Our Science Surgery series answers your cancer questions.
Cancer can develop in nearly every organ in the body, including vital organs like the brain and lungs. As organs go, the heart is pretty vital – it’s basically one big muscle whose purpose is to pump blood around our bodies. Despite its importance, you very rarely hear about tumours developing in the heart. Which begs the question, is there something special about the organ that shields it from cancer?
Dr Rohin Francis, cardiologist and researcher, says that it comes down to one very simple reason.
“It’s because the cancers are errors in cell division. And that’s just something that doesn’t happen very much in the heart. And I think this is perhaps something that a lot of people don’t realise is that heart cells – by which I mean myocardial muscle cells – don’t divide, to any real extent. This means that once they’re dead – they’re dead.”
So it’s a matter of division – the more the cells divide, the higher the chance of DNA errors (mutations) developing that can lead to cancer. So tissues where cells divide more frequently – the breast, liver and skin for example – are at higher risk.
Data has shown that cardiac tumours that develop in the heart (known as primary tumours) have an incidence of less than 0.02%. Of these, around 25% are cancerous and are most commonly sarcomas. This means that they develop in the connective tissue in the heart, rather than heart muscle cells themselves.
Another reason for the somewhat surprising lack of cancer is that the heart is quite protected, as Francis explains. “Organs like the lungs and skin are exposed to carcinogens such as tobacco smoke and ultraviolet light, contributing to lung cancer and skin cancer. However, the heart’s placement within the body means that it isn’t as exposed to the outside environment.”
So hearts don’t get tumours?
But just because the heart is protected from the outside world doesn’t mean that it’s protected from tumours growing in other parts of the body, as Francis explains.
“You get metastatic spread of cancers to the heart quite frequently. So probably about one fifth of metastatic cancer will have some trace that’s found in the heart, but those are all secondary. And the most common are lung cancers, which frequently spread to the heart because the lung directly drains to the heart.”
But despite this frequency, cancers spreading to the heart often go unnoticed. “Evidence of metastatic spread to the heart is only found in post-mortem studies in a lot of cases because it doesn’t really cause a lot of noticeable problems,” says Francis. “Classically, the organs where spread causes a lot of symptoms include the brain and the bones, where it can be extremely painful.”
Plop plop, who’s there?
This doesn’t mean that hearts don’t develop tumours at all – it’s just that they aren’t cancerous. Data from autopsy studies have shown that around 75% of all primary tumours found in the heart are benign. With the heart being made up of multiple different types of cells, these tumours are diverse.
One of the most common is a tumour known as a myxoma. This normally develops in the atrium and looks like “a blob of rubbery tissue on a stalk that has attached itself to the wall of the heart. When you examine it, you hear a plopping noise that is genuinely called a myxomatous plop.”
Despite being non-cancerous, these structures can still pose a threat.
Myxomas can be very mobile and so can move between the chambers of the heart, damaging the surrounding tissue. Without very rapid surgery, they can expand into the entire cardiac chamber and obstruct blood flow – causing issues like strokes, blood clots and even heart attacks. But these cases are fairly common and easily treated with surgery so “they’ve got a very good prognosis and it’s rare that someone will have a major problem.”
Then do we really need to study the heart?
Although, comparatively speaking, the heart isn’t at high risk of developing cancer, Francis emphasises the need for cancer researchers and specialists to study the heart.
Cancers and life-saving treatments for them (such as some chemotherapy drugs and radiotherapy) can have damaging effects to the heart. “This means that there’s now a sub-speciality of cardiology known as cardio-oncology, which hopes to understand these effects a bit more and bridge that gap between cancer specialists and cardiologists.”