Showing papers by "Takehiro Tomitani published in 2005"

Quantitative comparison of suitability of various beams for range monitoring with induced β+ activity in hadron therapy

Abstract: In radiation therapy with hadron beams, it is important to evaluate the range of incident ions and the deposited dose distribution in a patient body for the effective utilization of such properties as the dose concentration and the biological effect around the Bragg peak. However, there is some ambiguity in determining this range because of a conversion error from the x-ray CT number to the charged particle range. This is because the CT number is related to x-ray absorption coefficients, while the ion range is determined by the electron density of the substance. Using positron emitters produced in the patient body through fragmentation reactions during the irradiation has been proposed to overcome this problem. The activity distribution in the patient body can be deduced by detecting pairs of annihilation gamma rays emitted from the positron emitters, and information about the range of incident ions can be obtained. In this paper, we propose a quantitative comparison method to evaluate the mean range of incident ions and monitor the activity distribution related to the deposited dose distribution. The effectiveness of the method was demonstrated by evaluating the range of incident ions using the maximum likelihood estimation (MLE) method and Fisher's information was calculated under realistic conditions for irradiations with several kinds of ions. From the calculated Fisher's information, we compared the relative advantages of initial beams to determine the range of incident ions. The 16O irradiation gave the most information among the stable heavy ions when we measured the induced activity for 500 s and 60 s just after the irradiation. Therefore, under these conditions, we concluded that the 16O beam was the optimum beam to monitor the activity distribution and to evaluate the range. On the other hand, if the positron emitters were injected directly as a therapeutic beam, the 15O irradiation gave the most information. Although the relative advantages of initial beams as well as the measured activity distributions slightly varied according to the measurement conditions, comparisons could be made for different conditions by using Fisher's information.

Enhanced efficiency in cell killing at the penetration depths around the Bragg peak of a radioactive 9C-ion beam

Abstract: Purpose: To evaluate the potential importance of radioactive 9 C-ion beam in cancer radiotherapy. Methods and Materials: Human salivary gland (HSG) cells were exposed to a double-radiation-source 9 C beam at different depths around the Bragg peak. Cell survival fraction was determined by standard clonogenic assay. For comparison, the same experiment was conducted for a therapeutic 12 C beam. To determine relative biologic effectiveness (RBE) values, HSG cells were also irradiated with 60 Co γ-rays of fractionation scheme as the reference. Results: The 9 C beam was more efficient in cell killing at the depths around its Bragg peak than was the 12 C beam, which corresponded to the 9 C-ion stopping region and where delayed low-energy particles were emitted. The RBE value at 50% survival level for the 9 C beam varied from 1.38 to 4.23. Compared with the 12 C beam, the RBE values for the 9 C beam were always higher; an increase in RBE by a factor of up to 1.87 has been observed at the depths distal to the Bragg peak. Conclusion: The potential advantage of radioactive 9 C-ion beam in cancer therapy has been revealed at low dose rate in comparison with a therapeutic 12 C beam. This observation, however, remains to be investigated at therapeutic dose rates in the future.

A versatile control system for irradiation and measurement for secondary beam experiments in a heavy ion accelerator, HIMAC

Abstract: For spot-scanning irradiation, we have developed a new control system in the experiments involving secondary beams of short half-lives. Fast beam switching and spot-by-spot dose monitoring were incorporated into this system. The control system can interface with various instruments, perform sequencing of measurements, control the motion of instruments and data acquisition. Synchronized communications with the period of beam spills in the LAN are introduced in the system to avoid the conflicts among computer communications. As an example of measurement, the results of dose distribution in 3D spot-scanning irradiation are presented.

Simulation for position determination of distal and proximal edges for SOBP irradiation in hadron therapy by using the maximum likelihood estimation method

Abstract: In radiation therapy with hadron beams, conformal irradiation to a tumour can be achieved by using the properties of incident ions such as the high dose concentration around the Bragg peak. For the effective utilization of such properties, it is necessary to evaluate the volume irradiated with hadron beams and the deposited dose distribution in a patient's body. Several methods have been proposed for this purpose, one of which uses the positron emitters generated through fragmentation reactions between incident ions and target nuclei. In the previous paper, we showed that the maximum likelihood estimation (MLE) method could be applicable to the estimation of beam end-point from the measured positron emitting activity distribution for mono-energetic beam irradiations. In a practical treatment, a spread-out Bragg peak (SOBP) beam is used to achieve a uniform biological dose distribution in the whole target volume. Therefore, in the present paper, we proposed to extend the MLE method to estimations of the position of the distal and proximal edges of the SOBP from the detected annihilation gamma ray distribution. We confirmed the effectiveness of the method by means of simulations. Although polyethylene was adopted as a substitute for a soft tissue target in validating the method, the proposed method is equally applicable to general cases, provided that the reaction cross sections between the incident ions and the target nuclei are known. The relative advantage of incident beam species to determine the position of the distal and the proximal edges was compared. Furthermore, we ascertained the validity of applying the MLE method to determinations of the position of the distal and the proximal edges of an SOBP by simulations and we gave a physical explanation of the distal and the proximal information.