Particle induced X-ray emission and ion dose distribution in a biological micro-beam: Geant4 Monte Carlo simulations

The goal of a microbeam is to deliver a highly localized and small dose to the biological medium. This can be achieved by using a set of collimators that confine the charged particle beam to a very small spatial area of the order of microns in diameter. By using a system that combines an appropriate beam detection method that signals to a beam shut-down mechanism, a predetermined and counted number of energetic particles can be delivered to targeted biological cells. Since the shutter and the collimators block a significant proportion of the beam, there is a probability of the production of low energy X-rays and secondary electrons through interactions with the beam. There is little information in the biological microbeam literature on potential X-ray production. We therefore used Monte Carlo simulations to investigate the potential production of particle-induced X-rays and secondary electrons in the collimation system (which is predominantly made of tungsten) and the subsequent possible effects on the total absorbed dose delivered to the biological medium.We found, through the simulation, no evidence of the escape of X-rays or secondary electrons from the collimation system for proton energies up to 3MeV as we found that the thickness of the collimators is sufficient to reabsorb all of the generated low energy X-rays and secondary electrons. However, if the proton energy exceeds 3MeV our simulations suggest that 10keV X-rays can escape the collimator and expose the overlying layer of cells and medium. If the proton energy is further increased to 4.5MeV or beyond, the collimator can become a significant source of 10keV and 59keV X-rays. These additional radiation fields could have effects on cells and these results should be verified through experimental measurement. We suggest that researchers using biological microbeams at higher energies need to be aware that cells may be exposed to a mixed LET radiation field and be careful in their interpretation of data.Two other factors can affect the pattern of dose deposition in the biological medium: the phase space distribution of the beam particles and the production of secondary electrons (known as δ-rays). We investigated this by projecting simulated particles oriented at small angles with the beam axis. For lower fluence (2.6×104 protons mm−2), we determined that despite only the target cell being assumed to be hit by the particle beam, some significant level of radiation dose was, in fact, delivered to the adjacent cells. This was most probably due to secondary electrons. The simulation showed that two of the cells adjacent to the target cell received 42% and 5% of the dose delivered to the target cell per proton. When the incident fluence on the collimator was increased to 1.3×106 protons mm−2, it was observed that a significant number of protons deflected from the collimator spread into an area of 4340μm2. This is a significant spread when compared to the target area of 25μm2. The maximum number of particles that were delivered off-target was 25% of the particles delivered to the target cell. This equates to a probability of delivering 1 particle anywhere in an area of 4340μm2 for every 4 particles delivered to the target cell. This result has significant implications. Results of this work warrant a further investigation because if these results can be re validated, perhaps experimentally or through another simulation code, then they may have significant implications on the interpretation of published data from biological microbeam experiments.