A cigarette with a tumour? If only cancer was that simple …

In the government’s latest anti-smoking campaign (shot in those greyish tones peculiar to health promotion), something unexpected happens to a cigarette. As the smoker inhales, a pink mass starts to push through the side of his cigarette, protruding through the paper like a malformed mushroom. “Every 15 cigarettes you smoke will cause a mutation … and mutations are how cancer starts,” the voiceover explains. Damaged by the smoke passing through it, the cigarette itself has developed a tumour. It has cancer.

But what is a cancerous tumour? After all, a wart is also a kind of tumour, as is a cyst, as is a fibroid. A tumour is just a collection of cells growing in the wrong place. What makes it malignant rather than benign?

Malignant growth has several features. The body’s cells divide all the time, repairing damaged areas, replacing worn-out parts. But this growth follows rules; it’s an orderly process. Cancer occurs when the cells stop responding to the normal controls. Cancerous growth is usually disorganised. You can see this when you examine malignant tumours; they tend to share characteristic features regardless of where they come from. A cancerous tumour rarely forms a smooth, neat ball: instead it tends to be an untidy mass which pincers into its surroundings like the crab from which cancer takes its name. Invasiveness is one of the hallmarks of cancerous growth.

Cancerous growth also tends to be rapid: cancerous tumours grow more quickly than their benign counterparts. (A wart on your finger, for example, may remain for years, never growing or invading, but simply sitting there, unlovely but harmless. A melanoma on your finger has different goals. Having started out as a harmless mole, it gets bigger, changes shape and loses its uniformity, before starting to invade and spread – all signs of its malignant status.)

Another characteristic of cancer cells is their tendency to lose the distinctive features of their parent cells; that is, they become “undifferentiated”. A normal cell from the lining of the lung is a highly specialised thing, with the capacity to carry out useful lung jobs such as producing mucus. If the DNA of this normal cell is damaged – for example, by cigarette smoke – it may start to lose this functionality. The offspring of a damaged cell will be equally hopeless, incapable of much other than chaotic, useless growth.

Every cancer belongs to its parent cells – it is mutations in these cells that result in cancers. Lung cancers start in the cells of the lungs, kidney cancer in the cells of the kidney, and so on.

(The chemicals in cigarette smoke are damaging to many different cell types, which is why smoking is associated with more than a dozen different cancers including bowel, bladder, kidney, pancreatic and mouth cancer, as well as lung cancer.) But unlike most normal cells, cancerous cells can spread through the body, breaking off from their parent tissues and sailing round in the bloodstream or the lymphatic system until they find a new place to moor.

This is metastasis. It is the final hallmark of all the diseases we call “cancer” and the reason that cancer is so difficult to treat. If cancer only ever caused a single lump in a single location it would be vastly more curable. If it only buckled the side of your cigarette, or formed a tumour in part of your lung, it wouldn’t cause so much harm. But all malignant tumours have the potential to spread. The best way to prevent this is to protect your DNA from mutation if you possibly can.

It really is better not to smoke.

Sophie Harrison is a hospital doctor in South Yorkshire

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