It hardly needs to be said that a diagnosis of cancer, and subsequent treatment for the disease, can be an upsetting and harrowing experience that profoundly affects how we feel.
But over the past decade or more, researchers have begun to appreciate the depth of the link between cancer and our mental state.
Maybe, scientists have wondered, there’s something more profound going on than the seemingly common-sense explanation that “being told you have cancer makes you depressed”.
The story took a fascinating twist this week, when new US research suggested that cancer cells might actively release chemicals that can affect our mood, and make us more likely to be depressed.
Although the study only involved rats, not people, if the results are applicable in humans it would be a milestone in understanding the relationship between cancer and depression. Because understanding this link could allow scientists to develop ways to prevent or treat it.
Cancer and depression
The link between depression and cancer is a strong one. For example, nearly two-thirds of breast cancer patients experience some form of mood disorder, and similar figures have been found amongst patients with other forms of the disease.
But this has a greater cost than just human happiness. There’s strong evidence that depressed cancer patients actually fare worse than their ‘happier’ counterparts.
For example, a paper published last year found that, amongst 205 cancer patients given psychological assessments and followed up for 15 years,
Depressive symptomology was the most consistent psychological predictor of shortened survival time
There are several possible reasons for this. It might be that people who are already depressed before their diagnosis might not feel compelled to visit the doctor to go for screening, or report the symptoms of cancer until it’s too late for treatment to be effective.
Another more likely explanation is that people who become depressed during treatment, for whatever reason, are less likely to continue taking cancer drugs or proceed with invasive procedures, and there’s some evidence for this.
Either way, understanding the root cause of depression in cancer patients is key to preventing its effects.
How can cancer cause depression?
In a paper published this week in the Proceedings of the National Academy of Sciences, Dr Leah Pyter and her colleagues at the University of Chicago started with the idea that depression in cancer patients could have three main causes.
Firstly, as mentioned above, finding out you have cancer can, in and of itself, make you depressed – for reasons we can all understand and empathise with.
A second reason could be the unpleasant side-effects of cancer therapy. Chemotherapy, radiotherapy and surgery, despite saving lives, all have effects that can alter our mental well-being.
But the US researchers propose that chemicals produced by the cancer itself could be involved. Cancer cells secrete a range of chemicals that affect how our bodies work, so it’s not that far-fetched to think that they could affect how we think and feel.
Dr Pyter and her colleagues set out to see if they could detect any such mood-altering chemical changes.
In a series of experiments, they compared the behaviour and brain chemistry of healthy rats with those that had developed breast cancer.
Compared with healthy rats, Dr Pyter’s team found that rats that developed cancer also developed a range of behaviours believed to be symptoms of depression, such as burying marbles, and a failure to swim. But unlike rats with acute diseases such as a bacterial infection, they didn’t show other characteristic ‘sickness behaviours’ such as not eating, less social interaction or spending more time asleep, suggesting that the rats weren’t just ‘feeling ill’.
‘Sickness behaviours’ are now thought to be caused by chemicals called cytokines, produced by the immune system during infection. The fact that the rats were showing some behaviours, but not others, led the researchers to look at the exact cytokines being produced by the animals.
Rats with tumours all showed higher levels of three particular cytokines – IL-1β, IL-6 and TNFα – in their tumours, their blood and their brains. All of these have been previously linked to changes in behaviour following infection or brain injury.
As the researchers point out in their paper, raised levels of cytokines, at even moderate levels, have been linked to learning difficulties and emotional problems in humans. Dr Pyter’s team thinks that low level increases in cytokines over the period of time it takes for a tumour to grow (months or weeks) could be enough to significantly change our emotional state.
The team also found evidence of other brain changes after cancer developed, such as changes in levels of corticosterone, a chemical involved in responding to stress, and differences in the activity of certain behaviour-associated genes, called glucocorticoid receptors, in the brains of rats with cancer.
What does this all mean?
This research is some of the first to look at chemical changes in the brain, before and after cancer develops. As such, it provides tentative evidence that cancer cells in our bodies might trigger changes in our brains. And given the available evidence, it’s certainly plausible.
But as always, there’s a long way to go before we can be sure. Although we’re all part of the same tree of life, rats and humans have rather different brains. And it’s always worth exercising a note of caution when interpreting the results of an experiment designed to measure an animal’s feelings or mood. As the authors themselves say,
Caution must be taken in extrapolating these data to cancer patients.
Nevertheless, the idea that a tumour – in and of itself – might be able to trigger depression is one that might, paradoxically, offer a crumb of consolation. Because one of the hardest feelings to shake is that a particular problem is one’s own fault.
But it might turn out that feeling depressed about a diagnosis of cancer, or dreading that next round of chemo, isn’t your ‘own fault’ or weakness, but a treatable medical condition caused – or at least made worse – by abnormally growing cells releasing chemicals into your body.
And as we frequently point out on this blog – with new scientific and medical discoveries, understanding how something occurs is the first step on the road to stopping it.
- If you have cancer and are affected by depression, you may find the following pages of our CancerHelp UK website helpful
Pyter, L., Pineros, V., Galang, J., McClintock, M., & Prendergast, B. (2009). Peripheral tumors induce depressive-like behaviors and cytokine production and alter hypothalamic-pituitary-adrenal axis regulation Proceedings of the National Academy of Sciences DOI: 10.1073/pnas.0811949106