[Biophysical principals in radiotherapy of malignant tumors]

The successfulness of tumour radiotherapy depends before all on achieving the maximal effect of radiation on the tumour with contemporary minimalization of the injury of normal tissues in its vicinity. Such selectivity of radiation action may be realized by utilization of physical (kind and energy of radiation, irradiation conditions) as well as biological (modification of radiation effects) factors. The aim of this publication is to give a survey of fundamental biophysical and radiobiological principles conditioning the differential action of ionizing radiation on tumorous and normal cell populations from the standpoint of the relevancy of experimental radiobiology (radiation effect on the proliferative capacity of the tumorous and normal tissue) to radiotherapy. More accent than customary in medical literature is put on the physical interactions of radiations in matter and dosimetry as well as microdosimetry of radiation. The importance of microdosimetry will perhaps increase in future in connection with the therapeutic usage of non-conventional kinds of radiation (neutrons, heavy charged particles, pi-mezons). Basic biophysical machanisms of radiation action on cells and cell populations are described and explained in terms of quantum radiobiology (single- and multihit theory, target theory, stochastics of radiation injury development, etc.) as well as the main principles of modification of radiation effects (oxygen effect, radiosensitizers, fractionation of the dose). A brief interpretation of the Strandquist model of isoeffect curves for the fractionated irradiation is presented enabling to relate mutually the total absorbed dose to the total irradiation time and the number of fractions.