BY MATAKA BANDA (KEELE UNIVERSITY, UK) & DR CHERN EIN OON (UNIVERSITI SAINS MALAYSIA
What is cancer?
THE word cancer originates from the Greek physician Hippocrates, who used the terms carcinos and carcinoma to describe tumours. In Greek, these terms describe a crab, and they were used because a tumour resembles the shape of a crab. Later, these Greek terms
were translated into the Latin word for crab –‘cancer’ by the Roman physician Celsus. Cancer is the name given to a group of diseases where cells in the body undergo mutations, divide uncontrollably and spread into surrounding tissues. Cancer cells divide uncontrollably to form lumps or tumours because they ignore signals which tell cells to stop dividing. The behaviour and growth of tumours depends on whether they are cancerous (malignant) or non-cancerous (benign). A malignant tumour is one that can spread into and invade nearby tissues. As a malignant tumour grows, its cells can break away from the tumour and spread locally to nearby tissue. Furthermore, cancer can also spread to a new part of the body which is far away from the original tumour. This is known as metastatic cancer, and it leads to the formation of new tumours called metastases. The opposite is seen in benign tumours, which remain in one part of the body and rarely recur once removed.
How does cancer spread?
The area at which a cancer begins is called the primary site. When cancer spreads from this area to another part of the body, the cancer cells travel through either the lymphatic system or the bloodstream. The lymph vessels transport clear fluid and immune cells throughout the body via lymph nodes.
Cancer cells can flow to these lymph nodes after hijacking the lymph vessel before flowing via the lymph fluid to the lymph nodes. These cancer cells can grow within these lymph nodes or spread to other organs. More commonly, cancer cells will break away from a tumour and travel through the bloodstream and can be transported around the body, forming secondary tumours. Clearly, there must be a way that cancer cells are able to enter these vessels. The answer lies within a group of molecules called cell adhesion molecules.
As the name suggests, these molecules allow cancer cells to ‘adhere’ or stick to the walls of the blood and lymphatic vessels. These cancer cells can then enter the bloodstream or lymph fluid and lodge themselves at distant sites. Tumours can grow so rapidly that they outgrow their own blood supply and become deprived of oxygen. To counteract this, cancer cells promote the growth of new blood vessels which help to meet the oxygen requirements of the tumour.
Where does cancer spread to?
Different types of cancers have different tendencies to metastasise to different sites, depending on the closest ‘downstream’ organs or lymph nodes. The lungs are one area that cancer cells often target, and the reason is linked to the circulatory system. Deoxygenated blood from most parts of the body must flow back to the heart and then to the lungs, where it is oxygenated. Cancer cells in the blood take advantage of this free ride. This may lead to the formation of secondary cancer.
The liver is also another common metastatic site. Blood from the digestive system must circulate through the liver before it returns to the heart. The bones are also target areas for cancer metastasis. This may lead to fragile bones or nerve damage from spinal metastasis. Some types of cancer can also spread to the brain, such as breast cancer and lung cancer. Even when cancer has metastasised to a new site, it is still named after the primary cancer where it originated. For instance, liver cancer that spreads to the brain is called ‘metastatic liver cancer to the brain’.
What are the typical symptoms of cancer?
The symptoms caused by cancer may vary depending on the type of cancer. Some of the general signs include unexplained weight loss, fever, fatigue, pain, changes in bowel habits or bladder function, sores that do not heal, unusual bleeding, indigestion or trouble swallowing, and a persistent cough. The presence of any of these symptoms does not necessarily mean that you have cancer. However, if you do experience any of these for a long time or they begin to worsen, please see a doctor as soon as possible to get an accurate diagnosis. Some cancers are asymptomatic, leading to a late diagnosis and a poor outcome for patients. For example, ovarian and pancreatic cancers. Ovarian cancer is commonly detected at an advanced stage due its vague symptoms (e.g. difficulty eating, abdominal pain, indigestion).
Pancreatic cancer is usually diagnosed at a late stage because this is when the disease becomes symptomatic (e.g. jaundice and abdominal pain). The location of these organs, which are interior and not palpable, means that you may not feel a lump or tumour when the cancer is present.
Why do cancers relapse?
Despite receiving the best possible treatment and making significant progress, sometimes cancer can still return. One reason this may happen is because some of the original cancer cells survive the initial treatment and either form another tumour in the same region as the initial cancer, or have spread to another part of the body, prior to removal of the tumour. Some of these cells can also be so small that they are not detected during a patient’s follow-up immediately after treatment.
Cancer stem cells are cancer cells that can divide and give rise to many different cell types found within a tumour. They provide another explanation for cancer recurrence as they can exist within tumours but can avoid being destroyed by cancer treatments. This is because they are missed by conventional cancer chemotherapies and can therefore survive treatment, start to divide and give rise to new tumours.