Biophysical foundations of malignant neoplasm therapy with fast neutrons

The relationship of the radio sensitivity of tumor cells in nammals, the magnitude of oxygen intensity, and radiation LET has been shown. The results of the investigations of cellular and molecular mechanisms accounting for a different biological efficiency of X-rays and fast neutrons are given. It has been established that under the effect of neutrons the damage of both the DNA filaments in mother cells occurs far more often than in the case of X-rays. The yield of double-filament ruptures upon irradiation of cells with neutrons is twice as high as in the case of X-ray irradiation. It is shown that the yield of single-filament DNA ruptures declines with higher radiation LET, while the yield of double-filament ruptures goes through the maximum. A model of the formation of DNA ruptures in irradiation has been developed, which permits one to compute the yield of ruptures for secondary particles of different energies which arise in biological tissues during the passage of fast neutrons. A semiempirical theory of RBE is presented which indicates that the RBE of fast neutrons increases with the declining sensitivity of cells to gamma-radiation. The obtained results of biophysical analysis are likely to be useful in planning irradiation regimes in medicine, in the choice of neutrons energy and of the relationship between neutrons and gamma-rays in combined beams