How Cancer Grows and Metastasises

Tim Jackson

This is my own simplified impression of how cancer works, and may not agree in detail with medical text books, but it provides a reasonable explanation of how cancer behaves.

When a cancer starts it is one cell or a small group of cells which becomes damaged so that it becomes 'immortal' and never stops dividing. Every month or two the cell divides into two, two become four four become eight and so on. For a long time, several years, this doesn't do anything much. It is a microscopic dot. After a while we get to a million or so cells and things start to take off. Not only are the cells still dividing relentlessly, but the chemistry around and inside the tumour is changing, partly because if the genetic damage to the cells and partly simply because of the growth. One of the effects of the chemistry is to destroy surrounding cells. It is the debris of this destruction which leaves tiny crystals which show up on mammogram x-rays showing an faint outline of the tumour.

If the tumour is growing inside a duct or some other confining tough tissue, then it will get quite large before it breaks out, and may be detected and removed in that stage. Once it has done that, sooner or later it will start to destroy other structures. The tumour now develops the eponymous crab-like shape as it grows along the paths of least resistance. Lymph vessels are one of the most common and vulnerable structures in the body. These are a network of very fine tubes like very fine veins which go all over your body. They act in the body's hydraulic system rather like the earth wire in an electrical system, they drain away all the fluid which leaks through the tissues from high pressure areas (eg arteries) and feed it back into the bloodstream through a valve near your neck. The lymph flow is therefore generally upwards, but the lymph system is a multiconnected network and flow can happen in different directions. There will be a general flow away from any area of inflammation.

Once the tumour has destroyed the wall of a lymph duct, cancer cells are in direct contact with the lymph fluid, and some cells get washed away into the fluid. From the breast the main lymph flow is up to the shoulder and armpit, where there is a cluster of lymph nodes, like filters of sewage works, who's job is to 'process' or destroy any bacteria or toxins in the returning fluid. Any cancer cells in the lymph are likely to get trapped here. They are still dividing, month by month, and so another tumour starts here. Most of the transported cells wind up here, and so the state of the lymph nodes is a good indicator of how much cancer has escaped from the original tumour. The lymph nodes can be removed surgically, complete with cancer, and thus form a second line of defence. Once they have gone there is no filter between the breast or arm and the bloodstream, so there is increased risk of infection from cuts and scratches.

The lymph nodes are not perfect filters, and not all the lymph flows uphill, so a small fraction of the cancer cells go elsewhere, most likely they get into the bloodstream. The cancer might also invade a blood vessel and shed cells directly into the blood. There are also other paths, eg to the other breast and to part of the liver. These will end up trapped somewhere, and the favourite places seem to be the bone marrow or the lungs or brain. It is these which produce true 'distant metastases which is stage IV and is generally incurable, not because the secondary tumour itself is inoperable but because when the first one is found, we know that there must be many others growing somewhere.