Annual Report

21. Radiobiologial Effects of Heavy Ion Beams on Cells

Yoshiya Furusawa, Mizuho Aoki, Koichi Ando, Hideki Matsumoto1, Akihisa Takahashi2, Tetsuya Kawata3, Kerry George3 and Marco Durante4 (1Fukui Med. Univ.; 2Nara Med. Univ.; 3Johnson Space Center; 4Univ. Fedellico II.)

Keywords: heavy ion, LET, RBE, p53, apoptosis, chromosome aberration

LET-RBE spectra of cell inactivation of aerobic and hypoxic cells from three different cell lines by accelerated 3He-, 12C- and 20Ne-ion beams were investigated to design a spread- out Bragg peak beam for cancer therapy at HIMAC, prior to clinical trials. Cells that originated from a human salivary gland tumor (HSG cells) as well as V79 and T1 cells were exposed to 3He-, 12C- and 20Ne-ion beams with an LET ranging from approximately 20-600 keV/m under both aerobic and hypoxic conditions. Cell survival curves were fitted by equations from the linear-quadratic model and the target model to obtain survival parameters. RBE, OER, alpha and D0 were analyzed as a function of LET. The RBE increased with LET, reaching a maximum at around 200 keV/m, then decreased with a further increase in LET. Clear splits of the LET-RBE or -OER spectra were found among ion species and/or cell lines. At a given LET, the RBE value for 3He ions was higher than that for the other ions. The position of the maximum RBE shifted to higher LET values for heavier ions. The OER value was 3 for X rays but started to decrease at an LET of around 50 keV/m, passed below 2 at around 100 keV/m, and then reached a minimum above 300 keV/m; however, the values remained greater than 1. The OER was significantly lower for 3He ions than the others.

The relationship between the LET values and cell death, defined as either apoptosis or loss of reproductive integrity (reproductive death), was studied using V79 cells. The cells were irradiated with X-rays or carbon-ion beams from the HIMAC. Apoptosis was defined based on the morphological change upon treating cells with caffeine. The apoptotic index, the ratio of apoptotic cells to the total, after exposure to 2 Gy of X-rays was 0.043. Upon irradiation with carbon ion beams, the index was gradually increased with increasing LET values, reaching a maximum of 0.076 at 110 keV/m, and then decreased to 0.054 at 237 keV/m. An analogous pattern of the LET dependence was observed between reproductive death and apoptotic death. The cell survival values obtained after 2 Gy exposure (SF2) were 0.64, 0.13, and 0.24, respectively. A similar trend was found for the RBE values calculated from the initial slope for both apoptosis and reproductive death. These results strongly suggested that the target for both types of cell death was the same.

The dependence of p53 on the radiation enhancement of thermosensitivity at different LET was investigated. The aim of this study was to investigate the dependence of p53- gene status on the radiation enhancement of thermosensitivity at different levels of LET. We used two kinds of human glioblastoma transfectants of A- 172 cells bearing the wild-type p53 gene, A-172/neo cells with control vector containing the neo gene and A-172/mp53 cells with both the dominant negative mutated p53 gene and neo gene. We exposed these cells to X-rays and accelerated carbon ion beams (13-200 KeV/m) followed by heating at 44 degrees C. Cellular sensitivities were determined using clonogenic assay. The radiation enhancement of thermosensitivity was LET-dependent for the A-172/neo cells, but this was not clearly demonstrated in the A-172/mp53 cells. The supraadditive radiation enhancement of thermosensitivity was observed in A-172/neo cells at the LET range of 13 to 70 keV/m, though only an additive effect was observed at higher LET. In A-172/mp53 cells, only an additive effect was observed for all the LET examined. These results indicated that the radiation enhancement of thermosensitivity was p53- and LET-dependent. Our results suggested that the combined use of high-LET radiation and hyperthermia would provide useful applications for cancer therapeutic purposes.

Cytogenetic damage in lymphocytes in peripheral blood from cancer patients during tumor therapy X-rays and carbon ion beams was determined. Blood samples from patients diagnosed with esophageal or uterine cervical cancer were obtained before, during, and at the end of the radiation treatment. The novel technique of interphase chromosome painting was used to detect aberrations in prematurely condensed chromosomes 2 and 4. The fraction of aberrant lymphocytes was measured as a function of the dose to the tumor volume. For comparison, blood samples were also exposed in vitro to X-rays or to carbon ions accelerated at the HIMAC. The carbon ions were more efficient than X-rays in the induction of chromosomal aberrations in vitro. In patients with similar pathologies, tumor positions, and radiation field sizes, however, carbon ions induced a lower fraction of aberrant lymphocytes than X-rays during the treatment. The initial slope of the dose-response curve for the induction of chromosomal aberrations during the treatment was correlated to the relative decrease in the number of white blood cells and lymphocytes during the treatment. The carbon ions induced a lower level of cytogenetic damage in lymphocytes than X-rays, reducing the risk of bone marrow morbidity.

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11) Kawata, T., et al.: Int. J. Radiat. Biol. 77: 165-74, 2001.
12) Shigematsu, N., et al.: Int. J. Mol. Med. 7: 509-513, 2001.

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