title: Quantifying rigid and nonrigid motion of liver tumors during stereotactic body radiation therapy.
abstract: OBJECTIVE
  To quantify rigid and nonrigid motion of liver tumors using reconstructed 3-dimensional (3D) fiducials from stereo imaging during CyberKnife-based stereotactic body radiation therapy (SBRT).
  
  
  METHODS
  Twenty-three liver patients treated with 3 fractions of SBRT were used in this study. After 2 orthogonal kilovoltage images were taken during treatment, the 3D locations of the fiducials were generated by the CyberKnife system and validated using geometric derivations. A total of 4824 pairs of kilovoltage images from start to end of treatment were analyzed. For rigid motion, the rotational angles and translational shifts were reported by aligning 3D fiducial groups from different image pairs, using least-squares fitting. For nonrigid motion, we quantified interfractional tumor volume variations by using the proportional volume derived from the fiducials, which correlates to the sum of interfiducial distances. The individual fiducial displacements were also reported (1) after rigid corrections and (2) without angle corrections.
  
  
  RESULTS
  The proportional volume derived by the fiducials demonstrated a volume-increasing trend in the second (101.9% ± 3.6%) and third (101.0 ± 5.9%) fractions among most patients, possibly due to radiation-induced edema. For all patients, the translational shifts in left-right, anteroposterior, and superoinferior directions were 2.1 ± 2.3 mm, 2.9 ± 2.8 mm, and 6.4 ± 5.5 mm, respectively. The greatest translational shifts occurred in the superoinferior direction, likely due to respiratory motion from the diaphragm. The rotational angles in roll, pitch, and yaw were 1.2° ± 1.8°, 1.8° ± 2.4°, and 1.7° ± 2.1°, respectively. The 3D individual fiducial displacements with rigid corrections were 0.2 ± 0.2 mm and increased to 0.5 ± 0.4 mm without rotational corrections.
  
  
  CONCLUSIONS
  Accurate 3D locations of internal fiducials can be reconstructed from stereo imaging during treatment. As an effective surrogate to tumor motion, fiducials provide a close estimation of both rigid and nonrigid motion of liver tumors. The reported displacements could be further utilized for tumor margin definition and motion management in conventional linear accelerator-based liver SBRT.
authors.affiliation: Department of Radiation Oncology, MD Anderson Cancer Center at Cooper, Camden, New Jersey. Electronic address: xuqianyi@gmail.com.
authors.forename: Qianyi
authors.initials: Q
authors.lastname: Xu
authors.affiliation: Department of Radiation Oncology, MD Anderson Cancer Center at Cooper, Camden, New Jersey.
authors.forename: George
authors.initials: G
authors.lastname: Hanna
authors.affiliation: Department of Radiation Oncology, Holy Redeemer Hospital, Bott Cancer Center, Meadowbrook, Pennsylvania.
authors.forename: Jimm
authors.initials: J
authors.lastname: Grimm
authors.affiliation: Department of Radiation Oncology, MD Anderson Cancer Center at Cooper, Camden, New Jersey.
authors.forename: Gregory
authors.initials: G
authors.lastname: Kubicek
authors.affiliation: Department of Radiation Oncology, First Radiation and Oncology Group, Jacksonville, Florida.
authors.forename: Niraj
authors.initials: N
authors.lastname: Pahlajani
authors.affiliation: Department of Radiation Oncology, MD Anderson Cancer Center at Cooper, Camden, New Jersey.
authors.forename: Sucha
authors.initials: S
authors.lastname: Asbell
authors.affiliation: Department of Radiation Oncology, Fox Chase Cancer Center, Philadelphia, Pennsylvania.
authors.forename: Jiajin
authors.initials: J
authors.lastname: Fan
authors.affiliation: Department of Radiation Oncology, MD Anderson Cancer Center at Cooper, Camden, New Jersey.
authors.forename: Yan
authors.initials: Y
authors.lastname: Chen
authors.affiliation: Department of Radiation Oncology, MD Anderson Cancer Center at Cooper, Camden, New Jersey.
authors.forename: Tamara
authors.initials: T
authors.lastname: LaCouture
mesh_terms: D000465:Algorithms; D019583:Dose Fractionation, Radiation; D057918:Fiducial Markers; D006801:Humans; D021621:Imaging, Three-Dimensional / Q000379:methods*; D008099:Liver / Q000000981:diagnostic imaging*; D008113:Liver Neoplasms / Q000000981:diagnostic imaging* / Q000601:surgery*; D009038:Motion; D009068:Movement; D011859:Radiography; D016634:Radiosurgery / Q000295:instrumentation / Q000379:methods*; D012399:Rotation