Cancer Prevention
~ Part II ~
Four
cancer-related risk factors have been summarized in the previous article:
Here
are other things that we should aware, infections that link to cancer, and cancer
screening and vaccination.
5. Infections
and cancer
In
1981, only two infections that have links to cancer including the hepatitis B
virus (HBV) which is known to cause liver cancer, and the Epstein-Barr virus
(EBV) which was linked to Burkitt’s lymphoma. Today, 11 infectious agents are
well established as human carcinogens. In 2008, about two million of the 12.7
million new cancer cases worldwide, which means about 16% of the total, were
attribute to chronic infection.
There
are seven viruses, one bacterium, and three parasitic worms that act as cancer
agents.
1.
Epstein-Barr virus (EBV)
2.
Hepatitis B virus (HBV) and hepatitis C virus (HCV)
These
two are leading causes of the hepatocellular carcinoma, the most common form of liver cancer. Both of HBV and HCV can cause the long-term (chronic)
infections that increase a person’s chance of liver cancer. HBV and HCV are
spread from person to person through sharing needles (such as injection drug
use), unprotected sex, or childbirth. Once an infection is found, treatment and
preventive measures can be used to slow liver damage and reduce cancer risk.
3.
Human papilloma virus (HPV)
HPV
is an interesting group of viruses with more than 100 different types. Some of
them cause non-cancerous conditions such as warts on the hands and feet, and
genital warts. Others are sexually transmitted, and in some cases, cause cervical
cancer, cancers of the anus, vagina, penis, and vulva. 13 types out of 100 types
of HPV are classified as carcinogenic. HPV type 16 and 18 are nearly always
present in cervical cancer tumors and can also cause a type of throat cancer called
oropharyngeal cancer. Recent research shows that oropharyngeal cancer caused by
HPV infection can be considered a different form of cancer than that caused by
HPV negative oropharyngeal cancer. HPV positive and HPV negative tumors develop
along different molecular pathway and evidence suggests that patients with HPV
positive tumors survive longer after treatment compared than those with
patients with smoking related oropharyngeal carcinoma.
4.
Human T-cell lymphotropic virus type 1 (HTLV-1)
This
virus belogs to a class of viruses called retroviruses. Although it is
something like HIV which is another human retrovirus, HTLV-1 can not cause
AIDS. It is mainly endemic in Japan, the
Carribean, and Central Africa. It causes a type of lymphoma and leukemia in
about 5% of the people who are infected.
5.
Human Herpes virus eight (HHV-8, also known as Kaposi’s sarcoma)
Kaposi’s
sarcoma is a cancer that occurs mainly in people with weakened immunities such
as those with advanced AIDS or people who have undergone solid organ
transplantation or bone marrow transplantation. HHV-8 infection is life-long
(as with other herpes viruses), but it does not appear to cause disease in most
healthy people. Thus, having a weekened immune system appears to be one such
factor.
6.
HIV type one
HIV
does not appear to cause cancer directly. It infects and destroys white blood
cells known as helper T-cells, which weakens the body’s immune system. Advanced
HIV disease (AIDS) is defined by a severely damaged immune system. The loss of
immunity allows preexisting latent viral infections such as Kaposi’s sarcoma,
EBV, or HPV that has been suppressed by the immune system to become active.
This, in turn, can lead to AIDS-defining cancers.
7.
Merkel cell polyomavirus (MCV)
MCV
was discovered in 2008 in samples from a rare and aggressive type of skin
cancer called Merkel cell carcinoma. Nearly all Merkel cell cancers are now
thought to be linked with this infection.
8.
Helicobacter pylory
Helicobacter
pylory is a bacterium that affects the stomach or small intestines and causes
stomach cancer and gastric lymphoma, as well as gastric ulcers. Transmission
occurs through close personal contact. Other factors include poverty, poor
hygiene conditions, housing crowding, and the number of human children in the
household. H.Pylory can lead to cancer because the infection causes chronic inflammation
that induces growth and proliferation of stomach cels. This condition will
alter gene excpression and facilitates gene mutation.
9.
