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Why do some tissues appear to be more
radiosensitive? |
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| Although the general multistep nature of carcinogenesis is considered applicable
to all tissue types, the specific steps differ in different tissues. All
of the approximately 60 trillion cells that constitute an adult human body
originated from one cell--the fertilized egg. However, through a variety
of precise control signals, the same information contained in all the copied
DNA is used differently resulting in a variety of cell types and, thus,
tissues. Therefore, gene expression crucial for one type of tissue may
not be important in another and vice versa. More simply put, certain genes
expressed only in the thyroid are responsible for making the cells function
as a thyroid. Those genes are not expressed in the brain. Such differences
in gene expresssion will undoubtedly be reflected in a tissue's response
to ionizing radiation damage and its vulnerability to carcinogenesis. |
| Nevertheless, the term "tissue vulnerability"
should be used with care because epidemiologic studies of
A-bomb survivors do not demonstrate a large difference in
the relative risk of cancer between people exposed to radiation
and those not exposed except in the case of leukemia. Therefore,
it cannot be said that a specific cancer is prone to develop
due to radiation exposure. On the other hand, it is known
that in those exposed to radiation, incidence of a commonly
observed cancer is higher than for a rare cancer. Thus, it
may be said that a cancer more prone to develop from radiation
exposure is more prone to develop spontaneously. |
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