Microbeam irradiators have been developed to study the effects of radiation on single cells and the non-targeted cells around them.[1
] While most of this work has focused on charged particles, interest has developed on the effects of x-rays on this same scale. [4
] We have developed a 5 μm soft x-ray microbeam system base on proton induced x-ray emission (PIXE) that produces an x-ray beam with the photon energy of 4.5 keV (Ti-Kα
) at the Radiological Research Accelerator Facility (RARAF) at Columbia University. This energy is between the typical ultrasoft x-ray systems that operate in the ‘water window’ (280 eV to 500 eV) [4
] and the energies typically used at synchrotrons (5 keV to 100 keV)[5
] and broad beam irradiators of hard x-rays (50 keV and greater).
The long attenuation length (1/e is ~145 μm) of the 4.5 keV Ti-Kα x-rays will also make this an ideal tool for studying effects of localized irradiation of specific targets at depth in a tissue sample. This targeted irradiation is not available with lower energy x-rays as they will be absorbed in the initial layers of a tissues. Particle beams also cannot provide the depth targeting available here as the particles will scatter in the tissue losing their focused properties and irradiate many cells in the area making irradiated to non-irradiated distinctions difficult if not impossible. This x-ray energy, while still have good penetration characteristics for irradiating multiple layers into a tissue, also has enough absorbance in a single cell layer to deliver a required dose in a short period of time unlike higher energies. The single layer dose will have an even distribution across the thickness of a cell, unlike carbon x-rays which have a dose variation of nearly 30% across a cell thickness.
Soft x-rays are a well characterized form of radiation that causes specific types of damage to DNA and cellular structures.[6
] This characterization has happened in broad beam irradiations of many cells simultaneously and, in the past ten years, has come under single cell studies with the development of soft x-ray microbeams.[4
] The types of damage seen from soft x-rays are similar to that of high-LET radiation but limited to localized regions. Most high-LET radiation sources are heavy energetic particles with ionization tracks that are several micrometers wide and typically 10’s of micrometers long, usually traversing a whole cell creating a distribution of damage throughout the cell. Soft x-rays, on the other hand, are typically absorbed and generate a single free electron at the absorption location with a track typically less than a micrometer. These localized effects can have distinct effects on certain aspects of cells while allowing most of the cell to continue without any effect. This soft x-ray microbeam, with photon energy of 4.5 keV, will produce secondary electron tracks of range of ~400 nm presenting an average LET of 12.5 keV/μm. The traditional transition point from low to high LET regimes is 10 keV/μm, thus the PIXE microbeam will be directly probing the effects in this region. Soft x-rays, as they are absorbed, have very little scatter and short secondary electron lengths, allow a specific part of a cell to be irradiated while not irradiating any other part of the cell or its neighbors allowing evaluation of that cell and potential bystander effects in neighboring cells.
More recently, these moderate to soft energy x-rays are being developed into new apparatus for both medical imaging and homeland security.[7
] The different effects of low doses of soft x-rays on specific targets and non-targets will have significant input into the widespread use of these new systems and the potential effect on the population at large from these new sources of radiation.