An external perpendicular magnetic field does not influence survival and DNA damage after proton and carbon ion irradiation in human cancer cells

Abstract

Background and purpose: Magnetic field effects on the radiobiological effectiveness during treatment of magnetic resonance (MRI) guided particle therapy are being debated. This study aims at assessing the influence of a perpendicular magnetic field on the biological effects in two human cancer cell lines irradiated with proton or carbon ions.

Methods and materials: In vitro cell irradiations were performed in water inside a perpendicular magnetic field of 0 and 1T for both protons and carbon ions. Samples were located in the center of a spread-out Bragg peak at 8cm water equivalent depth with a dose averaged linear energy transfer (LET_d) of 4.2 or 83.4keV/μm for protons and carbon ions, respectively. Physical dose levels of 0, 0.5, 1, 2, 4 and 6Gy were employed. The irradiation field was shifted and laterally enlarged, to compensate for the beam deflection due to the magnetic field and ensure consistent and homogenous irradiations of the flasks. The human cancer cell lines SKMel (Melanoma) and SW1353 (chondrosarcoma) were selected which represent a high and a low (α/β)_x ratio cell type. Cell survival curves were generated applying a linear-quadratic curve fit. DNA damage and DNA damage clearance were assessed via γH2AX foci quantification at 1 and 24h post radiation treatment.

Results: Without a magnetic field, RBE₁₀ values of 1.04±0.03 (SW1353) and 1.51±0.06 (SKMel) as well as RBE₈₀ values of 0.93±0.15 (SW1353) and 2.28±0.40 (SKMel) were calculated for protons. Carbon treatments yielded RBE₁₀ values of 1.68±0.04 (SW1353) and 2.30±0.07 (SKMel) and RBE₈₀ values of 2.19±0.24 (SW1353) and 4.06±0.33 (SKMel). For a field strength of B=1T, RBE₁₀ values of 1.06±0.03 (SW1353) and 1.47±0.06 (SKMel) resulted from protons, while RBE₁₀ values of 1.70±0.05 (SW1353) and 2.37±0.08 (SKMel) were obtained for carbon ions. RBE₈₀ values were calculated to be 1.06±0.12 (SW1353) and 2.33±0.40 (SKMel) following protons and 2.13±0.25 (SW1353) and 4.29±0.35 (SKMel) following carbon treatments. Substantially increased γH2AX foci per nucleus were found in both cell lines 1h after radiation with both ion species. At the 24h time point only carbon treated samples of both cell lines showed increased γH2AX levels. The presence of the magnetic field did neither influence the survival parameters of either cell line, nor initial DNA damage and DNA damage clearance.

Conclusions: Applying a perpendicular magnetic field did not influence the cell survival, DNA repair, nor the biological effectiveness of protons or carbon ions in two human cancer cell lines.

Keywords: Biological effects; In-vitro; MR guided particle therapy; MR guided proton therapy; MRgPT; Magnetic fields.