PetCancerVet

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ONCOLOGY

Oncology is the study of cancer and it's treatments. It is a very large subject because of the large number of different tumours which can occur. While much experience from human oncology is used by veterinary oncologists, there are several tumours of pets which have much in common with human disease and their study can give information useful to human oncologists.

What is a tumour

Normal body cells live and die to a pattern and these cells grow and divide in a controlled way by which organs, in adult animals, maintain a constant size. This pattern is controlled by genes which are influenced by interactions between neighbouring cells, and by a programmed ageing process within each cell.

Tumour cells have lost the natural pattern of ageing and dying, and no longer respond to the state of their neighbours in the control of their growth and division. When these changes are relatively minor a benign tumour, composed of relatively normal cells which keep together in a lump, will develop, pushing normal cells away but not pushing between them, or invading blood and lymphatic vessels. Malignant cells have not only lost the control on life span and division rates but, to a degree which varies between tumours, also grow between surrounding cells to spread amongst normal tissue and to invade blood and lymphatic vessels.

The local spread of some tumours can be very extensive, with some having escaped cells over 2cms from the edge of the visible lump: these cells are microscopic and therefore cannot be seen at surgery. Therefore it can be necessary to remove a large amount of normal tissue around a tumour to have a reasonable chance of removing all the malignant cells from the area. There are some tumours which (usually) only spread locally and adequate surgery can cure these: obviously the earlier and smaller a tumour is removed, the better chance that effective surgery can be carried out without major cosmetic and/or functional effects.

Tumours which invade blood and lymphatic vessels can escape the area of the primary tumour, and travel to the next narrow vessels (usually the lungs or liver if via blood or the local lymph gland if via lymph). Here they can escape and grow into new (secondary or metastatic) tumours. The cells that escape are single or in microscopic groups of no more than a few dozen cells (approximately 100,000 cells take up the volume of a pin-head) so that there is no way that new spread can be detected (see Diagnostic methods page). These cells will not be removed by surgery, and other methods of treatment are used to control them (see Treatment page).

What makes oncology such a large subject is that any organ is made up of a variety of types of cell: the skin, for instance, has cells making it's surface (squamous cells), cells to strengthen it (fibrocytes), body defence cells of a number of types (lymphocytes, mast cells, plasma cells, histiocytes), basal cells, gland cells (sweat gland, sebaceous gland) and pigment cells (melanocytes). Each of these cells can produce either a benign or a malignant tumour, so that there are at least 20 types of tumour which could arise in the skin - and they all behave differently in terms of speed of growth and type of spread. Add to that the fact that no two melanomas behave exactly the same (and the same applies to other tumours) and that other organs all have their own special cell types and the complexity of oncology becomes clear.

Fortunately there are some general rules which apply to groups of tumours, and there is a large body of information about the tumours which occur reasonably frequently. However, one general rule of oncology - check every lump early - is the best life saver of all.

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