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Family
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Preview of Understanding Cancer
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Cancer is not a single
illness: there are very many different types. Some cancers may stay almost
unchanged for several years and have no impact on life expectancy. At
the other end of the spectrum there are rare cancers which may prove
fatal quite shortly after they have been discovered. In the same way
that the term ‘infection’
embraces illnesses as far apart as the common cold, a boil, malaria and
tuberculosis, the spectrum of malignant disease is almost equally varied,
in both behaviour and seriousness, although, of course, cancer is not contagious.
Loss of control
A lump of human tissue the size of a sugar cube may
contain a thousand million cells. These are the minute building blocks
from which our bodies are made, visible only down the microscope. It
is quite amazing that the billions of cells in a human body normally
function in perfect harmony, every cell knowing its place and doing the
job that it was designed to do. Most cells have a finite lifespan: millions
of new ones are produced every day to replace those lost through old
age or wear and tear.
New cells are produced when existing cells divide into
two, a process known as ‘mitosis’. Except in children, who
are growing, there is normally a perfect balance between the numbers
of the cells that are dying and those that are dividing. Normally exactly
the right amounts of new cells are produced to replace those that are
being lost. The control mechanisms involved are exceedingly complex.
Loss of control can lead to an excess of cells, resulting in a tumour.
However, it is important to realise that only a small
minority of tumours are cancerous. Most tumours are localised accumulations
of normal or fairly normal cells and are benign. A wart is a common example.
The development of a cancer involves a change in the
quality of the cells as well as an increase in quantity: they change
in both appearance and behaviour. They become more aggressive, destruc-tive
and independent of normal cells. They acquire the ability to infiltrate
and invade the surrounding tissues. In some instances the cells may also
invade lymphatic and blood vessels and thus spread away from the ‘primary’
growth to other places. In time these cells may cause the development
of secondary growths, known as ‘meta-stases’, in the lymph
glands and other organs such as lungs, liver and bones.
Position of
the main organs in a man. |
Position of
the main organs in a woman. |
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Genes
The behaviour of all cells is controlled by genes in
their central control unit, the nucleus. Each cell nucleus contains approximately
100,000 genes. The genes are minute, highly concentrated packets of information
and instructions stored in coded form in a complex chemical molecule
known as ‘DNA’. Large numbers of genes are grouped together
in strands looking rather like short pieces of string, which are just
visible under the microscope. These are the chromosomes, which are joined
to each other in pairs, 23 pairs in total.
A human being develops in the
uterus from a single cell. This first cell is formed by the fertilisation
of an ovum (egg) produced in one of the mother’s ovaries by a sperm
produced in one of the father’s testes. It divides to produce two ‘daughter’ cells
and then they divide, resulting in four cells. Successive divisions lead
to rapid growth. Mitosis involves replication of all the genetic information
so that all the cells in the developing microscopic organism or ‘embryo’
each have their own full comple-ment. This process continues as the embryo
develops into a ‘fetus’ and then eventually into a newborn
baby.
The genetic information con-tained within the first cell is what
determines the physical character-istics of the whole human being that
ultimately develops from it. However, once the body is fully formed,
most of this genetic information is no longer required by any particular
individual cell. All it needs is the information required to enable it
to perform its own designated role. Instructions on how to perform other
roles are redundant. The important pieces of information that are ‘switched
on’ in particular cells govern the characteristics and behaviour
of those cells and the properties of the particular tissues that they
constitute.
The behaviour
of all cells is controlled by genes in their central control
unit, the nucleus. |
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Cancer genes
There are particular genes known as ‘oncogenes’
which are present in normal cells, where they may either be dormant or
play a part in controlling cell behaviour and division. DNA damage
caused, for example, by tobacco smoke, ultra-violet light or certain
viruses can trigger abnormalities or ‘mutations’ in these
genes, resulting in increased and abnormal activity of the gene. This
can cause the cell to behave in an antisocial way and to become malignant
(cancerous).
In addition to oncogenes, each cell contains ‘tumour
suppressor genes’ whose normal job is to restrain it from dividing.
Many cancers are caused by damage which reduces the activity of a tumour
suppressor gene.
Genes are crucial not only to the development of malignancy,
but also to the subsequent behaviour of a cancer and its response to
treatment. For example, some genes are responsible for the manufacture
of proteins important for a cancer’s ability to invade adjacent
tissues and to spread to distant parts of the body, triggering the development
of metastases. Other genes can cause the cell to produce self-stimulating ‘growth
factors’ or to eliminate anti-cancer drugs. Even the process of
cell death is under genetic control. Genetic damage may result in cells
failing to die, which can be an important factor both in the development
of a cancer and in its resistance to treatment with radiotherapy or drugs.
The development of a cancer involves an accumulation of successive genetic
abnormalities over some years both before and after the cell starts behaving
in a malignant fashion. Further gene mutations after the cancer has started
can result in some of the cancerous cells behaving differently from others.
This may cause the growth to ‘change its spots’ at some stage.
The behaviour of the cancer and the long-term result of treatment depend
ultimately on those cells that have the most anti-social characteristics
and on those that are best able to resist treatment aimed at destroying
them.
