CARCINOMA IN SITU
toxin
is
present
in vehicle
exhaust
fumes and is produced by inefficient
burning of coal, gas, or oil.
POISONING
Carbon monoxide is toxic because it
binds with
haemoglobin
(the oxygen-
carrying molecule in red blood cells),
w hich prevents the blood from carry-
ing oxygen to the body tissues. As a
result, the tissues are deprived of oxy-
gen,
and
asphyxiation
occurs.
The
initial symptoms of acute high-level
carbon
monoxide
poisoning
include
dizziness, headache, nausea, and faint-
ness. Continued inhalation of the gas
may lead to loss of consciousness, per-
manent brain damage, and even death.
Low-level exposure to carbon m onox-
ide over a period of time may cause
fatigue, nausea, diarrhoea, abdominal
pain, and general malaise.
carbon tetrachloride (CQ
4
)
A colourless, poisonous, volatile chem-
ical w ith a characteristic odour. Carbon
tetrachloride consists
of one carbon
atom linked to four chlorine atoms and
has the chemical formula CCl4. Former-
ly used in domestic dry-cleaning fluids,
its use is now restricted to industry.
Carbon tetrachloride is an extremely
dangerous chemical, and it can cause
dizziness, confusion, and liver and kid-
ney damage if a significant amount of
the chemical is inhaled or swallowed.
(See also
household poisons
. )
carbuncle
A cluster of interconnected
boils
(pain-
ful,
pus-filled,
inflamed hair roots).
Carbuncles are usually caused by infec-
tion with the bacterium S
taphteococcus
aureus
. The back of the neck and the
buttocks are the most common sites.
The swellings mainly affect people with
reduced immunity, particularly those
with
diabetes mellitus
.
Treatment is usually with an
antibio-
tic drug
.
In addition, the application of
hot
compresses
may encourage the pus-
filled
heads
of the
boils
to
burst,
w hich relieves pain. Occasionally, in ci-
sion
and
drainage
(together
with
removal of the core of the carbuncle)
may be necessary if a carbuncle is per-
sistent, and drainage and healing have
not occurred spontaneously.
carcinogen
Any agent that is capable of causing
cancer
,
such as tobacco smoke, high-
energy
radiation
,
or asbestos fibres.
CHEMICAL CARCINOGENS
Chemicals form the largest group of
carcinogens. Major types include poly-
cyclic aromatic hydrocarbons (PAHs),
w hich occur in tobacco smoke, pitch,
tar fumes, and soot. Exposure to PAHs
may lead to cancer of the respiratory
system or skin. In addition, certain aro-
matic amines used in the chemical and
rubber industries may cause bladder
cancer after prolonged exposure.
PHYSICAL CARCINOGENS
The best-known physical carcinogen is
high-energy radiation, such as nuclear
radiation and
X-rays
.
Radiation may also
come from
ultraviolet light
,
for example
in sunlight. Another known physical
carcinogen is asbestos
(see
asbestos-
related diseases
) .
Exposure to radiation may cause can-
cerous changes in cells, especially in
cells that divide quickly; for example
changes in the
precursors
of white
blood cells in the bone marrow causes
leukaemia
.
The level of risk depends on
the dosage and duration of exposure
to the carcinogen. Over many years,
exposure to ultraviolet radiation in sun-
light can cause skin cancer.
BIOLOGICAL CARCINOGENS
Only a few biological agents are known
to cause cancer in humans. S
chistosoma
haematobium
, one of the blood flukes
responsible
for
the
tropical
disease
schistosomiasis
,
can cause cancer of the
bladder; and A
spergieeus feavus
, a fungus
that produces the poison
aflatoxin
in
stored peanuts and grain, is believed to
cause liver cancer.
Viruses that are associated with can-
cer include some strains of the human
papilloma virus, w hich are linked to
cancer of the cervix; the hepatitis B and
C viruses, w hich are linked to liver can-
cer; and some types of herpes virus,
w hich are associated with
Kaposi's sar-
coma
and
Burkitt’s lymphoma.
AVOIDANCE OF CARCINOGENS
In industry, known carcinogens may be
banned. Alternatively, as in the nuclear
industry and in hospital X-ray depart-
ments, they may be allowed only if
their
use
is
considered
essential,
if
exposure to them is strictly limited,
and if regular medical screening is pro-
vided for workers using them.
Outside industry, people are exposed
to very few known, unavoidable, high-
risk carcinogens. Any substance that
could possibly be carcinogenic, such as
a food additive, a cosmetic, or a chem i-
cal for use in drugs, must be carefully
screened by an official body (such as
the Medicines Control Agency, w hich
assesses drugs in the UK) before it is
allowed to be manufactured.
carcinogenesis
The development of a
cancer
caused by
the action of
carcinogens
(factors that
cause cancer) on normal cells.
Carcinogens are believed to alter the
DNA
in cells, particularly in
oncogenes
(genes that control the
growth and
division of cells). An altered cell divides
abnormally fast, passing on the genetic
changes to all of its offspring cells.
Thus, a group of cells is established that
is not affected by the body’s normal
restraints on growth.
carcinoid syndrome
A rare condition caused by an intestinal
or lung tumour,
called a carcinoid,
w hich secretes excessive amounts of
the hormone
serotonin
and often also
other
chemicals,
such
as
bradykinin
.
Carcinoid syndrome is characterized by
bouts of facial flushing, diarrhoea, and
wheezing, but symptoms usually occur
only if the tumour has spread to the
liver or has arisen in a lung.
Carcinoid tumours in the intestine,
lung, and, more rarely, the liver are
sometimes removed surgically, but, in
most cases, surgery is unlikely to be of
benefit. In these circumstances, symp-
toms may be relieved by drugs such as
octreotide
,
w hich
often
inhibits
the
growth of the tumour.
carcinoid tumour
A type of hormone-secreting cancerous
tumour. Carcinoids most often occur in
the small intestine or rectum but occa-
sionally also develop in the lungs. (See
also
carcinoid syndrome
. )
carcinoma
Any cancerous tumour (see
cancer
) that
arises from cells in the covering surface
layer or lining membrane of an organ.
A carcinoma is distinguished from a
sarcoma
,
w hich is a cancer arising in
bone, muscle, or connective tissue. The
most common cancers of the lungs,
breast, stomach, skin, cervix,
colon,
and rectum are carcinomas.
carcinoma in situ
The earliest, usually curable, stage of a
cancer. In this stage, the disease has not
yet spread from the surface layer of
cells in an organ or other tissue.
C
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