title: Intrafraction variation of mean tumor position during image-guided hypofractionated stereotactic body radiotherapy for lung cancer.
abstract: OBJECTIVE
  Prolonged delivery times during daily cone-beam computed tomography (CBCT)-guided lung stereotactic body radiotherapy (SBRT) introduce concerns regarding intrafraction variation (IFV) of the mean target position (MTP). The purpose of this study was to evaluate the magnitude of the IFV-MTP and to assess target margins required to compensate for IFV and postonline CBCT correction residuals. Patient, treatment, and tumor characteristics were analyzed with respect to their impact on IFV-MTP.
  
  
  METHODS
  A total of 126 patients with 140 tumors underwent 659 fractions of lung SBRT. Dose prescribed was 48 or 60 Gy in 12 Gy fractions. Translational target position correction of the MTP was performed via onboard CBCT. IFV-MTP was measured as the difference in MTP between the postcorrection CBCT and the posttreatment CBCT excluding residual error.
  
  
  RESULTS
  IFV-MTP was 0.2 ± 1.8 mm, 0.1 ± 1.9 mm, and 0.01 ± 1.5 mm in the craniocaudal, anteroposterior, and mediolateral dimensions and the IFV-MTP vector was 2.3 ± 2.1 mm. Treatment time and excursion were found to be significant predictors of IFV-MTP. An IFV-MTP vector greater than 2 and 5 mm was seen in 40.8% and 7.2% of fractions, respectively. IFV-MTP greater than 2 mm was seen in heavier patients with larger excursions and longer treatment times. Significant differences in IFV-MTP were seen between immobilization devices. The stereotactic frame immobilization device was found to be significantly less likely to have an IFV-MTP vector greater than 2 mm compared with the alpha cradle, BodyFIX, and hybrid immobilization devices.
  
  
  CONCLUSIONS
  Treatment time and respiratory excursion are significantly associated with IFV-MTP. Significant differences in IFV-MTP were found between immobilization devices. Target margins for IFV-MTP plus post-correction residuals are dependent on immobilization device with 5-mm uniform margins being acceptable for the frame immobilization device.
authors.affiliation: Department of Radiation Oncology, William Beaumont Hospital, Royal Oak, MI, USA.
authors.forename: Chirag
authors.initials: C
authors.lastname: Shah
authors.forename: Inga S
authors.initials: IS
authors.lastname: Grills
authors.forename: Larry L
authors.initials: LL
authors.lastname: Kestin
authors.forename: Samuel
authors.initials: S
authors.lastname: McGrath
authors.forename: Hong
authors.initials: H
authors.lastname: Ye
authors.forename: Shannon K
authors.initials: SK
authors.lastname: Martin
authors.forename: Di
authors.initials: D
authors.lastname: Yan
mesh_terms: D000368:Aged; D000704:Analysis of Variance; D001835:Body Weight; D002289:Carcinoma, Non-Small-Cell Lung / Q000000981:diagnostic imaging / Q000473:pathology / Q000601:surgery*; D054893:Cone-Beam Computed Tomography / Q000379:methods; D019583:Dose Fractionation, Radiation; D005260:Female; D056973:Four-Dimensional Computed Tomography; D006801:Humans; D007103:Immobilization / Q000295:instrumentation / Q000379:methods; D008175:Lung Neoplasms / Q000000981:diagnostic imaging / Q000473:pathology / Q000601:surgery*; D008297:Male; D009068:Movement; D011446:Prospective Studies; D016634:Radiosurgery / Q000295:instrumentation / Q000379:methods*; D011880:Radiotherapy Planning, Computer-Assisted / Q000379:methods; D061089:Radiotherapy, Image-Guided / Q000379:methods*; D012119:Respiration; D013997:Time Factors; D047368:Tumor Burden