What is a cancer?
Cancers can develop in any part of the body, but they are more common in some organs than in others. For example, the top five organs in which fatal cancers developed worldwide in 2008 were:
- lungs (1.4 million deaths)
- stomach (740,000 deaths)
- liver (700,000 deaths)
- colon and rectum (610,000 deaths)
- breast (460,000 deaths).
Each of these cancers began when a single normal cell in the organ (e.g. a breast) began to divide uncontrollably, forming a solid mass of new cells (Figure 3.2). A lump of new cells growing in an inappropriate location is known as a tumour (the general public often calls it a 'growth'). The original mass of cells is called the primary tumour to distinguish it from any 'break-away' tumours that may form later, if cells from the primary tumour escape and spread to other parts of the body.
Tumours can be either 'benign' or 'malignant' and this has a crucial impact on the person's chances of survival. Most benign tumours are rarely life-threatening, though some may grow very large over a long time and eventually interfere with the functioning of a vital organ, such as the liver, heart or brain. A malignant tumour is the medical name for a cancer. Some cells in a malignant tumour break away from the original primary mass of cells and spread around the body, carried in the blood stream or lymphatic vessels. The malignant cells become lodged in distant locations (e.g. in fine capillaries in organs like the lungs or breast) and begin to generate new secondary tumours. The habit of spreading to other parts of the body is the defining characteristic of cancers, and this is what makes them life-threatening and difficult to treat.
At this point, you may be asking yourself why cancer cells grow in an uncontrolled fashion, if they contain the same types of molecules and cellular structures as normal cells? Although cancer cells are very similar to their 'normal' counterparts, they differ in the activity of a few genes. These genes allow cancer cells to divide and produce new cancer cells much more often than normal cells are able to divide. The cancer genes also change the behaviour of cancer cells and give them other characteristics that promote their survival and their ability to spread beyond the original primary tumour.
Table 3.1 summarises some of the important differences between normal and cancer cells.
Table 3.1 The basic properties of normal and cancer cells.
| Normal cells | Cancer cells |
|---|---|
| Require energy to fuel chemical reactions inside the cells | Require more energy to fuel chemical reactions inside the cells |
| Divide and reproduce new cells only when appropriate signals are received | Divide and reproduce new cells continuously without needing the appropriate signals |
| Have specialised functions appropriate for their location (e.g. lung cells have special functions for their role in the lungs; breast cells have special functions for their role in the breast, etc.) | Lose part or all of their specialised functions (i.e. a cancer cell originating in the lungs no longer looks or behaves like a lung cell) |
| Age and die after a limited number of cell divisions (usually no more than eight cycles of cell division) | Survive an unlimited number of cell divisions (possibly hundreds of cycles of cell division) |
| Self-destruct and die when the appropriate signals are received (e.g. because the cell is ageing or developing abnormal characteristics) | May not self-destruct even when the appropriate signals are received, so abnormal cells survive and continue to divide |
| With some exceptions (e.g. blood cells) remain fixed in a location within tissues and organs | May escape from their original location and spread to other tissues and organs, where they can divide and reproduce new cancer cells |