Final answer:
Prolongation of overall treatment has little effect on rapidly dividing tissues (e.g., cancer cells), but a large effect on other tissues.
Step-by-step explanation:
The tissues most affected by large, whole-body exposures are bone marrow, intestinal tissue, hair follicles, and reproductive organs, all of which contain rapidly dividing cells. The susceptibility of rapidly dividing cells to radiation exposure explains why cancers are often treated by radiation. Because cancer cells divide faster than normal cells, they are destroyed preferentially by radiation.
Long-term radiation-exposure studies on fruit flies show a linear relationship between the number of genetic defects and both the magnitude of the dose and the exposure time. In contrast, similar studies on mice show a much lower number of defects when a given dose of radiation is spread out over a long period of time rather than received all at once.
Therefore, in the context of the question, prolongation of overall treatment has little effect on rapidly dividing tissues (e.g., cancer cells), but a large effect on other tissues.