RADIOBIOLOGICAL CHARACTERISTICS OF REACTOR NEUTRON BEAMS BASED ON REOXYGENATION PHENOMENA IN MURINE SOLID TUMORS

Masunaga S., Sakurai Y., Kobayashi T., Hasegawa Y., Takagaki M., Kinashi Y., Ono K.

C3H/He mice bearing SCC VII tumors and Balb/c mice bearing EMT6/KU tumors were irradiated with g-rays or a reactor neutron beam with a cadmium ratio of 150 or 1.0. The tumors from one group of these mice were excised, minced and trypsinized. Cell-survival curves for each irradiation condition were determined. The other group of mice further received a series of g-ray test irradiations 5 min through 72 h after the initial exposure, while alive or after being killed to obtain hypoxic cell fractions in the tumors. In both tumors, RBE of the neutron beam was greater at a cadmium ratio of 1.0 than 150, and reoxygenation occurred more rapidly after neutron beam irradiation than g-irradiation. Reoxygenation after neutron beam irradiation was induced slightly faster with a cadmium ratio of 1.0 than 150, in both tumors. Reoxygenation was more rapidly completed in acute hypoxia-rich SCC VII tumors than chronic hypoxia-rich EMT6/KU tumors. Both the quality of radiation and the character of irradiated tumors had an influence not only on RBE but also on reoxygenation following irradiation. Acute hypoxic tumor cells appeared to play a larger role in reoxygenation than chronic hypoxic tumor cells.

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