title: Dosimetric Implications of Residual Tracking Errors During Robotic SBRT of Liver Metastases.
abstract: Although the metric precision of robotic stereotactic body radiation therapy in the presence of breathing motion is widely known, we investigated the dosimetric implications of breathing phase-related residual tracking errors.
  
  
  
  In 24 patients (28 liver metastases) treated with the CyberKnife, we recorded the residual correlation, prediction, and rotational tracking errors from 90 fractions and binned them into 10 breathing phases. The average breathing phase errors were used to shift and rotate the clinical tumor volume (CTV) and planning target volume (PTV) for each phase to calculate a pseudo 4-dimensional error dose distribution for comparison with the original planned dose distribution.
  
  
  
  The median systematic directional correlation, prediction, and absolute aggregate rotation errors were 0.3 mm (range, 0.1-1.3 mm), 0.01 mm (range, 0.00-0.05 mm), and 1.5° (range, 0.4°-2.7°), respectively. Dosimetrically, 44%, 81%, and 92% of all voxels differed by less than 1%, 3%, and 5% of the planned local dose, respectively. The median coverage reduction for the PTV was 1.1% (range in coverage difference, -7.8% to +0.8%), significantly depending on correlation (P=.026) and rotational (P=.005) error. With a 3-mm PTV margin, the median coverage change for the CTV was 0.0% (range, -1.0% to +5.4%), not significantly depending on any investigated parameter. In 42% of patients, the 3-mm margin did not fully compensate for the residual tracking errors, resulting in a CTV coverage reduction of 0.1% to 1.0%.
  
  
  
  For liver tumors treated with robotic stereotactic body radiation therapy, a safety margin of 3 mm is not always sufficient to cover all residual tracking errors. Dosimetrically, this translates into only small CTV coverage reductions.
authors.affiliation: Department for Radiation Oncology, University Medical Center Schleswig-Holstein, Kiel, Germany; Tuen Mun Hospital, Hong Kong, China.
authors.forename: Mark
authors.initials: M
authors.lastname: Chan
authors.affiliation: Department for Radiation Oncology, University Medical Center Schleswig-Holstein, Lübeck, Germany; Institute for Robotics and Cognitive Systems, University of Lübeck, Lübeck, Germany.
authors.forename: Melanie
authors.initials: M
authors.lastname: Grehn
authors.affiliation: Department for Radiation Oncology, University Medical Center Schleswig-Holstein, Lübeck, Germany.
authors.forename: Florian
authors.initials: F
authors.lastname: Cremers
authors.affiliation: Department for Radiation Oncology, University Medical Center Schleswig-Holstein, Kiel, Germany.
authors.forename: Frank-Andre
authors.initials: FA
authors.lastname: Siebert
authors.affiliation: Saphir Radiosurgery Center Northern Germany, Güstrow, Germany; Department for Radiation Oncology, University Medicine Greifswald, Greifswald, Germany.
authors.forename: Stefan
authors.initials: S
authors.lastname: Wurster
authors.affiliation: Saphir Radiosurgery Center Northern Germany, Güstrow, Germany.
authors.forename: Stefan
authors.initials: S
authors.lastname: Huttenlocher
authors.affiliation: Department for Radiation Oncology, University Medical Center Schleswig-Holstein, Kiel, Germany; Department for Radiation Oncology, University Clinic Copenhagen, Copenhagen, Denmark.
authors.forename: Jürgen
authors.initials: J
authors.lastname: Dunst
authors.affiliation: Department for Radiation Oncology, University Medicine Rostock, Rostock, Germany.
authors.forename: Guido
authors.initials: G
authors.lastname: Hildebrandt
authors.affiliation: Institute for Robotics and Cognitive Systems, University of Lübeck, Lübeck, Germany.
authors.forename: Achim
authors.initials: A
authors.lastname: Schweikard
authors.affiliation: Department for Radiation Oncology, University Medical Center Schleswig-Holstein, Lübeck, Germany.
authors.forename: Dirk
authors.initials: D
authors.lastname: Rades
authors.affiliation: Institute for Robotics and Cognitive Systems, University of Lübeck, Lübeck, Germany.
authors.forename: Floris
authors.initials: F
authors.lastname: Ernst
authors.affiliation: Department for Radiation Oncology, University Medical Center Schleswig-Holstein, Kiel, Germany; Saphir Radiosurgery Center Northern Germany, Güstrow, Germany. Electronic address: blanck@saphir-rc.com.
authors.forename: Oliver
authors.initials: O
authors.lastname: Blanck
mesh_terms: D006801:Humans; D008113:Liver Neoplasms / Q000000981:diagnostic imaging / Q000532:radiotherapy* / Q000556:secondary*; D009038:Motion; D016634:Radiosurgery; D011879:Radiotherapy Dosage; D011880:Radiotherapy Planning, Computer-Assisted / Q000379:methods*; D061089:Radiotherapy, Image-Guided / Q000379:methods*; D015203:Reproducibility of Results; D015656:Respiratory Mechanics; D012371:Robotics / Q000379:methods*; D012680:Sensitivity and Specificity; D014057:Tomography, X-Ray Computed / Q000379:methods*; D016896:Treatment Outcome; D047368:Tumor Burden / Q000528:radiation effects