What is a cancer?

First we need play the definition game a little.

Neoplasias refer to the abnormal proliferation of cells in a tissue that tend to be somewhat autonomous (not under the body's control; we'll touch on this later). Neoplasms can be benign or malignant.

Benign neoplasms are (1) unable to invade surrounding tissues, and (2) unable to metastasise to distant tissues. An example of a benign tumour is uterine fibroids (leiomyomas), which most women have.

By contrast, malignant neoplasms usually break both of these boundaries. A cancer is a malignant neoplasm.

In is important to know, however, that these two categories (benign and malignant) are simply useful abstractions. There are, for instance, malignant tumours that can't metastasise (e.g. basal cell carcinoma), and benign tumours that keep growing until they kill you (e.g. some brain tumours). However, the distinction is still useful in theory to aid understanding.

All cancer is caused by alterations to the DNA of one of the body's cells. Incidentally, it is thought that it is usually ony one of the body's cells that goes bad to cause a cancer. When looking at a cancer, you are looking at the millions of descendants of that original cell, plus any other types of cell that have been recruited by it (e.g. cells to make blood vessels, or supporting fibrous tissue).

Typically, cancers display four attributes:

• They tend to be poorly differentiated (i.e. anaplastic). This means that they tend not to resemble the mature normal cells of the tissue of origin; rather they have "primitive-appearing, unspecialised cells." This is often accompanied by a number of additional changes visible to the light microscope, such as pleomorphism (variation in the size and shape of the nuclei), and hyperchromaticism (containing an abundance of DNA, so that they have large nuclei and stain darkly for the light microscope). Also potentially visible are numerous mitotic figures (reflecting the high metabolic rate of the cancer cells).

• They tend to show rapid growth. As a general rule, neoplasms grow at a rate proportional to their differentiation. Thus, a poorly differentiated cancer would, all things being equal, tend to proliferate very rapidly. This growth is out of keeping with the body's signals to the tissue - the body is not telling the cells to grow this fast. Rather, the cells achieve a degree of autonomy, dividing even when there is no signal to do so, unlike normal cells. There are many exceptions to the growth rule, though. For example, cancers of the prostate grow much slower than many benign neoplasms.

• As benign tumours grow, they tend to simply expand within the surrounding tissue, often compressing it into a fibrous capsule. Because they grow as one coherent mass, they usually show a well-demarcated edge, where the normal tissue is clearly distinguishable from the neoplastic one. By contrast, malignant tumours display local invasion, infiltration and destruction of the surrounding tissue. They don't simply expand; they invade in a multi-fronted and irregular manner. As a result, they are poorly differentiated.

• Cancers metastasise, benign tumours never do. Metastases are off-shoots of the original tumour that now implant in a distant area. It is the invasiveness (above) of cancer cells that allows for this: cancerous cells invade into blood vessels, lymphatics or body cavities, and are thus carried to remote areas. If they implant here, they'll form a local proliferation, or metastasis. In general, cancers that have metastasised are incurable. This is because to find a metastasis, it must already have grown to a substantial size, and there will doubtless be thousands (or more) of other smaller metastases that have simply not grown enough to be visible yet. It would be impossible in most cases to remove them all.

I need to emphasise again that there are many exceptions to any of the above rules. However, the points do still stand as characteristic of the vast majority of cancers.

In the next post, I'll discuss in more detail how cancers form, and then I'll tackle the question of how cancers kill you in the post after that.