What is Cancer? What
Causes Cancer?
Cancer is a class of diseases characterized by
out-of-control cell growth. There are over 100 different types of cancer, and
each is classified by the type of cell that is initially affected.
Cancer harms the body when damaged cells divide
uncontrollably to form lumps or masses of tissue called tumors (except in the
case of leukemia where cancer prohibits normal blood function by abnormal cell
division in the blood stream). Tumors can grow and interfere with the
digestive, nervous, and circulatory systems, and they can release hormones that
alter body function. Tumors that stay in one spot and demonstrate limited
growth are generally considered to be benign.
More dangerous, or
malignant, tumors form when two things occur:
a cancerous cell manages to move throughout the body using
the blood or lymph systems, destroying healthy tissue in a process called
invasion
that cell manages to divide and grow, making new blood
vessels to feed itself in a process called angiogenesis.
When a tumor successfully spreads to other parts of the body
and grows, invading and destroying other healthy tissues, it is said to have
metastasized. This process itself is called metastasis, and the result is a
serious condition that is very difficult to treat.
In 2007, cancer claimed the lives of about 7.6 million
people in the world. Physicians and researchers who specialize in the study,
diagnosis, treatment, and prevention of cancer are called oncologists.
What causes cancer?
Cancer is ultimately the result of cells that uncontrollably
grow and do not die. Normal cells in the body follow an orderly path of growth,
division, and death. Programmed cell death is called apoptosis, and when this
process breaks down, cancer begins to form. Unlike regular cells, cancer cells
do not experience programmatic death and instead continue to grow and divide.
This leads to a mass of abnormal cells that grows out of control.
Genes - the DNA type
Cells can experience uncontrolled growth if there are
damages or mutations to DNA, and therefore, damage to the genes involved in
cell division. Four key types of gene are responsible for the cell division
process: oncogenes tell cells when to divide, tumor suppressor genes tell cells
when not to divide, suicide genes control apoptosis and tell the cell to kill
itself if something goes wrong, and DNA-repair genes instruct a cell to repair
damaged DNA.
Cancer occurs when a cell's gene mutations make the cell
unable to correct DNA damage and unable to commit suicide. Similarly, cancer is
a result of mutations that inhibit oncogene and tumor suppressor gene function,
leading to uncontrollable cell growth.
Carcinogens
Carcinogens are a class of substances that are directly
responsible for damaging DNA, promoting or aiding cancer. Tobacco, asbestos,
arsenic, radiation such as gamma and x-rays, the sun, and compounds in car
exhaust fumes are all examples of carcinogens. When our bodies are exposed to
carcinogens, free radicals are formed that try to steal electrons from other
molecules in the body. Theses free radicals damage cells and affect their
ability to function normally.
Genes - the family
type
Cancer can be the result of a genetic predisposition that is
inherited from family members. It is possible to be born with certain genetic
mutations or a fault in a gene that makes one statistically more likely to
develop cancer later in life.
Other medical factors
As we age, there is an increase in the number of possible
cancer-causing mutations in our DNA. This makes age an important risk factor
for cancer. Several viruses have also been linked to cancer such as: human
papillomavirus (a cause of cervical cancer), hepatitis B and C (causes of liver
cancer), and Epstein-Barr virus (a cause of some childhood cancers). Human
immunodeficiency virus (HIV) - and anything else that suppresses or weakens the
immune system - inhibits the body's ability to fight infections and increases
the chance of developing cancer.
What are the symptoms
of cancer?
Cancer symptoms are quite varied and depend on where the
cancer is located, where it has spread, and how big the tumor is. Some cancers
can be felt or seen through the skin - a lump on the breast or testicle can be
an indicator of cancer in those locations. Skin cancer (melanoma) is often
noted by a change in a wart or mole on the skin. Some oral cancers present
white patches inside the mouth or white spots on the tongue.
Other cancers have symptoms that are less physically
apparent. Some brain tumors tend to present symptoms early in the disease as
they affect important cognitive functions. Pancreas cancers are usually too
small to cause symptoms until they cause pain by pushing against nearby nerves
or interfere with liver function to cause a yellowing of the skin and eyes
called jaundice. Symptoms also can be created as a tumor grows and pushes
against organs and blood vessels. For example, colon cancers lead to symptoms
such as constipation, diarrhea, and changes in stool size. Bladder or prostate
cancers cause changes in bladder function such as more frequent or infrequent
urination.
