Motion tracking using MV-fluoroscopy

For radiotherapy of organs influenced by respiratory motion using a scanned particle beam [1] rescanning [2], gating [3], and beam tracking [4] have been proposed. For gating and especially for beam tracking a high tumor conformity of the applied dose distribution can be achieved. One requirement to reach this goal is precise motion monitoring during treatment delivery and investigation of tumor motion before treatment to allow a suitable treatment planning. To date, most of the patients with tumors in moving organs (e.g. lung, liver and pancreas) are treated with MVphoton irradiation. Precise information of the tumor motion during treatment delivery can be determined by orthogonal kV x-ray fluoroscopy with the drawback of administering additional radiation dose to the patient. As an alternative with less spatial information but without additional dose MV-fluoroscopy can be used by detecting the irradiation exiting the patient during normal treatment [5]. To prepare data acquisition for patients an initial phantom study was performed. Using the SIEMENS ARTISTE photon accelerator at the Kopfklinik Heidelberg MVfluorocsopic images of a sliding table were taken. This sliding table is able to move sinusoidally with an amplitude of 1.5–2.5 cm and a period of 2–5 s. A few little screws were used as fiducial markers. The phantom was irradiated by a 20 × 20 cm field of 6 MV photons and the transmitting irradiation was detected with the ARTISTE’s flat panel detector (EPID s. fig. 2). Amplitude and period were varied. For data analysis software has been developed which extracts the frames from the fluoroscopy data and tracks within a predefined region of interest the marker in each frame. Figure 3 shows as an example four frames of a fluoroscopic image sequence. One can see that the code is able to track the marker (darker circle) which is true for all measured data. An example of an extracted trajectory is shown in figure 1.