Neuroendocrine tumours (NETs) are rare tumours of the neuroendocrine system, the system in the body that produces hormones. They can be cancerous or non-cancerous.
The tumour usually grows in the bowels or appendix, but it can also be found in the stomach, pancreas, lung, breast, kidney, ovaries or testicles. It tends to grow very slowly.
Neuroendocrine tumours are sometimes referred to as carcinoid tumours, particularly when they affect the small bowel, large bowel or appendix.
Carcinoid syndrome is the collection of symptoms some people get when a neuroendocrine tumour, usually one that has spread to the liver, releases hormones such as serotonin into the bloodstream.
About 2,900 people are diagnosed with a neuroendocrine tumour each year in the UK, but not everyone with a tumour will have carcinoid syndrome.
Cancer Research UK has more information about different types of neuroendocrine tumours.
In the early stages of having a neuroendocrine tumour, you may not have any symptoms.
You may also not have symptoms if the tumour is just in your digestive system, as any hormones it produces will be broken down by your liver.
If symptoms do develop, they tend to be fairly general and can be easily mistaken for signs of other illnesses.
Symptoms may result from both the tumour itself and from any hormones it releases into the bloodstream.
Symptoms will depend on where in the body the tumour develops:
Some tumours may not cause any symptoms and are discovered by chance.
For example, a tumour in the appendix may only be found when the appendix is being removed for another reason.
Typical symptoms of carcinoid syndrome include:
These symptoms may come on unexpectedly, as the hormones can be produced by the tumour at any time.
Some people may also develop carcinoid heart disease, where the heart valves thicken and stop working properly.
There's also a risk of developing a rare but serious reaction called a carcinoid crisis, which involves severe flushing, breathlessness and a drop in blood pressure.
It's not known exactly why neuroendocrine tumours develop, but it's thought that most occur by chance.
Your chances of developing a neuroendocrine tumour may be increased if you have:
You can read more about the risks and causes of neuroendocrine tumours on the Cancer Research UK website.
A neuroendocrine tumour may be found incidentally – for example, as a surgeon is removing an appendix.
In this case, the tumour will often be caught early and removed along with the appendix, causing no further problems.
Otherwise, people usually see their GP after they have developed symptoms.
A neuroendocrine tumour may be diagnosed after carrying out a series of scans and tests, which may include measuring the amount of serotonin in your urine and having an endoscopy.
If the tumour is caught early, it may be possible to completely remove it and cure the cancer altogether.
Otherwise, surgeons will remove as much of the tumour as possible (debulking).
You can read more about the surgery for neuroendocrine tumours on the Cancer Research UK website.
If the tumour can't be removed, but it's not growing or causing symptoms, you may not need treatment straight away – it might just be carefully monitored.
If it's causing symptoms, you may be offered one of the following treatments:
Symptoms of carcinoid syndrome can be treated with injections of octreotide and lanreotide.
You may also be given medication to widen your airways (to relieve wheezing and breathlessness) and anti-diarrhoea medication.
There are things you can do yourself to manage some of the symptoms of carcinoid syndrome.
Generally, you should avoid triggers of flushing, such as:
Some medications, such as selective serotonin reuptake inhibitor (SSRI) antidepressants, may make symptoms worse by further increasing your levels of serotonin.
But never stop taking medication without seeking medical advice.
If you have diarrhoea, it's important to keep drinking little and often to avoid dehydration.
If the whole tumour can be removed, this may cure the cancer and symptoms altogether.
But even if surgeons can't remove the entire tumour, it usually grows slowly and can be controlled with medication.
Overall, people with neuroendocrine tumours have a good life expectancy compared with many other cancers. Many people remain relatively well and lead active lives, with only occasional symptoms.
But as the tumour grows or spreads, it will produce more and more hormones, and it may eventually be difficult to completely control symptoms with medication. You may need further surgery or other treatments.
Unfortunately, life expectancy isn't as good for a cancerous tumour that's spread to other parts of your body because it won't usually be possible to remove all of it. But treatment can still control your symptoms and slow down the spread of cancer.
You can read more about the survival statistics for neuroendocrine tumours on the Cancer Research UK website.