A tumour develops when a group of cell escape from the normal orderly process of cell division and begin to multiply in an uncontrolled way. After a time enough of these abnormal cells will been produced to form a lump, which is called a growth or a tumour.

Tumours may be either benign or malignant. The two important differences between benign and malignant tumours are invasion and spread.

As they grow benign tumours simply push the surrounding normal tissues and organs out of their way. Sometimes pressure from a benign tumour may damage surrounding structures but the benign tumour never actually invades into those structures. By contrast malignant tumours eat into and destroy the normal tissue around them as they increase in size. This means that in some parts of the body benign tumours can grow quite large without causing any problems whereas a malignant tumour damages the tissue around it from the time it first begins to grow.

Benign tumours do not spread. They may grow to a large size but they do not go to other parts of the body. Malignant tumours have the ability to spread by sending off seedlings of tumour which can pass through the blood or lymphatic system to other parts of the body. These seedlings then settle in other organs and form what are called secondary tumours or metastases.

Whilst all malignant tumours have the ability to spread, different tumours vary in the speed with which they do so. Some aggressive malignant tumours begin to spread when they are very very small whilst other slow growing malignant tumours only spread very late in their lives. This means that many malignant tumours are discovered and treated before they have actually had the chance to form secondary tumours.

The word cancer only applies to malignant tumours. There is no such thing as a benign cancer. So, by definition, all cancers are malignant tumours.

The difference between benign and malignant tumours has nothing to do with frequency - in some organs of the body benign tumours are much more common than cancers, nor does it have anything to do with size, some benign tumours can be very large whilst many cancers are quite small.

There are many hundred different types of benign tumour that may develop in our bodies and also several hundred different types of malignant tumour that can occur. These different types of benign and malignant tumours behave in different ways but the fundamental rule that benign tumours never invade or spread always applies