Jack: Hi everyone.
In today's lecture, we'll be discussing cancer risk factors.
To begin with, we have three goals of today's lecture
to identify risk factors contributing to various kinds
of cancers to describe the unique role of tobacco use
in cancer ideology, and to summarize the importance
of including questions about exposures to various
risk factors in epidemiological studies.
So to begin with, cancer is caused by what we call
a web of causation.
So that just means that it's due to the cumulative
effects of multiple causal factors
that are going to eventually cause cancer.
It's difficult to establish a definitive proof
of causality but an international panel
of scientists created what's called the criteria
of judgment, which is a tool or a series of questions
that can be used to establish the causality
of a disease.
So in this web of causation we have five
criteria of judgment.
So the first is that multiple independent studies
have confirmed or are consistent about the direction
of an association.
The second is that the strength of the association
in dose response of increasing risk
with increasing exposure.
The third is that there's a temporal relationship
between exposure and pathogenesis.
That is the exposure happens before the event occurs
or the disease begins.
The fourth is that there's a biological plausibility
of this association, and the fifth is that there's coherence
of the association when the totality
of evidence is considered.
So these criteria really emphasize that when determining
whether something causes something else, you're looking
at a number of different independent epidemiological,
biological studies that are going to help determine
or paint a picture of what a causal effect might be.
So of all cancer related deaths, 30% are due
to chronic tobacco Smoking, 30 to 35%
are due to dietary factors.
15% to 20% are due to cancer causing infectious agents
and 3% to 4% are due to alcohol abuse.
It's also notable that only five to 10% of cancers
can be attributed to heritable genetic factors
whereas the remaining 90 to 95% are related
to environment and lifestyle factors.
So this graph just depicts what I just mentioned
and you can see infection diet and tobacco use are really
the three main drivers or causal factors of cancer.
So now let's dig into one of them.
So tobacco smoking is the dominant cancer risk factor.
During the 20th century, approximately 50 million
people died from tobacco-related cancers.
Approximately 50% of regular chronic smokers eventually
succumb to tobacco related cancer and most die prematurely.
And cigarette smoking causes about 30% of all cancer
death making tobacco smoke the single most
lethal source of carcinogens.
Tobacco use has unfortunately continued to rise
globally, and it is one of the greatest
global public health challenges of our time.
Recent survey data compiled from 193 member nations
of the WHO showed that one in three adults smoke globally.
So now we come to the 1964 US Surgeon General's Report
on "Smoking and Health."
And this was a landmark study that unequivocably
linked tobacco to lung cancer, laryngeal cancer
and a number of other severe adverse health outcomes.
This report highlighted the many delirious health
consequences of chronic cigarette smoking and tobacco use.
Specifically, it showed that chronic smoking
was linked to lung cancer, laryngeal cancer,
chronic bronchitis, emphysema and coronary heart disease.
The 1964 Surgeon General's report and many
subsequent reports, have had a major impact
on public attitudes and policy about smoking in America
and most other developed nations.
So after this report was published
the percentage of adults who smoked cigarettes
in the US declined from 42% in 1964 to only 18% in 2012.
This means that 8 million premature smoking
related deaths were prevented in the US population
during the past 50 years.
So after the 1964 report was published there was
a 2020 Surgeon General's report on smoking cessation
that was published.
This report makes it clear that one of the most
important actions people can take to improve
their health is to quit smoking.
This is true regardless of one's age or of how long
they've been smoking, there are proven treatments
and strategies to help people quit smoking successfully.
And this report really describes those efforts
and emphasizes the importance of treatment
use of medications and ultimately the health benefits
that can can accrue from quitting smoking tobacco use.
So next, this figure from your textbook shows the number
of worldwide deaths attributed to cigarette smoking
and 5.4 million of the 1.1 billion adults who are
chronically addicted to the smoking habit die annually
from lung cancer and other smoking related diseases,
like heart disease, chronic obstructive pulmonary disease
or COPD, stroke and other cancers.
Compared to non-smoker, chronic smokers have a 10 to 40
fold increased risk of developing lung cancer.
Worldwide approximately 90% of lung cancers
are attributable to cigarette smoking.
