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We determined the relative biological effectiveness (RBE) and oxygen enhancement ratio (OER) of micronuclei (MN) formation in clamped (hypoxic) and non-clamped (normoxic) solid tumors in mice legs following exposure to X-rays and heavy ions. Single-cell suspensions (aerobic) of non-irradiated tumors were prepared in parallel and used directly to determine the radiation response for aerobic cells. Squamous cell carcinoma (SCCVII) cells were transplanted into the right hind legs of syngeneic C3H/He male mice. Irradiation doses with either X-rays or heavy ions at a dose-averaged LET (linear energy transfer) of 14-192keV/μm were delivered to 5-mm diameter tumors and aerobic single-cells in sample-tubes. After irradiation, the tumors were excised and trypsinized to observe MN in single-cells. The single-cell suspensions were used for MN formation assays. The RBE values increased with increasing LET. The maximum RBE values for the three different oxygen conditions; hypoxic tumor, normoxic tumor, and aerobic cells, were 8.18, 5.30, and 3.76 at an LET of 192keV/μm, respectively. After X-irradiation, the OERh/n values (hypoxic tumor/normoxic tumor) were lower than the OERh/a (hypoxic tumor/aerobic cells), and were 1.73 and 2.58, respectively. We found that the OER for the in vivo studies were smaller in comparison to that for the in vitro studies. Both of the OER values at 192keV/μm were small in comparison to those of the X-ray irradiated samples. The OERh/n and OERh/a values at 192keV/μm were 1.12 and 1.19, respectively. Our results suggest that high LET radiation has a large biological effect even if a solid tumor includes substantial numbers of hypoxic cells. To conclude, we found that the RBE values under each oxygen state for non-MN fraction increased with increasing LET and that the OER values for both tumors in vivo and cells in vitro decreased with increasing LET., (Copyright © 2015 Elsevier B.V. All rights reserved.)

The aim of this study was to measure the RBE (relative biological effectiveness) and OER (oxygen enhancement ratio) for survival of cells within implanted solid tumors following exposure to 290MeV/nucleon carbon-ion beams or X-rays. Squamous cell carcinoma cells (SCCVII) were transplanted into the right hind legs of syngeneic C3H male mice. Irradiation with either carbon-ion beams with a 6-cm spread-out Bragg peak (SOBP, at 46 and 80keV/μm) or X-rays was delivered to 5-mm or less diameter tumors. We defined three different oxygen statuses of the irradiated cells. Hypoxic and normoxic conditions in tumors were produced by clamping or not clamping the leg to avoid blood flow. Furthermore, single-cell suspensions were prepared from non-irradiated tumors and directly used to determine the radiation response of aerobic cells. Single-cell suspensions (aerobic condition) were fully air-saturated. Single-cell suspensions were prepared from excised and trypsinized tumors, and were used for in vivo-in vitro colony formation assays to obtain cell survival curves. The RBE values increased with increasing LET in SOBP beams. The maximum RBE values in three different oxygen conditions; hypoxic tumor, normoxic tumor and aerobic cells, were 2.16, 1.76 and 1.66 at an LET of 80keV/μm, respectively. After X-ray irradiation the OERh/n values (hypoxic tumor/normoxic tumor) were lower than the OERh/a (hypoxic tumor/aerobic cells), and were 1.87±0.13 and 2.52±0.11, respectively. The OER values of carbon-ion irradiated samples were small in comparison to those of X-ray irradiated samples. However, no significant changes of the OER at proximal and distal positions within the SOBP carbon-ion beams were observed. To conclude, we found that the RBE values for cell survival increased with increasing LET and that the OER values changed little with increasing LET within the SOBP carbon-ion beams., (Copyright © 2013 Elsevier B.V. All rights reserved.)

We investigated the earliest possible chromosome break and repair process in normal human fibroblasts irradiated with low and high LET (linear energy transfer) heavy ion radiation using the modified premature chromosome condensation (PCC) technique utilizing wortmannin (WM) during the fusion incubation period [M. Okada, S. Saito, R. Okayasu, Facilitated detection of chromosome break and repair at low levels of ionizing radiation by addition of wortmannin to G1-type PCC fusion incubation, Mutat. Res., 562 (2004) 11-17]. The initial numbers of breaks were approximately 10/cell/Gy in X-irradiated samples, followed by carbon (LET: 70 keV/microm), neon, and the number was around 5/cell/Gy in silicon (LET: 70 and 200 keV/microm) and iron (LET: 200 keV/microm) samples. If WM was not used, the initial numbers of breaks with silicon and iron were higher than those of X-rays. To quantify these data, we used initial repair ratio (IRR) defined as the number of G1 PCC breaks with WM divided by the number of breaks without WM. X-irradiation gave the maximum IRR ( approximately 2.0), while iron as well as silicon irradiation showed the minimum IRR ( approximately 1.0), suggesting almost no rejoining at the initial stage. Although there is a comparatively good correlation between the IRR value and the cell survival, the survival fraction with the repair data at 2 or 6h correlates better statistically. Our data indicate that high LET heavy ion irradiation induces a lower number of initial chromosome breaks with minimal repair when compared with low LET irradiation. These results at the chromosome level substantiate and extend the notion that high LET radiation produces complex-type DNA double strand breaks (DSBs).

Biophysical models are commonly used to evaluate the effectiveness of shielding in reducing the biological damage caused by cosmic radiation in space flights. To improve and validate these codes biophysical experiments are needed. We have measured the induction of chromosomal aberrations in human peripheral blood lymphocytes exposed in vitro to 500 MeV/n iron ion beams (dose range 0.1-1 Gy) after traversing shields of different material (lucite, aluminium, or lead) and thickness (0-11.3 g/cm2). For comparison, cells were exposed to 200 MeV/n iron ions and to X-rays. Chromosomes were prematurely condensed by a phosphatase inhibitor (calyculin A) to avoid cell-cycle selection produced by the exposure to high-LET heavy-ion beams. Aberrations were scored in chromosomes 1, 2, and 4 following fluorescence in situ hybridization. The fraction of aberrant lymphocytes has been evaluated as a function of the dose at the sample position, and of the fluence of primary 56Fe ions hitting the shield. The influence of shield thickness on the action cross-section for the induction of exchange-type aberrations has been analyzed, and the dose average-LET measured as a function of the shield thickness. These preliminary results prove that the effectiveness of heavy ions is modified by shielding, and the biological damage is dependent upon shield thickness and material., (c2004 COSPAR. Published by Elsevier Ltd. All rights reserved.)