Parkinson’s disease and cancer: what’s the connection?

Researchers have long tried to understand the relationship between Parkinson’s disease (PD) and cancer, with multiple studies conducted over the years. Intuitively, PD and cancer are not at all similar, and can even be thought of as biochemically opposite – in one disease (PD), cells die unexpectedly and in the other (cancer), cells divide too often and are perpetuated for too long. In fact, there have been many epidemiological studies over past decades that have shown that patients who have had cancer have a lower risk of developing PD than the general population. Other studies have shown that people with PD have lower overall cancer risks than people without PD.

Despite many papers demonstrating this inverse relationship between PD and overall cancer risk, an occasional study has been conducted which concluded that people with PD have an increased risk of cancer. It can be confusing to understand the different studies and determine how the resulting information may or may not apply to you.

Here we highlight some of the more common cancer-related topics when it comes to PD:

Ethnicity: Does ethnicity play a role in the relationship between Parkinson’s and cancer?

A study, conducted in 2015 in Taiwan among people of Chinese heritage, indicated an increased risk of cancer in people with PD. In order to explain why this study did not conform to prior results, the researchers suggested that the relationship between cancer and PD may differ in different ethnic populations. Particular genes influence cancer risk as well as PD risk, so it should come as no surprise that these factors may change in different populations. However, much more work must be done to confirm this result, and currently there are no changes to cancer screening for PD patients of particular ethnicities.

What about cigarettes? Parkinson’s, smoking, and risk of cancer

As I mentioned in a previous blog, there is also an inverse relationship between cigarette smoking and PD. It is unclear whether that is because an ingredient of cigarette smoke protects smokers from PD, or because the lack of dopamine in people with PD make them more risk-averse as a group, and therefore less likely to smoke.

Regardless, because it is clear that smoking increases the risk of multiple types of cancer, any study that investigates the cancer rates of people with PD must always account for the fact that people with PD tend to smoke less than people without PD. The most recent study which analyzes the relationship between cancer risk and PD, as referenced above, takes great pains to remove this confounding factor in its statistical analysis. Even with taking differences in smoking rates into account, the conclusion of the study is that people who have had cancer are less likely to develop PD. It is also the case that rates of certain types of cancers that have not been linked to smoking are lower in people with PD. This fact supports the conclusion that the lower rates of smoking among PD patients is not the cause of the lower rates of cancer. Regardless of any study results, cigarette smoking is linked to numerous deadly diseases and must be avoided.

Parkinson’s disease and melanoma

Even in studies that have established a lower overall cancer risk in people with PD, the risk of certain specific types of cancers is higher than in the general population.

Melanoma is a type of skin cancer that has been consistently linked to PD. People who have had melanoma are at an increased risk for PD and people who have PD are at an increased risk of melanoma. The relationship between melanoma and PD is intriguing:

• The substantia nigra, or “the black substance,” is the area of the brain that contains the dopaminergic neurons that degenerate in PD. These neurons are full of neuromelanin, a dark pigment found in the brain which is related to melanin, a dark pigment found in the skin.
• Levodopa is a precursor for melanin and neuromelanin.

If a person regularly ingests Levodopa, he/she may be fueling the production of melanin, and possibly the production of melanin-containing cells, which in theory would increase the likelihood of melanoma. However, some studies have demonstrated that the increased melanoma risk is present in patients with PD even before Levodopa is started, suggesting that the relationship is not due to Levodopa intake but rather to a genetic link.

Epidemiological studies have shown an increased risk of non-melanoma skin cancers in PD patients as well.

What to do if you have an increased melanoma risk?

The increased risk of melanoma in people with PD over the general population is small and melanoma remains a relatively uncommon cancer, even in those with PD. However, it is a cancer that can be dangerous if caught at later stages and treatable if caught early, so focusing on prevention, screening and early detection makes sense.

You should know what other melanoma risk factors you have. These include:

• Increased age
• Male sex
• Personal history of melanoma or other skin cancers
• Family history of melanoma
• Fair skin, light eyes, freckles
• Exposure to UV rays, usually from sun exposure

These tips below are applicable to all adults with and without PD:

• Minimize sun exposure by wearing protective clothing and avoid being out in the sun at peak times, such as midday.
• Use a waterproof sunscreen that protects against UVA and UVB rays and that is at least SPF 30.
• Visit a dermatologist at least once a year for a skin check.
• Visit the National Institutes of Health website for more information on the types of skin changes that should raise concern.

Parkinson’s connections to breast and prostate cancer

There are some associations between PD and certain other cancers that you may have read about in the news, but these links remain tenuous and have not led to any change in screening procedures for people with PD. For example, some studies showed a small increase in breast cancer risk in people with PD, while other studies did not support this claim. Breast cancer screening recommendations for people with PD remain the same as for the general population.

During a clinical trial testing a medication called Entacapone, a member of a class of PD medications called COMT inhibitors, prostate cancer cases were slightly higher among those taking the drug as compared to those on placebo. This raised concerns about a connection between using Entacapone and developing prostate cancer. However, in subsequent studies, this association was not substantiated.

Tips and Takeaways

• Overall cancer risk is lower in people with PD as compared to the general population.
• It is important to note that although PD patients appear to have lower rates of many forms of cancer, this does NOT mean that people with PD can forgo their routine cancer screening. All cancers can develop in people with PD, so make sure to follow your doctor’s advice on the cancer screening program that is appropriate for you.
• The risk of melanoma is higher in people with PD than the general population. Care must be taken to minimize sun exposure and to undergo yearly skin checks with a dermatologist.
• Know your other melanoma risks including family history and share that information with your doctors.