When using the CyberKnife® (Accuray Incorporated, Sunnyvale, CA) Image-Guided Stereotactic Radiosurgery (SRS) System to treat soft-tissue tumors in anatomic sites other than intracranial or spinal locations - such as in the lung, liver, kidney, prostate, and pancreas - fiducial placement in or close to the tumors is necessary to assist patient alignment and target tracking for precise treatment delivery. Under the assumption of rigid transformation, at least three fiducial markers are required to obtain six-degreesof-freedom transformation parameters, i.e., three translations and three rotations. However, in most cases, soft tissue is highly deformable and non-rigid. This results in three possible scenarios: 1) the rigid body criteria fail and the rotational transformation cannot be obtained, 2) the tumor deformation results in unreliable computed rotational information, and 3) even when the fiducial array meets the rigid body criteria, the orientation of the tumor often has poor correlation with the global body orientation and thus results in dosimetric deviation.
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