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.
Malignant Tumours
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.
