A tumour is a mass of new tissue growth that does not
react to normal controls or the organizing influence of other
tissues, and it has no useful function in the body. This applies
to both types of tumours, malignant and benign. Malignant, also
known as, cancerous tumours, are additionally defined by their
invasion of local tissue and their ability to spread to other
parts of the body.
A benign tumour which is not cancerous, is less serious
than malignant tumours because they do not spread to other parts
of the body, but they may cause damage by local growth and
pressure on other structures, producing serious complications
such as bleeding. Benign tumours generally grow slowly and kill
the host only if it occupies or attaches to an organ so as to
interfere with a critical function. The cells of benign tumours
closely resemble the cells of the tissue of origin. Surface
benign tumours include warts and moles.
A malignant tumour always kills (unless treated)
because of its invasive and metastatic characteristics. The
tumour grows locally by spreading into surrounding tissues.
Solid tumours, which develop in the breast, colon, lung, and
other organs, contain an inner core with high pressure zones that
compress and collapse blood vessels, often preventing the
penetration of blood-borne cancer treatments. It spreads to
distant sites by the breaking off of malignant cells, which move
through the blood and lymphatic systems, attach themselves, and
begin to grow as new colonies. Malignant tumours are diagnosed
by examination of their vascularity, shapes, forms of cells
division, and differentiation. More than a hundred different
types have been identified in humans. In general, those derived
from epithelial tissue are carcinomas, and those from connective
tissue are sarcomas.
The most common form of malignant tumour of the
respiratory tract is lung cancer, which began increasing in
frequency at an alarming rate about 1940. In 1980 it was the
leading cause of cancer deaths in men and is also rapidly
increasing in woman. It is attributed to cigarette smoking and
environmental pollution: cancer of the lung is rare in
nonsmokers, and exposure to materials such as asbestos, chromium,
and radioactive substances increases the probability of
developing lung cancer.
Malignant tumours, also known as lymphomas, one of the
main types of lung cancer, arise in the lymph nodes related to
the lungs and other body tissue. the other main type of lung
cancer is Sarcomas, it may originate in the lungs or in some
other structure such as a bone. Sarcomas have a poor prognosis,
but recent advances in the treatment of lymphomas have increased
the long term survival.
The factors controlling tumour growth are poorly
understood, although genetics seems to play a role Tumours in
laboratory animals may be transplanted to a second host using
only a single tumour cell. This suggests that only one normal
cell needs to become cancerous for tumour growth to begin.
Tumours have been experimentally induced in animals by chemical,
physical, and viral agents, and by radiation.
Cancer researchers no longer believe that a single drug
will be able to cure cancer. Experts now believe that a
combination of drugs will be the best method to kill tumours.
One route being researched utilizes antibodies that bind to
specific receptors on the tumour cell, thereby inhibiting tumour
growth by blocking certain tumour-growth factors from entering
the cell. Tumour necrosis factor (TFN), an immune-system
protein, has been found effective in cutting off the blood supply
to tumours, although too much TFN causes severe side effects.
The work of physician Francis Peyton Rous, gave rise to
the virus theory of the causes of cancer. In 1960, Rous found
that he could transmit a cancerous tumour (sarcoma) from one hen
to another by using an injection of tumour filtrate. The sarcoma
virus was the first tumour virus identified, and it opened up a
whole new area of cancer research. Rous shared the 1966 Nobel
Prize for physiology or medicine for his work.