Every biological system is susceptible to high-energy radiation. The degree of response depends on the sensitivity of the biological system, the kind of radiation, and the quantity of radiation.
There are biological systems which react readily to irradiation with a few roentgens*; there are others which have to be exposed to thousands of roentgens before any reaction occurs. The response also depends on the reaction chosen as criterion for the effect. The Nobel Prize laureate, G. von Hevesy, showed that respiration is relatively insensitive to x-irradiation, while growth and cell division are among the most sensitive processes.
Little is known about the reasons for the different sensitivities of different species and tissues, and, since this holds also for different cancer tissues, we really face here one of the biggest riddles of radiation biology and radiation therapy. An answer to this problem would greatly assist the radiologist who tries to help