title: Analysis of components of variance determining probability of setup errors in CBCT-guided stereotactic radiotherapy of lung tumors.
abstract: OBJECTIVE
  Online tumor matching for SABR lung setup requires margins for inaccuracies due to intra-fraction variability of breathing-averaged tumor position (BATP) and CBCT image guidance. We studied intra-fraction variability during SABR delivery using VMAT, corrected these for measurement inaccuracies, and quantified the CBCT image-guidance uncertainties.
  
  
  METHODS
  For 193 fractions in 38 patients positioned without immobilization devices, CBCT scans were acquired before and after 2 arcs of a RapidArc treatment. A hidden marker test was performed to determine the accuracy of the CBCT system and an inter-observer test was performed to measure registration accuracy. Intra-fraction variability was calculated after correction for these components of variance, and the prediction interval for setup inaccuracies was determined.
  
  
  RESULTS
  Correction for measurement inaccuracies reduced the intra-fraction variability of the BATP from 1.9 to 1.6 mm in AP, from 1.7 to 1.4 mm in SI and from 1.5 to 1.1 mm in LR direction (1 SD). Intra-fraction variability in bony anatomy after correction was ≤ 1 mm (1 SD). The 95% prediction interval to account for CBCT image-guidance uncertainties and intra-fraction variability was determined, and was found to be within our institutional PTV margins of 5 mm.
  
  
  CONCLUSIONS
  Our findings show that it is essential to account for measurement and system inaccuracies when obtaining data for validating PTV margins from online CBCT image guidance.
authors.affiliation: Department of Radiation Oncology, VU University medical center, Amsterdam, 1081 HV, The Netherlands.
authors.forename: Johan P
authors.initials: JP
authors.lastname: Cuijpers
authors.affiliation: Department of Radiation Oncology, VU University medical center, Amsterdam, 1081 HV, The Netherlands.
authors.forename: Max
authors.initials: M
authors.lastname: Dahele
authors.affiliation: Department of Epidemiology and Biostatistics, VU University medical center, Amsterdam, 1081 HV, The Netherlands.
authors.forename: Marianne
authors.initials: M
authors.lastname: Jonker
authors.affiliation: Department of Radiation Oncology, VU University medical center, Amsterdam, 1081 HV, The Netherlands.
authors.forename: Bianca
authors.initials: B
authors.lastname: Kraan
authors.affiliation: Department of Radiation Oncology, VU University medical center, Amsterdam, 1081 HV, The Netherlands.
authors.forename: Suresh
authors.initials: S
authors.lastname: Senan
authors.affiliation: Department of Radiation Oncology, VU University medical center, Amsterdam, 1081 HV, The Netherlands.
authors.forename: Ben
authors.initials: B
authors.lastname: Slotman
authors.affiliation: Department of Radiation Oncology, VU University medical center, Amsterdam, 1081 HV, The Netherlands.
authors.forename: Wilko Far
authors.initials: WF
authors.lastname: Verbakel
mesh_terms: D000368:Aged; D000369:Aged, 80 and over; D000704:Analysis of Variance; D054893:Cone-Beam Computed Tomography / Q000295:instrumentation / Q000379:methods*; D005260:Female; D057918:Fiducial Markers; D006801:Humans; D008168:Lung / Q000000981:diagnostic imaging / Q000601:surgery; D008175:Lung Neoplasms / Q000000981:diagnostic imaging / Q000601:surgery*; D008297:Male; D008875:Middle Aged; D015588:Observer Variation; D056888:Patient Positioning; D019047:Phantoms, Imaging; D011336:Probability; D016634:Radiosurgery / Q000379:methods*; D061106:Radiotherapy Setup Errors; D061089:Radiotherapy, Image-Guided / Q000295:instrumentation / Q000379:methods*; D012189:Retrospective Studies; D035501:Uncertainty
keywords: SABR; SBRT; intra-fraction motion; lung cancer; setup errors