Chronic tobacco use markedly increases the risk
of cancer development at many other anatomic sites
including the oral cavity, larynx, pharynx, nasal cavity
esophagus, stomach, kidney, urinary bladder, pancreas,
liver and uterine cervix.
And finally, smoking has also been implicated
in the development of certain types of leukemia.
In the US, we're also seeing that more women now die
from smoking induced lung cancer than from breast cancer.
There's a huge rise in female smoking
and that's an important public health concern.
In the Nordic countries like Iceland and Denmark,
more women are now dying from lung cancer than men,
and in several European countries 50% of young women
are regular smokers, and such high rates will inevitably
produce an incredible burden of lung cancer
and other smoking related diseases
in women for centuries to come.
Diet and cancer in high income countries
is also a critical concern.
In high income countries, animal or saturated fat
in certain polyunsaturated fatty acids are associated
with cancers of the breasts, prostate
and lower gastrointestinal tract.
The colon and the rectum.
Diets high in fat and low in fiber are linked
to pathogenesis of colorectal cancer, prostate cancer,
breast cancer, endometrial cancer
and several other cancers.
We also see that the westernized diet
which is commonly described as the intake
of excessive calories from fat, refined carbohydrates
sugar and animal protein in combination
with a sedentary lifestyle can lead to energy
imbalance associated with a multitude
of chronic diseases like obesity, type two diabetes
cardiovascular disease, arterial hypertension, and cancer.
We also see that obesity is linked to a number of cancers.
In low and middle income countries, or LMICs.
We see differences in the dietary risks
than we see from high income countries.
So we see that there are certain types
of toxins that people can be exposed
to by ingesting moldy grains.
For example, aflatoxins when people consume them
are metabolized by the liver and become highly
carcinogenic epoxides.
We also see that chronic exposure to these toxins is related
to liver cancer, which is the leading cause of death
in some populations in southern China, southeast Asia
and coastal regions of Africa.
It's also important to note that some dietary factors
can interact with other cancer risk factors.
So aflatoxins and hepatitis viruses that is
hep B and Hep C interact synergistically
in the pathogenesis of liver cancer.
We also see that regular exposure to irritants, spices
and hot beverages can produce chronic inflammation
of the squamous cell lining of the esophagus
and can cause dysplastic lesions.
That's why in some communities where the consumption
of hot beverages like tea or coffee are common
especially extremely hot temperatures, like right
after the water boils, we see elevated rates
of esophageal cancer or other cancers
of the upper digestive tract.
So as I noted, the practice of ingesting hot
spicy beverages is commonplace in many cultures
in communities in East Asia, in the Middle East
which contributes to esophageal cancer rates.
Turning to alcohol, chronic alcohol use is a risk factor
for a number of cancers, including cancer of the oral cavity
the esophagus, the liver, and the pancreas.
Alcohol can also interject synergistically
with chronic cigarette smoking that can again
increase the risk of cancers in some sites.
Next, we see that in low and middle income countries
more than 20% of cancers and related deaths are caused
by infectious agents that cause cervical cancer
liver cancer, stomach cancer, and other kinds of cancers.
Especially in China, Asia and Africa we see various
infectious agents playing a role.
These include hepatitis viruses human papilloma viruses
or HPV, Helicobacter pylorus, schistosomiasis,
co-infection by HIV and herpes B
or herpes virus eight rather, and malaria,
which can be linked to Burkett's Lymphoma.
In regions with high HPV infection rates
and cervical screening programs or a lack rather
of cervical screening programs.
Cervical cancer ranks first as a cause of cancer death.
We also see that more than 80% of all cervical
cancer deaths today occur in low and middle income countries
due to this duality of high rates of HPV
and low rates of screening.
Finally, we see the chronic infections account
for less than 10% of all malignancies in high income
countries, and that's due to more screening programs,
vaccination, et cetera.
So stomach cancer declined in many nations
due to the replacement of irritating preservatives
by refrigeration and ready access to fruits and vegetables.
So lowering the chance of infection with bacteria
or molds or other agents.
Cervical cancer rates have decreased due to an increase
in the use of the Pap test.
And finally rates of liver cancer have declined
in China and Thailand due to effective
immunization programs to prevent viral hepatitis
and improve storage conditions of staple foods
to reduce exposure to toxins.