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

Purpose: 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. Methods and materials: Human cancer and normal cell lines were exposed to low (12 keV/μm) and high (50 keV/μm) linear energy transfer (LET) carbon-ion beams under the longitudinal magnetic fields of B// = 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% (D10s) were determined for each cell line and magnetic field. Results: For cancer cells exposed to the low (high)-LET beams, D10 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 D10 was observed for higher magnetic fields. For normal cells, decreases in D10 of comparable magnitudes were observed by applying the magnetic fields. Conclusions: Significant decreases in D10, i.e. significant enhancements of the biological effectiveness, were observed in both cancer and normal cells by applying longitudinal magnetic fields of B// ≥ 0.1 T. These effects were enhanced with LET. Further studies are required to figure out the mechanism underlying the observed results.