TO FIND THE WAY TO OVERCOME THE TUMOR ACCELERATED PROLIFERATION DURING FRACTIONATIONS OF RADIATION THERAPY

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The tumor repopulation (accelerated proliferation) during fractionation is thought to be a factor causing insufficient radiation effects. Using low LET irradiation and mouse jejunal crypts, we studied the mechanism of accelerated proliferation. To summarize the previous results as follows ; 1) accelerated proliferation is observed during multiple fractions of 2 Gy dose, 2) the extent of proliferation depend on the total dose per a day and 3) there are 2 possible mechanisms for accelerated proliferation-shortening of potential doubling time and reduction of cell loss factor.The purpose of this study is to clarify whether accelerated proliferation found in jejunal crypt is observed during multiple doses of high LET beams such as carbon ion beam. Carbon ion beam of LET 70 keV/μm (290 MeV/u and 6 cm SOBP) was used. Endpoint was Jejunal colony assay. After obtaining RBE, fractionation effects were compared between X-ray irradiation and carbon ion beam. Experiments were performed as follows ; 1) a single dose, 2) two split dose at an interval of 4 h and 3) fractionation for 1 to 3 days with a single top-up dose. Fraction size of carbon ion beams and X-rays are 1 Gy and 2 Gy, respectively.The results showed colony survival obtained from carbon ion beam was shifted to the sensitive side compared with that obtained from X-rays. This is either due to incomplete repair or inhibition of accelerated proliferation. The former could be ruled out, since split dose study showed repair was completed between 4 h interval. From this study, it is concluded that fractionation of carbon ion beam could inhibit, in some extent, accelerated proliferation in jejunal crypt.Some other investigations about the human cancer were also studied.This study regarding the use of carbon ion beam is performed under a Research Project with Heavy Ions at NIRS-HIMAC.