Introduction: One of advantage in the use of heavy charged particles for radiotherapy is the high potential for concentrating a uniform prescribed dose to the target volume while minimizing irradiation to surrounding normal tissues. The compensator and collimator statically define a shape of the beam to conform the irradiation filed at each patient. However the target around thorax and abdomen moves physiologically in association with heartbeat, respiration and peristaltic function. These motions cause the decline in the dose concentration to the target volume. Prior discussions about organ motions have mainly focused on inferior-superior movement with respiration. In contrast, the anterior-posterior change of the target depth along the beam path is very important in particle radiotherapy, because the shape end of the beam (Bragg peak) is so sharp as to be matched to the distal edge of the target. In addition, depth variations for particle radiotherapy must be presented in water equivalent path length (WEL) using CT numbers. The purpose of this study is to analyze depth variations of organs from the aspect of respiratory-gated irradiation.\nMethod: A set of sequential CT images was reconstructed from continuous X-ray projection data accumulated by dynamic scanning. At the same time of CT data acquisition, the respiratory signal of patient was measured. The timing of each CT image is related with the phase of respiration waveform. Then organ movements on these CT image are analyzed, and investigated the correlation with respiration waveform.\nResults: In case of liver around diaphragm, the target depth along beam path was remarkably changed depending on the respiratory organ movement. The magnitude of depth variations was comparable to the inferior-superior movement. For example, changes in WEL from the iso-center to the body surfaces of the front and the back were 6.2 mm and 18.9 mm, respectively. These variations were estimated to reduce to less than 2 mm by gated irradiation.In lung case, the correlation between the change in WEL of lung size and the respiratory signal appeared not to be clear. The change in WEL had two frequency components, one was respiration and other was heartbeat. In pancreas case, the position of pancreas itself didn’t move so much. However air cavities in intestine along the beam path changed, and affected the target depth in WEL.World Congress on Medical Physics and Biomedical Engineering 200
