Purpose: Radiotherapy machines deliver intrinsically divergent x-ray radiation, requiring collimation to a diseased region. The penumbra and power efficiency limit the minimal size of the field to ~5 millimeters for conventional photon and charged particle beams. Therefore, many medical conditions that may be responsive to local radiation cannot be effectively treated because of their small size and susceptibility to intolerable collateral damages to surrounding critical organs. To overcome the fundamental issues of collimated radiation, we are developing a focused kV technique based on novel usage of polycapillary x-ray lenses, which can focus x-ray beams to <0.2 mm in diameter with a dose peak well below the skin region.
Methods: Unlike diffraction lenses, polycapillary optics are broadband, based on total external reflection. We simulated the optics using Monte Carlo (MC)-based ray tracing. Multi-arc scanning was simulated to deliver highly conformal radiation treatment to 3 types of small targets of a few millimeters size. Because the lens output focal spot is smaller than the x-ray tube focal spot, we also used the lens output focus as a virtual x-ray source to acquire phantom and rat head x-ray and CBCT images (60 kVp) to demonstrate improved resolution.
Results: The simulated beam dosimetric characteristics were verified by EBT3 film measurements within 10%. MC simulated treatments of choroidal melanoma and age-related macular degeneration demonstrated more conformal target coverage and substantially reduced critical structure doses compared to standard treatments. Simulations also demonstrated irradiating 1-2 mm neurofunctional regions in rat heads with sharp penumbras. CBCT of a thin wire showed 33% reduction of the FWHM of a reconstructed wire with vs without lens. CBCT of a rat head could distinguish ~0.3mm fine bony structure with lens, but not without.
Conclusion: Focused kV x-ray technique offers the capability of treating mm size targets and improved x-ray imaging resolution.