Dose profiles and x-ray energy optimization for microbeam radiation therapy by high-dose, high resolution dosimetry using Sm-doped fluoroaluminate glass plates and Monte Carlo transport simulation.

Microbeam radiation therapy (MRT) utilizes highly collimated synchrotron generated x-rays to create narrow planes of high dose radiation for the treatment of tumors. Individual microbeams have a typical width of 30-50 µm and are separated by a distance of 200-500 µm. The dose delivered at the center of the beam is lethal to cells in the microbeam path, on the order of hundreds of Grays (Gy). The tissue between each microbeam is spared and helps aid in the repair of adjacent damaged tissue. Radiation interactions within the peak of the microbeam, such as the photoelectric effect and incoherent (atomic Compton) scattering, cause some dose to be delivered to the valley areas adjacent to the microbeams. As the incident x-ray energy is modified, radiation interactions within a material change and affect the probability of interactions, as well as the directionality and energy of ionizing particles (electrons) that deposit energy in the valley regions surrounding the microbeam peaks. It is crucial that the valley dose between microbeams be minimal to maintain the effectiveness of MRT. Using a monochromatic x-ray source with x-ray energies ranging from 30 to 150 keV, a detailed investigation into the effect of incident x-ray energy on the dose profiles of microbeams was performed using samarium doped fluoroaluminate (FA) glass as the medium. All dosimetric measurements were carried out using a purpose-built fluorescence confocal microscope dosimetric technique that used Sm-doped FA glass plates as the irradiated medium. Dose profiles are measured over a very a wide range of x-ray energies at micrometer resolution and dose distribution in the microbeam are mapped. The measured microbeam profiles at different energies are compared with the MCNP6 radiation transport code, a general transport code which can calculate the energy deposition of electrons as they pass through a given material. The experimentally measured distributions can be used to validate the results for electron energy deposition in fluoroaluminate glass. Code validation is necessary for using transport codes in future treatment planning for MRT and other radiation therapies. It is shown that simulated and measured micro beam-profiles are in good agreement, and micrometer level changes can be observed using this high-resolution dosimetry technique. Full width at 10% of the maximum peak (FW@10%) was used to quantify the microbeam width. Experimental measurements on FA glasses and simulations on the dependence of the FW@10% at various energies are in good agreement. Simulations on energy deposited in water indicate that FW@10% reaches a local minimum around energies 140 keV. In addition, variable slit width experiments were carried out at an incident x-ray energy of 100 keV in order to determine the effect of the narrowing slit width on the delivered peak dose. The microbeam width affects the peak dose, which decreases with the width of the microbeam. Experiments suggest that a typical microbeam width for MRT is likely to be between 20-50 µm based on this work.