Three parasitic worms
Two
of the worms are liver flukes, endemic in China, North and South Korea, and
Southeast Asia. They can cause liver cancer. The flukes are transmitted through
consumption of raw or undercooked infected fish includes dried, pickled, or
salted fish.
The
third wom is schistosomiases haematobium which is found on the African
continent, and the Middle-East. It infects the upper small intestine and the
bladder, causes urinary scistosomiases, an infection that can progress to
bladder cancer.
6. Vaccination
and Screening
Now,
we have realized the importance of preventing cancer whenever possible. And
then, when cancer does happen, it is most treatable if detected earlier.
Vaccines are available and can protect agains two infections that are important
cause of cancer worldwide. Hepatitis B virus (HBV) which is responsible for 50
to 90% of liver cancer, and Human papilloma virus (HPV) which causes virtually
all cancers of the cervix and some cancers of the anus, vulva, vagina, penis,
and oral pharynx. HPV types 16 and 18 are among the most common cancer causing
infections. HBV vaccination is usually part of childhood vaccination. In 2010,
179 countries reported including HBV vaccination in their childhood vaccination
program and nearly 70% of children wordwide receive three doses of the HBV
vaccine. This should significantly reduce the incidence of liver cancer in
coming decades. For HPV vaccine, it is best given to boys and girls at ages 11
to 12, but it can be given to those 9 to 26 years old. The vaccines are most
effective when given to pre-teens, before they become sexually active.
Checking
for cancer or for conditions that might lead to cancer in people who have no
symptoms is called screening. Screening tests are not meant to diagnose cancer.
Sometimes screening test can find signs or symptoms of cancer before the person
knows of the problem. In these cases, the person will be referred for further
diagnostic test. Detecting cancer early is the key to surviving cancer. Chances
of complete cure can be 95% or higher, for some cancers, if the cancer is found
early. The evidence is strong that regular screening for breast, colorectal and
cervical cancer can find cancer early. So if a person does have cancer, the longer
he or she waits to get screened, the more likely that they will be diagnosed at
later stage of cancer. And the later that cancer is diagnosed, the lower the
chances of survival.
Cancers
we can screen for include breast cancer, cervival cancer, and colorectal
cancer. The most common method to screen for breast cancer is mammography. Women
age 40 to 49 years old should talk with their healthcare provider about their
risk of breast cancer and how often to have a mammogram. In addition, women
should get regular clinical breast exams by their physicians as part of their
annual exams. Cervival cancer is the fourth most common cancer in women
worldwide. There are two tyes of approved cervival cancer screening test: PAP
test and HPV test. New cervical cancer screening guidelines recommend annual
PAP testing every three years for women age 21 to 65. Colorectal cancer is the
third most common cancer wordwide. A stool blooed test (a test that identifies
the presence of blood in a small sample of stool) should be done annually alone
or with sigmoidoscopy or colonoscopy (uses a tube with a light to examine the
lower one-third of the colon or the entire colon). If polyps are detected, the
can be removed during the test and sent to a laboratory to test for cancer.
Here
are some reasons commonly hear to avoid cancer screening test. Fear of finding
cancer, fear of treatment, the cost of screening, not knowing what test to get
or where to go for testing, embarrassment,waiting for symptoms to appear before
going to get checked, thinking they are not at risk because they are older, or
because they do not have family history.
Please
note, cancer screening shows that most people usually do not have cancer, and
this leads to peace of mind. For most of cancers, when caught earlier, the
treatment is much less intensive. Also remember that being healthy and cancer
free is beneficial to the whole family. Many cancers take years to grow in your
body without causing pain or other symptoms, so it is important not to wait
before you have symptoms. Only a small of percentage or people who develop
cancer have a family history of cancer. In fact, three out of four people who
are diagnosed with cancer do not have a family history. Thus, you should get
screened even if no one in your family had cancer. Cancer does not just impact
you. A cancer diagnosis can impact the whole family. Please remember that cancer screening
can save your life.
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Summary
from the lectures: “Introduction to the Science of Cancer” – The Ohio State
University Comprehensive Cancer Center-James
*
Additional data source:
Images:
Kathryn
- Tokyo
