title: Feasibility study on image guided patient positioning for stereotactic body radiation therapy of liver malignancies guided by liver motion.
abstract: BACKGROUND
  Fiducial markers are the superior method to compensate for interfractional motion in liver SBRT. However this method is invasive and thereby limits its application range. In this retrospective study, the compensation method for the interfractional motion using fiducial markers (gold standard) was compared to a new non-invasive approach, which does rely on the organ motion of the liver and the relative tumor position within this volume.
  
  
  METHODS
  We analyzed six patients (3 m, 3f) treated with SBRT in 2014. After fiducial marker implantation, all patients received a treatment CT (free breathing, without abdominal compression) and a 4D-CT (consisting of 10 respiratory phases). For all patients the gross tumor volumes (GTVs), internal target volume (ITV), planning target volume (PTV), internal marker target volumes (IMTVs) and the internal liver target volume (ILTV) were delineated based on the CT and 4D-CT images. CBCT imaging was used for the standard treatment setup based on the fiducial markers. According to the patient coordinates the 3 translational compensation values (t x , t y , t z ) for the interfractional motion were calculated by matching the blurred fiducial markers with the corresponding IMTV structures. 4 observers were requested to recalculate the translational compensation values for each CBCT (31) based on the ILTV structures. The differences of the translational compensation values between the IMTV and ILTV approach were analyzed.
  
  
  RESULTS
  The magnitude of the mean absolute 3D registration error with regard to the gold standard overall patients and observers was 0.50 cm ± 0.28 cm. Individual registration errors up to 1.3 cm were observed. There was no significant overall linear correlation between the respiratory motion and the registration error of the ILTV approach.
  
  
  CONCLUSIONS
  Two different methods to calculate the translational compensation values for interfractional motion in stereotactic liver therapy were evaluated. The registration accuracy of the ILTV approach is mainly limited by the non-rigid behavior of the liver and the individual registration experience of the observer. The ILTV approach lacks the accuracy that would be desired for stereotactic radiotherapy of the liver.
authors.affiliation: Department of Radiation Oncology, LMU Munich, 81377, Munich, Germany. Christian.Heinz@med.uni-muenchen.de.
authors.forename: Christian
authors.initials: C
authors.lastname: Heinz
authors.affiliation: Department of Radiation Oncology, LMU Munich, 81377, Munich, Germany.
authors.forename: Sabine
authors.initials: S
authors.lastname: Gerum
authors.affiliation: Department of Radiation Oncology, LMU Munich, 81377, Munich, Germany.
authors.forename: Philipp
authors.initials: P
authors.lastname: Freislederer
authors.affiliation: Department of Radiation Oncology, LMU Munich, 81377, Munich, Germany.
authors.forename: Ute
authors.initials: U
authors.lastname: Ganswindt
authors.affiliation: Department of Radiation Oncology, LMU Munich, 81377, Munich, Germany.
authors.forename: Falk
authors.initials: F
authors.lastname: Roeder
authors.affiliation: Department of Radiation Oncology, LMU Munich, 81377, Munich, Germany.
authors.forename: Stefanie
authors.initials: S
authors.lastname: Corradini
authors.affiliation: Department of Radiation Oncology, LMU Munich, 81377, Munich, Germany.
authors.forename: Claus
authors.initials: C
authors.lastname: Belka
authors.affiliation: Department of Radiation Oncology, LMU Munich, 81377, Munich, Germany.
authors.forename: Maximilian
authors.initials: M
authors.lastname: Niyazi
mesh_terms: D006528:Carcinoma, Hepatocellular / Q000378:metabolism / Q000473:pathology / Q000601:surgery*; D054893:Cone-Beam Computed Tomography; D005240:Feasibility Studies; D057918:Fiducial Markers; D005500:Follow-Up Studies; D056973:Four-Dimensional Computed Tomography; D006801:Humans; D008113:Liver Neoplasms / Q000473:pathology / Q000601:surgery*; D009038:Motion; D056888:Patient Positioning; D011379:Prognosis; D016634:Radiosurgery; D012119:Respiration; D012189:Retrospective Studies; D025321:Surgery, Computer-Assisted / Q000379:methods*
keywords: CBCT; Fiducial marker; Liver; Patient positioning; Stereotactic body radiation therapy