Cancer is a group of diseases in which cells are
aggressive (grow and
divide
without respect to normal limits), invasive
(invade and destroy adjacent tissues), and sometimes
metastatic (spread to other
locations in the body). These three
malignant
properties of cancers
differentiate them from
benign tumors, which are
self-limited in their growth and don't invade or metastasize
(although some benign tumor types are capable of becoming
malignant). Cancer may affect people at all ages, even
fetuses,
but risk for the more common varieties tends to increase with
age.[1]
Cancer causes about 13% of
all deaths.[2]
According to the
American Cancer Society, 7.6
million people died from cancer in the world during 2007.[3]
Apart from humans, forms of cancer may affect other animals and
plants.
Nearly all cancers are caused by abnormalities in the
genetic material
of the
transformed
cells. These abnormalities may be due to the effects
of
carcinogens,
such as
tobacco smoke,
radiation,
chemicals, or
infectious agents. Other
cancer-promoting genetic abnormalities may be randomly acquired
through errors in
DNA replication, or are
inherited, and thus present in
all cells from birth. Complex interactions between carcinogens
and the host
genome
may explain why only some develop cancer after
exposure to a known carcinogen. New aspects of the genetics of
cancer pathogenesis, such as
DNA methylation, and
microRNAs
are increasingly being recognized as important.
Genetic abnormalities found in cancer typically affect two
general classes of genes. Cancer-promoting
oncogenes
are often activated in cancer cells, giving those
cells new properties, such as hyperactive growth and division,
protection against
programmed cell death, loss of
respect for normal tissue boundaries, and the ability to become
established in diverse tissue environments.
Tumor suppressor genes
are often inactivated in cancer cells, resulting in
the loss of normal functions in those cells, such as accurate
DNA replication, control over the
cell cycle, orientation and
adhesion within tissues, and interaction with protective cells
of the
immune system.
Cancer is usually classified according to the tissue from which
the cancerous cells originate, as well as the normal cell type
they most resemble. These are location and histology,
respectively. A definitive diagnosis usually requires the
histologic
examination of a tissue
biopsy
specimen by a
pathologist, although the
initial indication of malignancy can be symptoms or
radiographic imaging
abnormalities. Most cancers can be treated and some cured,
depending on the specific type, location, and
stage. Once diagnosed, cancer
is usually treated with a combination of
surgery,
chemotherapy
and
radiotherapy. As research
develops, treatments are becoming more specific for different
varieties of cancer. There has been significant progress in the
development of
targeted therapy
drugs that act specifically on detectable molecular
abnormalities in certain tumors, and which minimize damage to
normal cells. The prognosis of cancer patients is most
influenced by the type of cancer, as well as the stage, or
extent of the disease. In addition, histologic grading and the
presence of specific molecular markers can also be useful in
establishing prognosis, as well as in determining individual
treatments.