Dosimetry by Means of In-Beam PET with RI Beam Irradiation

In situ visualization of deposited dose distribution is necessary to exploit the advantages of Carbon-ion therapy. Therefore we are developing the world's first, open-type PET "OpenPET" to verify the field irradiated. In addition, a method of utilizing activity measurement in the target irradiated with the beam with positron-emitting radioisotopes (RI) such as C-11 and C-10 has been proposed. This method has advantage in the amount of activity as well as direct visualization of primary particles themselves, compared with the irradiation with stable nuclei C-12 beam. With the RI beam, however, the profile of the dose was still different from that of the activity. Therefore interpretation from activity distribution to dose distribution is necessary to confirm the irradiation field precisely. In this paper, we developed a method of estimating the dose distribution from PET measurements. To utilize the activity measurement of the target, we used the code which calculated the activation distribution and dose distribution taking initial beam energy as a free parameter. Then the maximum likelihood parameter of the initial energy was determined by comparing the measured and the calculated distributions. By using the parameter determined, the dose distribution in the target was calculated as the estimated distribution for the actual one. The uniform PNEVIA target was irradiated with C-11 beam for 10 s. The target was measured from the time beam irradiation started and to 60 s after finishing the irradiation by using a small single-ring OpenPET prototype developed for a proof-of-concept of the in-beam monitoring for charged particle therapy. As a result, the dose distribution, which was originally different from primary particles distribution, was successfully estimated. The peak depth of the estimated dose distribution was in good agreement with the measured dose peak.