title: Accuracy of the dose-shift approximation in estimating the delivered dose in SBRT of lung tumors considering setup errors and breathing motions.
abstract: BACKGROUND
  Geometrical uncertainties can result in a delivered dose to the tumor different from that estimated in the static treatment plan. The purpose of this project was to investigate the accuracy of the dose calculated to the clinical target volume (CTV) with the dose-shift approximation, in stereotactic body radiation therapy (SBRT) of lung tumors considering setup errors and breathing motion. The dose-shift method was compared with a beam-shift method with dose recalculation.
  
  
  METHODS
  Included were 10 patients (10 tumors) selected to represent a variety of SBRT-treated lung tumors in terms of tumor location, CTV volume, and tumor density. An in-house developed toolkit within a treatment planning system allowed the shift of either the dose matrix or a shift of the beam isocenter with dose recalculation, to simulate setup errors and breathing motion. Setup shifts of different magnitudes (up to 10 mm) and directions as well as breathing with different peak-to-peak amplitudes (up to 10:5:5 mm) were modeled. The resulting dose-volume histograms (DVHs) were recorded and dose statistics were extracted.
  
  
  RESULTS
  Generally, both the dose-shift and beam-shift methods resulted in calculated doses lower than the static planned dose, although the minimum (D98%) dose exceeded the prescribed dose in all cases, for setup shifts up to 5 mm. The dose-shift method also generally underestimated the dose compared with the beam-shift method. For clinically realistic systematic displacements of less than 5 mm, the results demonstrated that in the minimum dose region within the CTV, the dose-shift method was accurate to 2% (root-mean-square error). Breathing motion only marginally degraded the dose distributions.
  
  
  CONCLUSIONS
  Averaged over the patients and shift directions, the dose-shift approximation was determined to be accurate to approximately 2% (RMS) within the CTV, for clinically relevant geometrical uncertainties for SBRT of lung tumors.
authors.affiliation: a Department of Medical Radiation Physics and Nuclear Medicine , Karolinska University Hospital , Stockholm , Sweden.|b Department of Oncology-Pathology , Karolinska Institutet , Stockholm , Sweden.
authors.forename: Kristin
authors.initials: K
authors.lastname: Karlsson
authors.affiliation: a Department of Medical Radiation Physics and Nuclear Medicine , Karolinska University Hospital , Stockholm , Sweden.|b Department of Oncology-Pathology , Karolinska Institutet , Stockholm , Sweden.
authors.forename: Ingmar
authors.initials: I
authors.lastname: Lax
authors.affiliation: a Department of Medical Radiation Physics and Nuclear Medicine , Karolinska University Hospital , Stockholm , Sweden.|b Department of Oncology-Pathology , Karolinska Institutet , Stockholm , Sweden.
authors.forename: Elias
authors.initials: E
authors.lastname: Lindbäck
authors.affiliation: a Department of Medical Radiation Physics and Nuclear Medicine , Karolinska University Hospital , Stockholm , Sweden.|b Department of Oncology-Pathology , Karolinska Institutet , Stockholm , Sweden.
authors.forename: Gavin
authors.initials: G
authors.lastname: Poludniowski
mesh_terms: D006801:Humans; D008175:Lung Neoplasms / Q000601:surgery*; D016634:Radiosurgery; D011879:Radiotherapy Dosage; D011880:Radiotherapy Planning, Computer-Assisted / Q000379:methods; D061106:Radiotherapy Setup Errors; D050397:Radiotherapy, Intensity-Modulated / Q000379:methods; D012119:Respiration