인체내 양성자 빔 비정 확인을 위한 즉발감마선 측정시스템 개발

Abstract

In radiation therapy, most research has focused on reducing unnecessary radiation dose to normal tissues and critical organs around the target tumor volume. Proton therapy is considered to be one of the most promising radiation therapy methods with its physical characteristics in the dose distribution, delivering most of the dose just before protons come to rest at the so-named Bragg peak; that is, proton therapy allows for a very high radiation dose to the tumor volume, effectively sparing adjacent critical organs. However, the uncertainty in the location of the Bragg peak, coming from not only the uncertainty in the beam delivery system and the treatment planning method but also anatomical changes and organ motions of a patient, could be a critical problem in proton therapy. In spite of the importance of the in vivo dose verification to prevent the misapplication of the Bragg peak and to guarantee both successful treatment and patient safety, there is no practical methodology to monitor the in vivo dose distribution, only a few attempts have been made so far. The present dissertation suggests the prompt gamma measurement method for monitoring of the in vivo proton dose distribution during treatment. As a key part of the process of establishing the utility of this method, the verification of the clear relationship between the prompt gamma distribution and the proton dose distribution was accomplished by means of Monte Carlo simulations and experimental measurements. First, the physical properties of prompt gammas were investigated on the basis of cross-section data and Monte Carlo simulations. Prompt gammas are generated mainly from proton-induced nuclear interactions, and then emitted isotropically in less than 10-9 sec at energies up to 10 MeV. Simulation results for the prompt gamma yield of the major elements of a human body show that within the optimal energy range of 4-10 MeV the highest number of prompt gammas is generated from oxygen, whereas over the entire energy range, it is from calcium. Second, to verify the relationship between the proton dose distribution and the prompt gamma distribution, the present study developed a proof-of-principle measurement system (the PGS system) employing a scanning process. The first-time experimental study verified not only that prompt gammas can be measured during treatment, but also that their distribution has a clear relationship with the proton dose distribution for therapeutic proton beams. Third, for the clinical application, a small array-type prompt gamma measurement system for use without the problematic scanning process was designed, and its optimal dimensions for effective reduction of background gammas were determined (by Monte Carlo simulations): 3-mm scintillation thickness; 2-mm slit width; 2-mm septal thickness; 150-mm slit length. To accelerate the simulations, the present study employed the parameterized source term that improved the calculation speed by a factor of 300. Finally, the performance of the array-type measurement system for clinical applications was evaluated with the test measurement system composed of a multi-slit collimation system, a CsI(Tl) scintillation detector, and a precise motion system. To quantitatively determine the location of the distal dose edge from the prompt gamma distribution, a methodology based on a sigmoidal curve fitting is here proposed, and this methodology proves that the distal dose edge could be accurately determined within about 4 mm for therapeutic proton beams. Additionally, the phantom effect on the prompt gamma distribution and the analysis of background gammas are studied by Monte Carlo simulations.