Epidemiology
Peter Boyle, Michael J S Langman
In countries with a westernised lifestyle about half of
all deaths
are caused by circulatory disease and a quarter by
cancer.
Cancer is an important problem in both public health
and
political terms worldwide, irrespective of a country’s
development. The most recent estimates of the global
cancer
burden suggest that there were 8.1 million new cases,
excluding
non-melanoma skin cancer, worldwide in 1990. About
10 million new cases are now diagnosed each year.
Colorectal cancer is the fourth commonest form of
cancer
occurring worldwide, with an estimated 783 000 new
cases
diagnosed in 1990, the most recent year for which
international
estimates are available. It affects men and women
almost
equally, with about 401 000 new cases in men annually
and
381 000 in women. The number of new cases of colorectal
cancer worldwide has been increasing rapidly since 1975
(when
it was 500 000).
Worldwide, colorectal cancer represents 9.4% of all incident
cancer in men and 10.1% in women. Colorectal cancer,
however, is not equally common throughout the world. If
the
westernised countries (North America; those in
northern,
southern, and western Europe; Australasia; and New
Zealand)
are combined, colorectal cancer represents 12.6% of all
incident
cancer in westernised countries in men and 14.1% in
women.
Elsewhere colorectal cancer represents 7.7% and 7.9% of
all
incident cases in men and women respectively.
Large differences exist in survival, according to the
stage of
disease. It is estimated that 394 000 deaths from
colorectal
cancer still occur worldwide annually, and colorectal
cancer is
the second commonest cause of death from any cancer in
men
in the European Union. Substantial differences in
cancer
survival seem to exist between Great Britain, Europe as
a whole,
and the United States. This variation in survival is
not easily
explained but could be related to stage of disease at
presentation or treatment delivery, or both of these.
The numbers of new cases of colorectal
cancer worldwide has increased rapidly
since 1975
Age at diagnosis (years)
Rate per 100 000 population
0
200
300
400
500
600
Men
100
0-4
15-19
10-14
5-9
20-24
35-39
30-34
25-29
40-44
55-59
50-54
45-49
60-64
75-79
70-74
65-69
80-84
>85
Women
Estimated incidence of colorectal cancer in United
Kingdom, by
age and sex, 1995
England and
Wales
Colon cancer
Men Women
Rectal cancer
Men Women
0 10 20 30 40 50 60 70 80 90 100 0 10 20 30 40 50 60 70 80
90 100
Relative survival (%) Relative survival (%)
Scotland
Europe
United States
England and
Wales
Scotland
Europe
United States
International comparison of five year relative
survival for colon and rectal cancer in adults aged 15-99 at diagnosis (based
on Coleman et al,
Cancer survival trends in England and Wales,
1971-1975; Berrino et al, Survival of cancer patients in Europe: the
EUROCARE-2 study;
and Surveillance
Epidemiology and End Results (SEER) programme,
National Cancer Institute, 1998)
1
Survival and deprivation
The relation between poverty and ill health has been
researched
for more than 100 years. In Scotland, since the 1851
census, all
cause occupational mortality has been routinely
reported, and
since 1911, inequalities in health, as shown by
mortality, have
been examined in decennial reports classified by social
class
(based on occupation) and by occupational group alone.
No single, generally agreed definition of deprivation
exists.
Deprivation is a concept that overlaps but is not
synonymous
with poverty. Absolute poverty can be defined as the
absence of
the minimum resources for physical survival, whereas
relative
poverty relates to the standards of living in a
particular society.
Deprivation includes material, social, and multiple
deprivation.
In Scotland the Carstairs and Morris index of
deprivation was
derived from 1981 census data with the postcode sector
as the
basic geographical unit (covering a population of about
5000).
This index describes a deprivation category on a scale
of 1 (least
deprived) to 7 (most deprived) for each household
address in
Scotland.
The incidence of colorectal cancer is higher in men
than
women among each of the seven deprivation categories in
Scotland, although incidence varies little with
deprivation
category. Survival, however, clearly improves with
decreasing
deprivation. At each milestone, there is a notable
gradient in
survival, with the most affluent doing best and the
least affluent
doing worst. The reasons that such variations exist are
unclear
and highlight an important priority for research.
Descriptive epidemiology
Different populations worldwide experience different
levels of
colorectal cancer, and these levels change with time.
Populations living in one community whose lifestyles
differ
from those of others in the same community also
experience
different levels of colorectal cancer. Groups of
migrants quickly
lose the risk associated with their original home
community
and acquire the patterns of the new community, often
starting
within one generation of arrival.
Ethnic and racial differences in colorectal cancer, as
well as
studies on migrants, suggest that environmental factors
play a
major part in the aetiology of the disease. In Israel
male Jews
born in Europe or the United States are at higher risk
of colon
cancer than those born in Africa or Asia. Risk in the
offspring
of Japanese populations who have migrated to the United
States has changed—incidence now approaches or surpasses
that in white people in the same population and is
three or four
times higher than among the Japanese in Japan.
