Lung
Cancer
Cancer of the lung, like all cancers, results from an abnormality in the body’s
basic unit of life, the cell. Normally, the body maintains a system of checks
and balances on cell growth so that cells divide to produce new cells only when
needed. Disruption of this system of checks and balances on cell growth results
in an uncontrolled division and proliferation of cells that eventually forms a
mass known as a tumor.
Tumors can be benign or malignant; when we speak of
“cancer” we refer to those tumors that are considered malignant. Benign tumors
can usually be removed and do not spread to other parts of the body. Malignant
tumors, on the other hand, grow aggressively and invade other tissues of the
body, allowing entry of tumor cells into the bloodstream or lymphatic system
which spread the tumor to other sites in the body. This process of spread is
termed metastasis; the areas of tumor growth at these distant sites are called
metastases. Since lung cancer tends to spread, or metastasize, very early in its course, it is a very
life-threatening cancer and one of the most difficult cancers to treat. While
lung cancer can spread to any organ in the body, certain organs – particularly
the adrenal glands, liver, brain, and bone - are the most common sites for lung
cancer metastasis.
The lung is also a very common site for metastasis from tumors in other parts of
the body. Tumor metastases are made up of the same type of cells as the
original, or primary, tumor. For example, if prostate cancer spreads via the
bloodstream to the lungs, it is metastatic prostate cancer in the lung and is
not lung cancer.
The principal function of the lungs is the exchange of gases between the air we
breathe and the blood. Through the lung, carbon dioxide is removed from the body
and oxygen from inspired air enters the bloodstream. The right lung has three
lobes while the left lung is divided into two lobes and a small structure called
the lingula that is the equivalent of the middle lobe. The major airways
entering the lungs are the bronchi, which arise from the trachea. The bronchi
branch into progressively smaller airways called bronchioles that end in tiny
sacs known as alveoli, where gas exchange occurs. The lungs and chest wall are
covered with a thin layer of tissue called the pleura.
Lung cancers can arise in any part of the lung. Ninety to 95% of cancers of the
lung are thought to arise from the epithelial, or lining cells of the larger and
smaller airways (bronchi and bronchioles); for this reason lung cancers are
sometimes called bronchogenic carcinomas. Cancers can also arise from the pleura
(the thin layer of tissue that surrounds the lungs), called mesotheliomas, or
rarely from supporting tissues within the lungs, for example, blood vessels.
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