New cells are
produced when existing cells divide into two, a process known
as mitosis. |
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Speed of growth
Most cells divide at the most every few days, some
much more slowly. Given that almost all cancers start off as a result
of a genetic abnor-mality in a single cell, and that there may be a thousand
million cells in a sugar cube-sized lump, it follows that most cancers
begin a very long time before they become apparent. Most cancers are
rather larger than the size of a sugar lump when they are discovered
and many will by then have been present, growing slowly, for 10 to 20
years. There is, however, a large variation in the time a tumour takes
to double in size. This ‘doubling time’ may vary from a few
days to many years, although for most of the more common cancers the
average is about two to three months.
Effects of cancer
It can sometimes be difficult to understand just how
an excessive number of abnormal cells can in certain circumstances be
a threat to life. The serious effects of malignant disease occur as a
result of progressive infiltration and destruc-tion of the surrounding
normal tissues and/or other parts of the body to which the cancer has
spread, such as the liver, bones or lungs, disrupting their normal function.
It is unusual for localised cancers to be fatal. Most of those who die
from cancer have widespread or metastatic disease. However, in addition
to these ‘physical’ processes, cancers can cause progressive
debilitation by producing a wide range of toxic chemicals acting both
locally and, via the bloodstream, throughout the body. We do not understand
this process fully, but it is these chemicals that can be responsible
for symptoms such as loss of weight in some patients.
Classification
Cancers are graded according to the extent to which
the cells are different from normal. In well-differentiated (sometimes
called ‘grade 1’) cancers some of the normal cellular architecture
is maintained and the cells do not seem to be dividing frequently. Some
may still be retaining some ability to perform their original specific
tasks. At the other end of the spectrum are poorly dif-ferentiated (‘grade
3’) cancers in which the cells have changed so much that they are
now very different from normal cells and have completely lost their ability
to perform their appointed tasks. These cancers tend to be faster growing
and more aggressive, and to carry a less favourable prognosis. Moderately
differentiated cancers are in between.
Cancers are classified according to the type of normal
cell from which they originated, not according to the tissues into which
they may have spread. This is what might be called the primary classification.
Cancers in each category will be graded as described above, and their
growth and extent of spread will also be assessed in a process known
as
‘staging’. As far as primary classification is concerned, almost
all cancers can be placed in one of the following groups.
Carcinomas
These are by far the most common types of cancer. They
originate from cells lining body surfaces, including the skin and a wide
variety of internal linings. Among these are those of the mouth, throat,
bronchi (the tubes carrying air in and out of the lungs), oesophagus
(the swallowing tube or gullet), stomach, bowel, bladder, uterus (womb)
and ovaries, and the linings of ducts in the breasts, prostate gland
and pancreas.
There are different types of carcinomas named according
to the appearance of the normal cells from which they arose. ‘Squamous
carcinomas’ arise particularly in the skin, mouth, throat, oesophagus
and lung; ‘adenocarcinomas’ arise particularly in the lower
oesoph-agus, stomach, bowel, breast and ovary; ‘transitional cell
carcinomas’ arise chiefly in the bladder and ‘small cell carcinomas’
also occur in the lung.
Sarcomas
These arise from supportive rather than surface lining
tissues, such as those of bone, fat, muscle and the strengthening ‘fibrous
tissue’ found in most parts of the body.
Lymphomas
These originate from cells known as ‘lymphocytes’
which are found throughout the body, particularly in the lymph glands
and blood. They are a very important component of the body’s immune
system. Lymphomas are divided into ‘Hodgkin’s disease’
and the ‘non-Hodgkin’s lymphomas’, according to the cell
type affected.
Leukaemias
These arise from the cells in the bone marrow which
make the white blood cells. The white blood cells are crucial to the
body’s defence system against infection. In leukaemia there is a
greatly increased concentration of abnormal white cells in the blood,
which causes problems both because the abnormal cells often don’t
function properly and because they restrict the space within the bone
marrow for new normal blood cells to be made.
Myeloma
This is a malignancy of the ‘plasma cells’
in the bone marrow which produce antibodies – the proteins that
help to fight infections.
Germ cell tumours
These develop from those cells in the testes and ovaries
responsible for the production of ova and sperm. They include ‘teratomas’
and ‘seminomas’.
Melanoma
This type of skin cancer arises from the skin pigment-producing
cells or ‘melanocytes’.
Gliomas
These develop from cells of the supportive tissue of
the brain or spinal cord.
Pre-cancers
Finally, it is important to mention the fairly common
potentially pre-cancerous conditions, which are diagnosed mainly in apparently
healthy people who undergo ‘screening’ tests such as cervical
smears and mammograms (breast X-rays). These conditions particularly
affect the surfaces of the cervix (neck of the womb) and breast ducts
and are referred to as ‘carcinoma in situ’. This means that
the cells on the very surface have a malignant appearance when seen though
a microscope, but show no sign of having begun to behave malignantly
by invading any of the tissue immediately beneath the surface lining.
Carcinoma in situ has no ability to spread via the
lymphatic or blood systems and in itself poses absolutely no threat to
life. It does, however, carry a risk of eventually becoming truly cancerous
if left untreated.
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KEY POINTS
- Cancers start off as a consequence of gene damage
in a single cell and usually take many years to become apparent
- Cancers vary enormously in appearance, behaviour
and prognosis (outlook)
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