Ryonfa Lee

Complex exchanges are responsible for the increased effectiveness of C-ions compared to X-rays at the first post-irradiation mitosis

The purpose of the present study was to investigate as to what extent differences in the linear energy transfer (LET) are reflected at the chromosomal level. For this study human lymphocytes were exposed to 9.5 MeV/u C-ions (1 or 2 Gy, LET=175 keV/microm) or X-rays (1-6 Gy), harvested at 48, 72 or 96 h post-irradiation and aberrations were scored in first cycle metaphases using 24 color fluorescence in situ hybridization (mFISH). Additionally, in selected samples aberrations were measured in prematurely condensed G2-phase cells. Analysis of the time-course of aberrations in first cycle metaphases showed a stable yield of simple and complex exchanges after X-ray irradiation. In contrast, after C-ion exposure the yields profoundly increased with harvesting time complicating the estimation of the frequency of aberrations produced by high LET particles within the entire cell population. This is especially true for the yield of complex exchanges. Complex aberrations dominate the aberration spectrum produced by C-ions. Their fraction was about 50% for the two measured doses. In contrast, isodoses of X-rays induced smaller proportions of complex aberrations (i.e. 5% and 15%, respectively). For both radiation qualities the fraction of complexes did not change with harvesting time. As expected from the different dose deposition of high and low LET radiation, complex exchanges produced by high LET C-ions involved more breaks and more chromosomes than those induced by isodoses of X-rays. Noteworthy, C-ions but not X-rays induced a small number of complex chromatid-isochromatid exchanges that are not expected for cells exposed in the G0-phase. The results obtained so far for cells arrested in G2-phase confirm these patterns. Altogether our data show that the increased effectiveness of C-ions for the induction of aberrations in first cycle cells is determined by complex exchanges, whereas for simple exchanges the relative biological effectiveness is about one.

Cellular response in normal human cells exposed to chronically low-dose radiation in heavy-ion radiation field

Abstract We have been studying the biological effects in normal human fibroblasts exposed to chronically low-dose radiation in a heavy-ion radiation field. The cells were cultured in a CO2 incubator, which was placed in the irradiation room for biological study of the heavy ions in the Heavy Ion Medical Accelerator in Chiba (HIMAC), at the National Institute of Radiological Sciences (NIRS). We measured the absorbed dose, which was determined by a thermoluminescence dosimeter (TLD) and a Si-semiconductor detector, to be 1.4 mGy per day when operating the HIMAC machine. The total population doubling number (tPDN) of the exposed cells reduced to 77–94% of the non-exposed control cells. Furthermore, the shortening of the speed of the telomere length in the exposed cells was much higher than that in the non-exposed cells at the 26th passage, during which period the exposed cells were accumulated at 0.22 Gy. These findings show that the acceleration of senescence in normal human cells occurs by the chronically low-dose irradiation in a heavy-ion radiation field.