Magnetic field effects on the nanoscopic cluster-size distribution for therapeutic proton beams

Protons interact in a cellular environment producing a large spectrum of δ-electrons, many with energies of less than 10 keV. Low energy δ-electrons have high-LET and as such are extremely biologically effective, causing cell kill through interactions with DNA and the surrounding molecules. Studies have shown that a magnetic field can affect the path of secondary electrons with energies down to 1 keV. Thus, a magnetic field may also influence the nanoscopic spatial distribution of δ-electrons. Multiple ionisations occurring in close proximity on a DNA scale increase the chance of biological damage for an absorbed dose. Spatial redistribution of δ-electron tracks from a magnetic field may increase local clustering of DNA damage and result in an enhanced biological effect. Previous studies have found a magnetic field to enhance the biological effects of radiation. Further investigation into these effects is relevant for consideration in MRI-guided radiotherapy and also as a possible means of increasing the therapeutic ratio in radiotherapy techniques. Geant4 Monte Carlo was used to study the nanoscopic spatial distribution of δ-electron tracks produced from proton irradiation to determine whether a magnetic field will cause an increase in clustered damage on a DNA scale. This study found no evidence that a transverse magnetic field applied during proton irradiation causes spatial redistribution of δ-electron tracks as measurable by a change nanoscopic cluster size. From this study we can infer that the experimentally observed enhancement of radiobiological effectiveness produced by a magnetic field is due to a reason other than the spatial redistribution of delta-electrons.