As cancer cells use the body's energy and interfere with
normal hormone function, it is possible to present symptoms such as fever,
fatigue, excessive sweating, anemia, and unexplained weight loss. However,
these symptoms are common in several other maladies as well. For example,
coughing and hoarseness can point to lung or throat cancer as well as several
other conditions.
When cancer spreads, or metastasizes, additional symptoms
can present themselves in the newly affected area. Swollen or enlarged lymph
nodes are common and likely to be present early. If cancer spreads to the
brain, patients may experience vertigo, headaches, or seizures. Spreading to
the lungs may cause coughing and shortness of breath. In addition, the liver
may become enlarged and cause jaundice and bones can become painful, brittle,
and break easily. Symptoms of metastasis ultimately depend on the location to
which the cancer has spread.
How is cancer
classified?
There are five broad groups that are used to classify
cancer.
Carcinomas are characterized by cells that cover internal
and external parts of the body such as lung, breast, and colon cancer.
Sarcomas are characterized by cells that are located in
bone, cartilage, fat, connective tissue, muscle, and other supportive tissues.
Lymphomas are cancers that begin in the lymph nodes and
immune system tissues.
Leukemias are cancers that begin in the bone marrow and
often accumulate in the bloodstream.
Adenomas are cancers that arise in the thyroid, the
pituitary gland, the adrenal gland, and other glandular tissues.
Cancers are often referred to by terms that contain a prefix
related to the cell type in which the cancer originated and a suffix such as
-sarcoma, -carcinoma, or just -oma. Common prefixes include:
Adeno- = gland
Chondro- = cartilage
Erythro- = red blood cell
Hemangio- = blood vessels
Hepato- = liver
Lipo- = fat
Lympho- = white blood cell
Melano- = pigment cell
Myelo- = bone marrow
Myo- = muscle
Osteo- = bone
Uro- = bladder
Retino- = eye
Neuro- = brain
How is cancer
diagnosed and staged?
Early detection of cancer can greatly improve the odds of
successful treatment and survival. Physicians use information from symptoms and
several other procedures to diagnose cancer. Imaging techniques such as X-rays,
CT scans, MRI scans, PET scans, and ultrasound scans are used regularly in
order to detect where a tumor is located and what organs may be affected by it.
Doctors may also conduct an endoscopy, which is a procedure that uses a thin
tube with a camera and light at one end, to look for abnormalities inside the
body.
Extracting cancer cells and looking at them under a
microscope is the only absolute way to diagnose cancer. This procedure is
called a biopsy. Other types of molecular diagnostic tests are frequently
employed as well. Physicians will analyze your body's sugars, fats, proteins,
and DNA at the molecular level. For example, cancerous prostate cells release a
higher level of a chemical called PSA (prostate-specific antigen) into the
bloodstream that can be detected by a blood test. Molecular diagnostics,
biopsies, and imaging techniques are all used together to diagnose cancer.
After a diagnosis is made, doctors find out how far the cancer
has spread and determine the stage of the cancer. The stage determines which
choices will be available for treatment and informs prognoses. The most common
cancer staging method is called the TNM system. T (1-4) indicates the size and
direct extent of the primary tumor, N (0-3) indicates the degree to which the
cancer has spread to nearby lymph nodes, and M (0-1) indicates whether the
cancer has metastasized to other organs in the body. A small tumor that has not
spread to lymph nodes or distant organs may be staged as (T1, N0, M0), for
example.
TNM descriptions then lead to a simpler categorization of
stages, from 0 to 4, where lower numbers indicate that the cancer has spread
less. While most Stage 1 tumors are curable, most Stage 4 tumors are inoperable
or untreatable.
How is cancer
treated?
Cancer treatment depends on the type of cancer, the stage of
the cancer (how much it has spread), age, health status, and additional
personal characteristics. There is no single treatment for cancer, and patients
often receive a combination of therapies and palliative care. Treatments
usually fall into one of the following categories: surgery, radiation,
chemotherapy, immunotherapy, hormone therapy, or gene therapy.
Surgery
Surgery is the oldest known treatment for cancer. If a
cancer has not metastasized, it is possible to completely cure a patient by
surgically removing the cancer from the body. This is often seen in the removal
of the prostate or a breast or testicle. After the disease has spread, however,
it is nearly impossible to remove all of the cancer cells. Surgery may also be
instrumental in helping to control symptoms such as bowel obstruction or spinal
cord compression.
