Enhancement of biological effectiveness of carbon-ion beams by applying a longitudinal magnetic field

<b>Purpose:</b> A magnetic field longitudinal to an ion beam will potentially affect the biological effectiveness of the radiation. The purpose of this study is to experimentally verify the significance of such effects. <b>Methods and materials:</b> Human cancer and normal cell lines were exposed to low (12 keV/<i>μ</i>m) and high (50 keV/<i>μ</i>m) linear energy transfer (LET) carbon-ion beams under the longitudinal magnetic fields of <i>B</i>_// = 0, 0.1, 0.2, 0.3, or 0.6 T generated by a solenoid magnet. The effects of the magnetic fields on the biological effectiveness were evaluated by clonogenic cell survival. Doses that would result in a survival fraction of 10% (<i>D</i>₁₀s) were determined for each cell line and magnetic field. <b>Results:</b> For cancer cells exposed to the low (high)-LET beams, <i>D</i>₁₀ decreased from 5.2 (3.1) Gy at 0 T to 4.3 (2.4) Gy at 0.1 T, while no further decrease in <i>D</i>₁₀ was observed for higher magnetic fields. For normal cells, decreases in <i>D</i>₁₀ of comparable magnitudes were observed by applying the magnetic fields. <b>Conclusions:</b> Significant decreases in <i>D</i>₁₀, i.e. significant enhancements of the biological effectiveness, were observed in both cancer and normal cells by applying longitudinal magnetic fields of <i>B</i>_// ≥ 0.1 T. These effects were enhanced with LET. Further studies are required to figure out the mechanism underlying the observed results.