In this chapter you will learn about:

A malignant growth is characterised by a continuing, purposeless, unwanted, uncontrolled and damaging growth of cells that differ structurally and functionally from the normal cells from which they developed.

All living plants and animals are composed of living cells that often need to divide to produce more cells for growth and development and also to replace cells that have been damaged or have died. The process of cell proliferation (cell division and cell growth) is controlled by genes in the DNA of the cell nucleus. The genes are inherited from parents and bestow particular features in the offspring, including height, colour, weight and countless other distinctive features and functions of the tissues. The process is normally under remarkably well-balanced control. A cancer forms when this genetic control is damaged or lost in one or more cells, which then continue to divide and divide again, producing more abnormal cells that continue to divide and increase in number when and where they should not. The masses of unwanted dividing cells cause damage to other cells and tissues in the body. They are no longer controlled by normal genes that stop division after normal body needs have been met. They just go on dividing in spite of causing damage to other tissues and body functions. This is a cancer. All the causes of cancer are now known to directly, or indirectly, damage these normal genes that regulate cell division.

One obvious factor is that the longer we live, the more chance there is for the genes that regulate cell proliferation to become damaged by exposure to agents that damage the genetic blueprint, DNA. So most cancers become more common the longer we live; most cancers are more common in old age. Another factor is the rate of division for growth and replacement of tissues. Tissues like skin, bowel lining or lining of air passages (especially in the lungs) and blood cells are constantly being shed and replenished. Breast cells are constantly changing due to hormone activity over a woman’s years of fertile life. With all this constant cell proliferation, there is more likelihood of mistakes being made in the process of copying the genetic blueprint to daughter cells, especially as the process becomes less accurate as we get older. A mistake or error in copying the genetic blueprint is called a genetic mutation. These then are the tissues most likely to undergo malignant change. Bone growth is greatest in growing young people, and testicular activity is greatest in young adult males, and these are the periods of life most prone to cancers of these tissues. As men grow old, the slow but constant changes in the prostate gland make it more likely that factors causing a change in cells might go wrong after years of exposure to the driving force of male hormones. So prostate cancer becomes increasingly common in old age.

The remarkable thing is not that something goes wrong from time to time in the delicate process of cell division but that things don’t go wrong more often. In all life, there is a continuous delicate living process involving countless generations of cell division. The better we care for our bodies with good living practices, the greater the likelihood of preventing something, possibly uncontrollable, from seriously going wrong.

These good living practices include having good nutrition, healthy exercise, safe sex and avoiding exposure to potentially damaging agents in our environment. All of these practices serve to reduce the exposure of the genetic material in cells to agents that could cause changes in the genetic blueprint.

Whilst most normal body tissues are composed of cells that have the ability to grow or reproduce, they normally only do so when there is a need. When this need has been satisfied, they stop reproducing. In the normal cell, there is a braking mechanism to stop cell division when the need for more cells has been satisfied. Cells in such tissues as the skin or blood or the lining of the mouth, throat or alimentary tract wear out quickly and are constantly being replaced. They are normally replaced only to meet the immediate need of the body, after which reproduction stops. Also, after injury or cell death, surrounding cells reproduce to replace and repair damaged tissues, but there is an inbuilt mechanism that stops the cell reproduction once the injury has been repaired and the wound has healed. The “switch-on” and “switch-off” mechanisms are governed by two different types of genes, whose functions are either to promote or to suppress cell division and cell growth. These are called proto-oncogenes and tumour suppressors. Proto-oncogenes respond to growth signals and are positive regulators of cell proliferation, only in the presence of appropriate growth signals. Tumour suppressor genes conversely act as negative regulators of cell growth and suppress or check the unregulated growth of cells. So in the normal cell, the “switch-off” mechanism is the response to the absence of specific growth signals.

Some, but not all, body tissues retain a lifelong ability to replicate themselves to meet the body needs. For example, after surgical removal of as much as three quarters of a normal liver, the remaining liver will grow back to its original size within about 6 weeks and then stop. The nature of the “switch-off” mechanism is not fully understood – but it is clearly a critically important process that is normally under genetic control.

In the case of a malignancy, there is no “switching-off” mechanism. Some of the proto-oncogenes have now acquired mutations that mean they promote cell growth even in the absence of appropriate growth signals, i.e. they become oncogenes (cancer-promoting genes), and some of the tumour suppressor genes are inactivated, such that abnormal growth goes unchecked. Abnormal and unwanted cells then invade into surrounding tissues and possibly blood and lymph vessels, or body cavities, thereby spreading to other parts of the body, where they establish new damaging colonies of unwanted growing cells. These colonies are called secondary or metastatic cancers, known as secondaries or metastases.