[IV. Studies on cell biological experiments to the relative biological effectiveness (RBE) of fast neutrons in different phantom depths (author's transl)].

From carcinomas of the collum uteri and from human embryos (10 weeks) primary cell cultures were obtained, irradiated with 60Co-gamma-rays and fast neutrons (6,2 mev), respectively, in doses ranging from 50 to 300 rad in the phantom depth of 3 and 12 cm at 37 degrees C, and the number of mitotically surviving cells was determined. From the survival curves, the values of D37, Do and Dq were determined. The studies have shown that embryonic cells have higher Dq values than the tumor cells under study, while there were no distinct differences in primary radiation sensitivity Do between normal and tumor cells when considering biological variability. The relative biological effectiveness (RBE) of the 6,2 mev neutrons proved clearly dependent on the level of the single dose or of percent survival rate, respectively. The minima of the RBE values were obtained refering to Do (2,2-2,7). From the quotient RBE (tumor cells)/RBE (embryonic cells) ranging from 0.88 to 1.23 depending on the reference system, the different phantom depth and the cell cultures used can be concluded that the stronger biological effectiveness of fast neutrons is not necessarily an additional therapeutical advantage, but that the "anoxic gain factor" and the ensueing increased killing of hypoxic tumor cells has to be considered as the main advantage of neutron therapy. Using the neutron therapy in a phantom depth of 12 cm, a slow increase of the RBE-values can be registered in comparison to the radiation in a depth of 3 cm, if for the formation of the RBE-values the pure dose of neutrons without reference to the gamma-ray part will be employed. When we use for the formation of the RBE-values the neutron-dose including the dose for the corresponding gamma-ray part, a change of the RBE on the phantom depth tested here is not detectable.