer patients just as they do to the general population, including:
- History of head injury (OR = 6.23) 6
- Family history of Parkinson's disease (OR = 6.08) 6
- Pesticide exposure 4
- History of melanoma 4
Protective factors that reduce Parkinson's disease risk include physical activity, caffeine consumption, and higher serum urate concentrations, which should be promoted for general health benefits regardless of breast cancer history 4.
Cancer Surveillance in Parkinson's Disease Patients
Conversely, women with Parkinson's disease should receive appropriate breast cancer screening given their modestly elevated risk 1. This includes:
- Standard mammography screening per age-appropriate guidelines 5
- Awareness that breast tumors in Parkinson's disease patients may differ in grade of malignancy compared to the general population 1
- Recognition that the overall cancer risk in Parkinson's disease is actually decreased (SIR = 0.86), but specific cancers including breast cancer, melanoma, and non-melanoma skin cancer show increased incidence 1, 2
Common Pitfall to Avoid
Do not confuse the directionality of this association. The question asks whether breast cancer increases Parkinson's disease risk, but the established relationship is the opposite: Parkinson's disease modestly increases breast cancer risk 1, 3. This distinction is critical for appropriate patient counseling and risk stratification.