C. H. Choi1,2, J. I. Kim1,2, H. Park2, S. H. Heo3, S. Y. Heo4, M. Choi5, and J. Y. Shin1; 1Institute of Radiation Medicine, Seoul National University Medical Research Center, Seoul, Korea, Republic of (South), 2Department of Radiation Oncology, Seoul National University Hospital, Seoul, Korea, Republic of (South), 3PAPRICALAB CO.,LTD, Seoul, Korea, Republic of (South), 4Papricalab Co.,ltd, Seoul, Korea, Republic Of, 5Papricalab Co., Ltd, Seoul, Korea, Republic of (South)
Purpose/Objective(s): One of the most common diseases in those over 65 in Korea is osteoarthritis (OA) with incident rate approximating 37.8%. Low dose radiotherapy (LDRT) have been reported as effective to cure the disease but there is not available treatment machine in current hospital. The aim of this study was to investigate the optimal design of the dedicated machine to develop the clinically available machine. Materials/
Methods: The GEometry ANd Tracking 4 tool-kit was used to define the optimal geometry of each component inside the dedicated RT machine for the OA. It consisted of radiation isotope (source), parallel-hole collimator and housing that are filled with tungsten, and other mechanical components. The OA treatment can be proceeded by delivering therapeutic beam with a single-arc rotation of the source. Firstly, radionuclide to be used in the machine was decided among those being used in current radiotherapy by comparing their physical characteristics. In the second, thickness of the collimator and housing was investigated by simulating gamma transmission for the selected radionuclide according to different tungsten thicknesses from 0 to 4 cm. Subsequently, dimensions of the source were decided in terms of its diameter and height. The rotation interval of the source was also optimized. Both were carried out by comparing dose distributions in cylindrical water phantom with diameter of 25 cm and height of 20 cm according to different source dimensions and rotation intervals. Results: 60Co was considered as the most appropriate for the LDRT among radionuclides investigated in this study. It has higher energy than other radionuclides, such as 192Ir, and 137Cs as well as the longest half-life except the 137Cs. It also has sufficient specific activity to deliver enough dose to the treatment region. The gamma rays from 60Co showed transmission rate less than 5% when tungsten thickness was larger than 3 cm, which was the optimal for thickness of the collimator and housing. The dose distributions in the water phantom showed that the source diameter should be 27 cm rather than 50 cm. The dose distributions showed similar dose coverage while larger diameter results in higher source activity. This requires thicker radiation shields for the radiation safety management. For the case of the source height to decide dose coverage along superior-inferior direction, the dose distribution should have steep fall-off as possible. It was due to that broader fall-off region could produce hot or cold spots at the field junctions. Therefore, height of the source was decided as 1 cm. The dose distribution was almost identical when the rotation interval was smaller than 60 degrees. However, all of the results showed dose uniformity less than 10% at central region. Conclusion: The results of this study is expected to contribute development of the OA treatment machine, which can improve treatment effect of the OA. Further study will be carried out on manufacturing its hardware and developing its treatment planning algorithm.
