title: Required target margins for image-guided lung SBRT: Assessment of target position intrafraction and correction residuals.
abstract: OBJECTIVE
  With increased use of stereotactic body radiotherapy (SBRT) for early-stage lung cancer, quantification of intrafraction variation (IFV) is required to develop adequate target margins.
  
  
  METHODS
  A total of 409 patients with 427 tumors underwent 1593 fractions of lung SBRT between 2005 and 2010. Translational target position correction of the mean target position (MTP) was performed via onboard cone-beam computed tomography (CBCT). IFV was measured as the difference in MTP between the post-correction CBCT and the post-treatment CBCT and was calculated on 1337 fractions.
  
  
  RESULTS
  Mean IFV-MTP was 0.0 ± 1.7 mm, 0.6 ± 2.2 mm, and -1.0 ± 2.0 mm in the mediolateral (ML), anteroposterior (AP), and craniocaudal (CC) dimensions, and the vector was 3.1 ± 2.0 mm; 67.8% of fractions had an IFV vector greater than 2 mm, and 14.3% greater than 5 mm. Weight, excursion, forced expiratory volume in the first second of expiration, diffusing capacity of the lung for carbon monoxide, and treatment time were found to be significant predictors of IFV-MTP greater than 2 mm and 5 mm. Significant differences in IFV-MTP were seen between immobilization devices with a mean IFV of 2.3 ± 1.4 mm, 2.7 ± 1.6 mm, 3.0 ± 1.7 mm, 3.0 ± 2.5 mm, 3.3 ± 1.7 mm, and 3.3 ± 2.2 mm for the body frame, hybrid device, alpha cradle, body fix, wing board, and no immobilization, respectively (P < .001). Estimated required target margins for the entire cohort were 4.3, 6.1, and 6.0 mm in the ML, AP, and CC dimensions, with differences in margins based on immobilization.
  
  
  CONCLUSIONS
  IFV is dependent on several factors: immobilization device, treatment time, pulmonary function, and bodyweight. These factors are responsible for a significant portion of target margins with a mean IFV vector of 3 mm. Target margins of 6 mm or greater are required to encompass IFV in all dimensions when using four-dimensional CT with CBCT without respiratory gating or compression.
authors.affiliation: Department of Radiation Oncology, William Beaumont Hospital, Royal Oak, Michigan.
authors.forename: Chirag
authors.initials: C
authors.lastname: Shah
authors.affiliation: Department of Radiation Oncology, William Beaumont Hospital, Royal Oak, Michigan.
authors.forename: Larry L
authors.initials: LL
authors.lastname: Kestin
authors.affiliation: Princess Margaret Hospital, Toronto, Ontario, Canada.
authors.forename: Andrew J
authors.initials: AJ
authors.lastname: Hope
authors.affiliation: Princess Margaret Hospital, Toronto, Ontario, Canada.
authors.forename: Jean-Pierre
authors.initials: JP
authors.lastname: Bissonnette
authors.affiliation: University of Würzburg, Würzburg, Germany.
authors.forename: Matthias
authors.initials: M
authors.lastname: Guckenberger
authors.affiliation: Thomas Jefferson University and Hospitals, Philadelphia, Pennsylvania.
authors.forename: Ying
authors.initials: Y
authors.lastname: Xiao
authors.affiliation: Netherlands Cancer Institute, Amsterdam, Netherlands.
authors.forename: Jan-Jakob
authors.initials: JJ
authors.lastname: Sonke
authors.affiliation: Netherlands Cancer Institute, Amsterdam, Netherlands.
authors.forename: Jose
authors.initials: J
authors.lastname: Belderbos
authors.affiliation: Department of Radiation Oncology, William Beaumont Hospital, Royal Oak, Michigan.
authors.forename: Di
authors.initials: D
authors.lastname: Yan
authors.affiliation: Department of Radiation Oncology, William Beaumont Hospital, Royal Oak, Michigan. Electronic address: igrills@beaumont.edu.
authors.forename: Inga S
authors.initials: IS
authors.lastname: Grills