Ngo F Q, Schroy C B, Jia X L, Kalvakolanu I, Roberts W K, Blue J W, Antunez A R, Higgins P D, Tefft M
Laboratory of Radiobiology, Cleveland Clinic Foundation, Ohio 44195.
Radiat Res. 1991 Oct;128(1 Suppl):S94-102.
The radiobiological properties of a cyclotron-produced 43-MeV (p----Be) fast-neutron beam relative to gamma rays have been investigated using Chinese hamster V79 cells in culture. As expected, the relative biological effectiveness (RBE) of this neutron beam for cell killing was shown to increase as dose decreased, and the effectiveness per unit dose was slightly less compared to a 25-MeV (d----Be) neutron beam. By tracing single cells that formed microcolonies after irradiation, we found cell proliferation kinetics to be retarded to a greater extent by fast neutrons than by gamma irradiation. Following either neutron or gamma irradiation, a fraction of the irradiated cells failed to divide in the first postirradiation division and another fraction could produce as many as four generations of progeny before proliferation stopped. The properties of these cells presumed to be destined for death suggest that more than one mechanism and/or multistep process underlies the radiation-induced proliferative death. The fast-neutron beam was also found to be more effective quantitatively than gamma rays in producing DNA double-strand breaks (DSBs, measured by nondenaturing filter elution), and G1-phase chromosome fragments (measured by the premature chromosome condensation technique). However, the reverse was observed for DNA single-strand breaks (SSBs, measured by alkaline filter elution or hydroxylapatite uncoiling). Interestingly, both fast neutrons and gamma rays produced a large component of SSBs and DSBs with a fast-rejoining time constant of about 2-5 min, which appears to be independent of dose. The latter results could not resolve the possibility of lengthening the repair-time constant by increasing radiation dose within the range that is reflected by the shoulder of the survival curve, and consequently did not support the idea of repair saturation as a mechanism for the presence of the shoulder. The RBE for the hypoxanthine phosphoribosyl transferase mutation frequency per survivor at the 10% survival level was estimated to be 2.5, a value that is comparable to the RBE (2.1) for cell killing at the same survival level. Although most of the above-mentioned findings are compatible qualitatively with the relatively high-LET (linear energy transfer) nature associated with the fast-neutron beam, the significance of the action attributable to the mixture of LET could not be delineated in these experiments. Further, the biological significance of DSBs and chromosome aberration and the molecular mechanisms responsible for the repair and expression of these damaging processes remain to be elucidated.
利用培养的中国仓鼠V79细胞，研究了回旋加速器产生的43兆电子伏（p→Be）快中子束相对于γ射线的放射生物学特性。正如预期的那样，该中子束对细胞杀伤的相对生物效能（RBE）显示随着剂量降低而增加，并且与25兆电子伏（d→Be）中子束相比，单位剂量的效能略低。通过追踪辐照后形成微集落的单细胞，我们发现快中子比γ射线照射更能抑制细胞增殖动力学。在中子或γ射线照射后，一部分受辐照细胞在首次辐照后分裂时未能分裂，另一部分在增殖停止前可产生多达四代子代。这些推测注定死亡的细胞的特性表明，辐射诱导的增殖性死亡有不止一种机制和/或多步骤过程作为基础。还发现快中子束在产生DNA双链断裂（通过非变性滤膜洗脱测量）和G1期染色体片段（通过早熟染色体凝聚技术测量）方面在数量上比γ射线更有效。然而，对于DNA单链断裂（通过碱性滤膜洗脱或羟基磷灰石解旋测量），观察到的情况则相反。有趣的是，快中子和γ射线都产生了很大一部分具有约2 - 5分钟快速重新连接时间常数的单链断裂和双链断裂，这似乎与剂量无关。后一结果无法解决在存活曲线肩部所反映的剂量范围内通过增加辐射剂量来延长修复时间常数的可能性，因此不支持修复饱和作为肩部存在机制的观点。在10%存活水平下，每个存活者的次黄嘌呤磷酸核糖转移酶突变频率的RBE估计为2.5，该值与相同存活水平下细胞杀伤的RBE（2.1）相当。尽管上述大多数发现定性上与快中子束相关的相对高传能线密度（LET）性质相符，但在这些实验中无法确定归因于LET混合物作用的意义。此外，双链断裂和染色体畸变的生物学意义以及负责这些损伤过程修复和表达的分子机制仍有待阐明。
