Development of the real-time four-dimensional cone beam CT reconstruction technique for interactive radiotherapy

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We have proposed a novel method for four-dimensional (4D) cone-beam computed tomography (CBCT) reconstruction with real-time tumor tracking technique. In the real-time tumor tracking, a fiducial marker inserted near the tumor is detected in the orthogonal fluoroscopic images. The three-dimensional position of the marker is calculated every 33 msec with an accuracy of 1 mm. According to the position of the marker, projection images can be classified into some phases. Four dimensional CBCT reconstruction is therefore available in each phase. Feasibility of the 4D CBCT reconstruction was evaluated through the experience using the testing machine, which has the same imaging devices in the proton therapy system in Hokkaido University Hospital. Additionally, characteristics of intrafractional baseline shift of tumor motion were analyzed to investigate efficacy of the 4D CBCT reconstruction method. Through the study, the feasibility and efficacy of the 4D CBCT reconstruction was confirmed.