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Why do some tissues appear to be more radiosensitive?

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.