Radiation
Radiation treatment, also known as radiotherapy, destroys
cancer by focusing high-energy rays on the cancer cells. This causes damage to
the molecules that make up the cancer cells and leads them to commit suicide.
Radiotherapy utilizes high-energy gamma-rays that are emitted from metals such
as radium or high-energy x-rays that are created in a special machine. Early
radiation treatments caused severe side-effects because the energy beams would
damage normal, healthy tissue, but technologies have improved so that beams can
be more accurately targeted. Radiotherapy is used as a standalone treatment to
shrink a tumor or destroy cancer cells (including those associated with
leukemia and lymphoma), and it is also used in combination with other cancer
treatments.
Chemotherapy
Chemotherapy utilizes chemicals that interfere with the cell
division process - damaging proteins or DNA - so that cancer cells will commit
suicide. These treatments target any rapidly dividing cells (not necessarily
just cancer cells), but normal cells usually can recover from any
chemical-induced damage while cancer cells cannot. Chemotherapy is generally
used to treat cancer that has spread or metastasized because the medicines
travel throughout the entire body. It is a necessary treatment for some forms
of leukemia and lymphoma. Chemotherapy treatment occurs in cycles so the body
has time to heal between doses. However, there are still common side effects
such as hair loss, nausea, fatigue, and vomiting. Combination therapies often
include multiple types of chemotherapy or chemotherapy combined with other
treatment options.
Immunotherapy
Immunotherapy aims to get the body's immune system to fight
the tumor. Local immunotherapy injects a treatment into an affected area, for
example, to cause inflammation that causes a tumor to shrink. Systemic
immunotherapy treats the whole body by administering an agent such as the
protein interferon alpha that can shrink tumors. Immunotherapy can also be
considered non-specific if it improves cancer-fighting abilities by stimulating
the entire immune system, and it can be considered targeted if the treatment
specifically tells the immune system to destroy cancer cells. These therapies
are relatively young, but researchers have had success with treatments that
introduce antibodies to the body that inhibit the growth of breast cancer
cells. Bone marrow transplantation (hematopoetic stem cell transplantation) can
also be considered immunotherapy because the donor's immune cells will often
attack the tumor or cancer cells that are present in the host.
Hormone therapy
Several cancers have been linked to some types of hormones,
most notably breast and prostate cancer. Hormone therapy is designed to alter
hormone production in the body so that cancer cells stop growing or are killed
completely. Breast cancer hormone therapies often focus on reducing estrogen
levels (a common drug for this is tamoxifen) and prostate cancer hormone
therapies often focus on reducing testosterone levels. In addition, some
leukemia and lymphoma cases can be treated with the hormone cortisone.
Gene therapy
The goal of gene therapy is to replace damaged genes with
ones that work to address a root cause of cancer: damage to DNA. For example,
researchers are trying to replace the damaged gene that signals cells to stop
dividing (the p53 gene) with a copy of a working gene. Other gene-based
therapies focus on further damaging cancer cell DNA to the point where the cell
commits suicide. Gene therapy is a very young field and has not yet resulted in
any successful treatments.
How can cancer be
prevented?
Cancers that are closely linked to certain behaviors are the
easiest to prevent. For example, choosing not to smoke tobacco or drink alcohol
significantly lower the risk of several types of cancer - most notably lung,
throat, mouth, and liver cancer. Even if you are a current tobacco user,
quitting can still greatly reduce your chances of getting cancer.
Skin cancer can be prevented by staying in the shade,
protecting yourself with a hat and shirt when in the sun, and using sunscreen.
Diet is also an important part of cancer prevention since what we eat has been
linked to the disease. Physicians recommend diets that are low in fat and rich
in fresh fruits and vegetables and whole grains.
Certain vaccinations have been associated with the
prevention of some cancers. For example, many women receive a vaccination for
the human papillomavirus because of the virus's relationship with cervical
cancer. Hepatitis B vaccines prevent the hepatitis B virus, which can cause
liver cancer.
Some cancer prevention is based on systematic screening in
order to detect small irregularities or tumors as early as possible even if
there are no clear symptoms present. Breast self-examination, mammograms,
testicular self-examination, and Pap smears are common screening methods for
various cancers.
