We all have ideas about cancer – the “C” word, the big “C”, fundraising, marathon runners in fancy dress, we’ll “beat” cancer, coloured ribbons, tumours, chemotherapy, hair-loss, sickness, survival – or not. With 1 in 2 of us likely to get cancer during our lifetimes, it seems that cancer awareness is everywhere. But what actually is this disease?
What is "normal"?
An adult human body contains around 37 trillion cells, you could imagine it as a huge and complex community. For that community to work, every cell has to be a team player. It should do the job it is programmed to do, not take more than its share of resources, stay in the tissue or organ it belongs to and only make more copies of itself when they are needed for normal growth or to replace worn out or damaged cells. If damaged beyond repair, or no longer needed they should die through a carefully controlled process. In order to do all of this our cells essentially follow “rules” that are laid down by the instructions held in our genetic code.
Tumours - not always sinister.
Sometimes cells will start to behave oddly. They divide when they shouldn’t, making more and more cells until there are so many that a lump or tumour can be felt or seen on a scan. Tumours can get big enough that they squash or press on organs or important blood vessels and nerves. This can affect the way that organ functions or can cause pain or numbness.
Not all tumours are cancer. They can still grow big enough to cause problems depending where they are, but if they can be removed by surgery, they won’t usually grow back or spread. These are called benign tumours, and are not cancer.
Not always a tumour.
Although we usually think of cancer as having one or more tumours somewhere in the body that isn’t always the case. The one exception is blood cancer. Leukaemias, lymphomas and myelomas are cancers where the excess cells are released into the bloodstream, lymphatic system or build up in the bone marrow.
Is it malignant?
The key feature that defines cancer cells is that they can spread or metastasise. They can unstick themselves from their neighbours and invade nearby tissues or spread to other parts of the body through the blood vessels or the lymphatic system (a series of tubes that resemble the blood network but carry immune system cells and fluids). Tumours that can metastasise are described as malignant, even if they have not yet spread, or are a slow growing type that is less likely to spread.
The diagram shows the difference between benign and malignant tumours. The benign tumour has(red) cells that look relatively normal (blue cells), are growing more than they should, but remain contained within their original tissue or organ. The malignant tumour has cells that are increasingly different from normal cells and are beginning to break out from the original site to spread to other places.
Damaged, but surviving.
When someone has cancer some of their cells are damaged so that the genetic instructions for how to behave are corrupted. Controls on when they divide are lost which allows them to produce many new cells. Some fast growing cancers do this at an astonishing rate.
Because the cancer cells’ genetic code is damaged and they are dividing often, this increases the chance of getting more damaged. Sometimes they look and behave more like the original type of cell the cancer started in, scientists describe this as well differentiated. If the cancer has more changes, or certain important ones the cells become unrecognisable as their original type. This is described as undifferentiated.
You might think that all of this chaos and excessive growth would mean that the cancer cells couldn’t survive, but they are remarkably good at adapting both themselves and their environment.
Tumours growing quickly need a lot of resources. Materials needed to build new cells and glucose and oxygen for energy are delivered through the bloodstream. As tumours grow, the existing network of blood vessels is not enough to supply their needs so they release signals that make new blood vessels grow towards them.
Normal cells can repair themselves, they have mechanisms that repair genetic damage or if they are too damaged will trigger a self-destruct mechanism called apoptosis. In cancer cells this system is broken, so they not only survive, but can accumulate more damage each time they divide.
The last trick they have to help them survive is that they can hide. Normally your immune system recognises cells that are damaged or potentially cancerous through changes on their surfaces. This marks them out for destruction. Successful cancer cells can release chemicals that switch off part of the immune system, remove the surface changes that mark them as “bad” or interact directly with immune cells to stop them attacking.
But it’s just a lump - why is cancer so harmful?
Sometimes the original tumour, or the secondary tumours that form when the cancer has spread are in places where they can’t be removed by surgery and damage important organs.
Many cancers develop secondary (metastatic) tumours in bones. This affects your immune system because many of the cells we need are produced in the bone marrow. Fewer immune cells mean you are more likely to suffer from serious infections and are less likely to recover. Tumours in bone can also cause the bones to release calcium into your blood, bones contain a lot of calcium, and such high calcium levels can be poisonous to us.
If the tumours are in or near the digestive system they can block it and stop us absorbing essential nutrients leading to malnutrition. In the lungs they can reduce the amount of oxygen we can take in causing breathlessness or harmfully low oxygen levels.
Our livers have around 500 different functions including some essential for our digestion and making and breaking down thousands of different chemicals. Damage caused by tumours in the liver eventually stop it working properly.
Finally, tumours can also cause problems by damaging blood vessels leading to internal bleeding.
This all sounds pretty awful - is there any good news?
Today cancer is much more treatable and many people are cured of their disease. Over half of the people now diagnosed with cancer are still alive ten years later and even if the disease can’t be cured it is possible to slow it down for many years.
Scientists are learning more about cancer every day. Every aspect of cancer from the initial genetic mutations to the way it spreads, grows blood vessels and hides from the immune system is being unpicked and new treatments are being developed that exploit every difference between cancer and normal cells.
What about rare cancers? Whilst a great deal of research effort is still focussed on the “big four” cancers (lung, breast, prostate and colon), the emphasis is changing towards treating every cancer as an individual disease through fast genetic analysis of tumours and personalised medicine. Internet based patient communities are increasingly supporting access to clinical trials by bringing patients and researchers together. This will help to ensure that new treatments can be properly tested in enough patients with rare cancer that they too can benefit from the latest advances in medicine.