Reconstruction of Proton Image with Ion Recombination Compensation

Pencil Beam Scanning (PBS) of proton therapy is an advanced radiation technique for cancer treatment. Due to its effective planning and control for particle-beam, the narrow pulse train and extremely high radiation dose delivering becomes more practical toward the development of flash delivery of proton beam. Benefits due to flash therapy are reducing uncertainty caused by the movement of breath and depress damage to healthy parts near tumor. But, flash therapy usually applies high proton current density to achieve high dose rate. This causes underestimation in dose measurement, because of the recombination effect of the electron-ion pairs inside ionization chamber that leads to the received electronic signal underestimated. The study proposed a novel algorithm to reconstruct the 2D image of the flash delivery of proton/particle beam. The Boag's model of high energy physics is adopted for the estimation of the charge collecting efficiency. Based on the assumption of spatial symmetry and numerical methods, the proton beam is expected to be aligned. Consequently, in preliminary simulation, the flash proton radiation could be converted into a two-dimensional image, and the effective spatial resolution could be achieved to 1 mm2. The simulation also revealed the charge collection efficiency will affect the proposed image reconstruction algorithm. The results matched the IAEA TRS-398 dosimetry protocol when the charge collection efficiency is greater than 0.7. It shows the proposed novel algorithm of 2D image reconstruction could be valuable for the assistance for the medical instrument quality assurance.