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

在分离过程中肿瘤的再生（加速增殖）被认为是导致辐射效果不足的一个因素。利用低LET照射和小鼠空肠隐窝，研究了加速增殖的机制。将之前的结果总结如下；2)增殖的程度取决于每天的总剂量；3)增殖的加速可能有两种机制——缩短潜在加倍时间和降低细胞损失因子。本研究的目的是阐明在多剂量高LET束（如碳离子束）下是否观察到空肠隐窝中发现的加速增殖。采用LET 70 keV/μm （290 MeV/u, SOBP 6 cm）的碳离子束。终点是空肠菌落试验。获得RBE后，比较x射线辐照与碳离子束的分馏效果。实验方法如下：1)单次给药，2)间隔4小时分两次给药，3)分次给药1 - 3天，每次补加一次。碳离子束和x射线的分数尺寸分别为1gy和2gy。结果表明，与x射线相比，碳离子束获得的菌落存活率向敏感侧偏移。这是由于不完全修复或抑制加速增殖。前者可以排除，因为分剂量研究显示修复在间隔4小时之间完成。本研究表明，碳离子束的分离可以在一定程度上抑制空肠隐窝细胞的加速增殖。其他一些关于人类癌症的研究也被研究了。这项关于碳离子束使用的研究是在NIRS-HIMAC重离子研究项目下进行的。

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.