Application of double scattering method using a dual-ring to formation of irradiation field for proton cancer therapy

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A method of applying the efficient double scattering method using a dual-ring 2nd scatterer to proton cancer therapy has been clarified. Since insertion of beam modifying devices such as a range shifter and a range modulator (a bar ridge filter) modifies the lateral dose distribution, it is impossible to cover the whole depth range used for cancer treatment by a single scatterer combination. It is shown that we can obtain the uniform lateral dose distribution for the whole depth region if the covered depth of irradiation is divided into eight ranges and for each range, the appropriate combination of the first and the second scatterers are used.The method of applying the method to beam with relatively large emittance has been clarified. The six parameters specifying the lateral dose distribution are approximately optimized by changing three free parameters so that those six parameters are near to those which are optimum solution of the zero-emittance case. It is also shown that the dose distribution is rather sensitive to the selection of beam phase space parameters even if the emittance is the same. Namely focused, or diverging, or converging beams will produce different dose distributions.Since the method is sensitive to the beam misalignment, it is necessary to monitor the flatness of the distribution. For that purpose, we manufactured an ionization chamber with four identical collection electrodes placed at four quadrants of a single plane, each of them was electrically isolated from others. We tested the monitor chamber for the pulsed proton beam (5Onsec FWHM) and found that it can be used to monitor the inhomogeneity of less than 1 %.By the research, we clarified that this method can be used for proton cancer therapy effectively.