For reasons such as these, colorectal cancer is widely
believed to be an environmental disease, with “environmental”
defined broadly to include a wide range of ill defined
cultural,
social, and lifestyle practices. As much as 70-80% of
colorectal
cancers may owe their appearance to such factors; this
clearly
identifies colorectal cancer as one of the major
neoplasms in
which causes may be rapidly identified, and a large
portion of
the disease is theoretically avoidable.
The move from theoretically avoidable causes to
implementation of preventive strategies depends on the
identification of risk factors, exposures that have
been
associated with an increased (or decreased) risk of
colorectal
cancer, and the smaller subset of risk determinants,
whose
alteration would lead directly to a reduction in risk.
From
analytical epidemiology some clear ideas have now
emerged
about measures for reducing the risk of colorectal
cancer.
Highest incidence of colorectal cancer in men
worldwide around 1990
Registry Age standardised
incidence per 100 000
US (Hawaii: Japanese), 1988-92 53.48
New Zealand (non-Maori), 1988-92 51.30
Japan (Hiroshima), 1986-90 50.99
France (Haut-Rhin), 1988-92 49.90
Italy (Trieste), 1989-92 49.37
France (Bas-Rhin), 1988-92 49.24
Canada (Yukon), 1983-92 48.98
US (Detroit: black), 1988-92 48.32
Czech Republic, 1988-92 48.23
US (Los Angeles: black), 1988-92 47.89
Canada (Nova Scotia), 1988-92 47.84
Canada (Newfoundland), 1988-92 47.29
Australia (New South Wales), 1988-92 46.92
US (San Francisco: black), 1988-92 46.82
Israel (Jews born in America or Europe),
1988-92
46.79
Data taken from Parkin et al, eds (Cancer incidence in five continents. Vol 7. IARC
Scientific Publications, 1997:120)
Deprivation category
Incidence
1 2 3 4 5 6 7
0
20
30
40
50
60
70
10
Men Women
Incidence according to deprivation category in
Scotland, 1998
(1=least deprived, 7=most deprived)
Survival
Incidence
1 year 2 years 3 years 4 years 5 years
0
20
30
40
50
60
70
1
10
2 3 4 5
Survival according to deprivation category in
Scotland, 1998
(1=least deprived, 5=most deprived)
ABC of Colorectal Cancer
2
Dietary and nutritional practices
Evidence from epidemiological studies seems to show
consistently that intake of dietary fat and meat is
positively
related to risk of colorectal cancer. This evidence is
obtained
from ecological studies, animal experiments, and
case-control
and cohort studies.
In 1990 Willett et al published the results from the US
nurses health study involving follow up of 88 751 women
aged
34-59 years who were without cancer or inflammatory
bowel
disease at recruitment. After adjustment for total
energy intake,
consumption of animal fat was found to be associated
with
increased risk of colon cancer. The trend in risk was
highly
significant (P = 0.01), with the relative risk in the
highest
compared with the lowest quintile being 1.89 (95%
confidence
interval 1.13 to 3.15). No association was found with
vegetable
fat. The relative risk in women who ate beef, pork, or
lamb as a
main dish every day was 2.49 (1.24 to 5.03) compared
with
women reporting consumption less than once a month. The
authors suggested that their data supported the
hypothesis that
a high intake of animal fat increases the risk of colon
cancer,
and they supported existing recommendations to
substitute fish
and chicken for meats high in fat.
Intake of vegetables, fruit, and fibre
Dietary fibre has been proposed as accounting for the
differences in the rates of colorectal cancer between
Africa and
westernised countries—on the basis that increased intake of
dietary fibre may increase faecal bulk and reduce
transit time.
Various other factors, related to risk of colorectal
cancer, are
now thought to contribute to explaining these
differences.
Fibre has many components, each of which has specific
physiological functions. The components are most
commonly
grouped into insoluble, non-degradable constituents
(mainly
found in cereal fibre) and soluble, degradable
constituents, such
as pectin and plant gums (mainly found in fruits and
vegetables). Epidemiological studies have reported
differences
in the effect of these components. Many studies,
however, found
no protective effect of fibre in cereals but have consistently
found a protective effect of fibre in vegetables and
perhaps
fruits. This might reflect an association with other
components
of fruits and vegetables, with fibre intake acting
merely as an
indicator of consumption.
Physical activity, body mass index, and energy intake
Evidence from epidemiological studies is strong that
men with
high occupational or recreational physical activity
seem to have
a decreased risk of colon cancer. Such evidence comes
from
follow up studies of cohorts who are physically active
or who
have physically demanding jobs, as well as from
case-control
studies that have assessed physical activity by, for
example,
measurement of resting heart rate or questionnaire. The
association remains even after potential confounding
factors,
such as diet and body mass index, are controlled for.
