Effect of external magnetic fields on biological effectiveness of proton beams

Background and Purpose The purpose is to verify experimentally whether application of magnetic fields longitudinal and perpendicular to a proton beam alters the biological effectiveness of the radiation. Methods and Materials Proton beams with linear energy transfer (LET) of 1.1 and 3.3 keV/μm were irradiated onto human cancer and normal cells under the longitudinal (perpendicular) magnetic field of BL (BP) = 0, 0.3, or 0.6 T. Cell survival curves were constructed to evaluate the effects of the magnetic fields on the biological effectiveness. The ratio of dose that would result in a survival fraction of 10% without the magnetic field Dwo to the dose with the magnetic field Dw, R10 = Dwo / Dw, was determined for each cell line and magnetic field. Results For cancer cells exposed to the 1.1- (3.3-) keV/μm proton beams, R10s were increased to 1.10±0.07 (1.11±0.07) and 1.11±0.07 (1.12±0.07) by the longitudinal magnetic fields of BL = 0.3 and 0.6 T, respectively. For normal cells, R10s were increased to 1.13±0.06 (1.17±0.06) and 1.17±0.06 (1.30±0.06) by the BLs. In contrast, R10s were not changed significantly from 1 by the perpendicular magnetic fields of BP = 0.3 and 0.6 T for both cancer and normal cells exposed to 1.1- and 3.3-keV/μm proton beams. Conclusions The biological effectiveness of proton beams was significantly enhanced by the longitudinal magnetic fields of BL = 0.3 and 0.6 T, while the biological effectiveness was not altered by the perpendicular magnetic fields of the same strengths. This enhancement effect should be taken into account in MRI guided proton therapy with a longitudinal magnetic field.