Many soft-tissue tumors targeted with extracranial SRS move during respiration. New imaging technologies, motion compensation strategies, and treatment planning algorithms are being developed which enable tracking and treatment of moving tumors in real-time. For this chapter we reviewed the literature to determine known tumor motion amplitudes for lung, liver, and pancreas. Then we analyzed predicted tumor motion for 36 patients and 117 treatment fractions that were previously saved in CyberKnife® (Accuray Incorporated, Sunnyvale, CA) treatment logfiles. These represent 27 tumors in the lung (16 upper lung, 4 middle lung, 7 lower lung) and 9 pancreas patients. For each treatment, the location of the target at end inspiration and end expiration was determined in the patient coordinate system. The origin of the patient coordinate system is at the center of mass of the fiducials as marked on the simulation CT, +x is patient inferior, +y patient left, and +z anterior in a right-handed coordinate system. The mean and variance of respiratory cycle extrema positions were calculated using a program written in MatLab code. Observed motion ranges for all sites except pancreas agree very well with the literature. The largest motion amplitudes of up to 38.7 mm were observed in the lower lung. Twenty-five percent of tumors in the upper lung could have been treated without Synchrony® (Accuray Incorporated, Sunnyvale, CA) with a PTV margin of 2 mm, because the uncertainty is in the range of the technical tracking accuracy of Synchrony of 1.5 mm. Possible causes of large fluctuations around the mean motion could be fiducial tracking errors or irregular breathing. We concluded that a subset of all patients could have been treated using skeletal structure tracking, rather than implanted fiducials, and a PTV margin in the range of the stated tracking accuracy for Synchrony. Defining meaningful parameters to characterize the effects of free breathing is part of ongoing research, since published data from non-dynamic SBRT is limited to short fluoroscopic studies or Cine-CT. The results can be transferred to other treatment modalities to determine PTV margins in standard external beam treatments as well as defining the PTV in the third dimension for 2D motion compensation .
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