The available data, however, show no consistent
association
between obesity and risk of colorectal cancer (analysis
and
interpretation of this factor is difficult in
retrospective studies,
where weight loss may be a sign of the disease),
although
evidence now suggests an association between obesity
and
adenomas. This increased risk associated with energy
intake
does not seem to be the result merely of overeating; it
may
reflect differences in metabolic efficiency. If the
possibility that
the association with energy intake is a methodological
artefact
is excluded (as such a consistent finding is unlikely
to emerge
from such a variety of study designs in diverse
population
groups), it would imply that individuals who use energy
more
efficiently may be at a lower risk of colorectal
cancer.
Physical activity and colorectal cancer
x
Giovannucci et al examined the
role of physical activity, body mass
index, and the pattern of adipose distribution in the
risk of
colorectal adenomas
x
In the nurses health study, 13
057 female nurses, aged 40-65 years
in 1986, had an endoscopy during 1986-92. During this
period,
adenoma of the distal colorectum was newly diagnosed in
439
nurses
x
After age, prior endoscopy,
parental history of colorectal cancer,
smoking, aspirin use, and dietary intake were
controlled for,
physical activity was associated inversely with the
risk of large
adenomas (>1 cm) in the distal colon (relative risk 0.57 (95%
confidence interval 0.30 to 1.08)) when high and low
quintiles of
average weekly energy expenditure from leisure
activities were
compared
x
Much of this benefit came from
activities of moderate intensity, such
as brisk walking
Intake of dietary fat and meat may increase risk of
colorectal
cancer
Fruits are a good source of fibre and may protect
against cancer
Epidemiology
Hormone replacement therapy
Increasing evidence supports an (originally unexpected)
association between hormone replacement therapy and a
reduced risk of colorectal cancer.
Of 19 published studies of hormonal replacement therapy
and risk of colorectal cancer, 10 support an inverse
association
and a further five show a significant reduction in
risk. The risk
seems lowest among long term users. Although some
contradictions still exist in the available literature,
hormone
replacement therapy seems likely to reduce the risk of
colorectal cancer in women. The risk seems to halve
with 5-10
years’ use. The role of unopposed versus combination hormone
replacement therapy needs further research.
Whether this association is causal or is associated
with some
selection factor that directs women to using hormone
replacement therapy is, however, not known. This
question is
important; if the link is indeed causal, women who are
at high
risk of colorectal cancer could be offered the therapy
to lower
their risk.
Control of colorectal cancer
Prospects for preventing death from colorectal cancer
are now
more promising than even 10 years ago. To achieve this
goal
public health decisions have to be taken, and part of
this
decision process involves deciding at which point
enough
epidemiological evidence is available to change focus
comfortably from information generation to health
actions.
To turn research findings into public health strategies
for
controlling the incidence of and mortality from
colorectal
cancer requires a profound change of mentality in the
epidemiological community. It is easy to say that more
studies
are needed, but they would be unlikely to alter
existing
conclusions. Moreover, the implementation of strategies
to
control cancer must be considered separately from
research
into the control of cancer.
One consequence of epidemiological research into the
contribution of lifestyle factors to cancer risk has
been to blame
the individual who develops cancer. Smoking, alcohol,
dietary
imprudence, and exposure to sunlight tend to assign
responsibility to the individual. The individual is
often not
principally responsible for decisions about factors
that influence
his or her risk of cancer, and society—including
government
and industry—could do more to discourage lifestyles associated
with cancer risk. Government legislation, including
taxation
policy and other actions, could have profound effects
on
smoking habits, for example.
The goal of all cancer research and treatment is to
prevent
people dying from the disease. Knowledge has been
accruing
rapidly about actions and interventions that could lead
to a
reduction in death from colorectal cancer by reducing
the risk
of developing the disease, identifying the disease at a
stage
when it is more curable, or improving the outcome of
treatment.
How individuals can reduce their risk of colorectal
cancer
x
Increase intake of vegetables and
fruits (eat five servings of fruits
and vegetables each day); replace snacks such as
chocolate, biscuits,
and crisps with an apple, orange, or other fruit or
vegetable
x
Reduce intake of calories (animal
fats in particular); often replace
beef, lamb, and pork with fish and poultry
x
Increase physical activity—by
activities of moderate intensity, such as
brisk walking
x
Participate in population screening
programmes; when these are
not in place, strongly consider having a colonoscopy
with polyp
removal once between ages 50 and 59
x
Consult a doctor as soon as
possible if a noticeable and
unexplained change in bowel habits occurs, blood is
present in the
stool, colicky pain occurs in the abdomen, or a
sensation of
incomplete evacuation after defecation recurs
