title: A moving target: Image guidance for stereotactic body radiation therapy for early-stage non-small cell lung cancer.
abstract: OBJECTIVE
  Precise patient positioning is critical due to the large fractional doses and small treatment margins employed for thoracic stereotactic body radiation therapy (SBRT). The goals of this study were to evaluate the following: (1) the accuracy of kilovoltage x-ray (kV x-ray) matching to bony anatomy for pretreatment positioning; (2) the magnitude of intrafraction tumor motion; and (3) whether treatment or patient characteristics correlate with intrafraction motion.
  
  
  METHODS
  Eighty-seven patients with lung cancer were treated with SBRT. Patients were positioned with orthogonal kV x-rays matched to bony anatomy followed by cone-beam computed tomography (CBCT), with matching of the CBCT-visualized tumor to the internal gross target volume obtained from a 4-dimensional CT simulation data set. Patients underwent a posttreatment CBCT to assess the magnitude of intrafraction motion.
  
  
  RESULTS
  The mean CBCT-based shifts after initial patient positioning using kV x-rays were 2.2 mm in the vertical axis, 1.8 mm in the longitudinal axis, and 1.6 mm in the lateral axis (n = 335). The percentage of shifts greater than 3 mm and 5 mm represented 39% and 17%, respectively, of all fractions delivered. The mean CBCT-based shifts after treatment were 1.6 mm vertically, 1.5 mm longitudinally, and 1.1 mm laterally (n = 343). Twenty-seven percent and 10% of shifts were greater than 3 mm and 5 mm, respectively. Univariate and multivariable analysis demonstrated a significant association between intrafraction motion with weight and pulmonary function.
  
  
  CONCLUSIONS
  Kilovoltage x-ray matching to bony anatomy is inadequate for accurate positioning when a conventional 3-5 mm margin is employed prior to lung SBRT. Given the treatment techniques used in this study, CBCT image guidance with a 5-mm planning target volume margin is recommended. Further work is required to find determinants of interfraction and intrafraction motion that may help guide the individualized application of planning target volume margins.
authors.affiliation: Department of Radiation Oncology, Hospital of the University of Pennsylvania, Philadelphia, Pennsylvania. Electronic address: corradetti@uphs.upenn.edu.
authors.forename: Michael N
authors.initials: MN
authors.lastname: Corradetti
authors.affiliation: Department of Biostatistics and Epidemiology, University of Pennsylvania Perelman School of Medicine, Philadelphia, Pennsylvania.
authors.forename: Nandita
authors.initials: N
authors.lastname: Mitra
authors.affiliation: Department of Radiation Oncology, Hospital of the University of Pennsylvania, Philadelphia, Pennsylvania.
authors.forename: Lara P
authors.initials: LP
authors.lastname: Bonner Millar
authors.affiliation: Department of Radiation Oncology, Hospital of the University of Pennsylvania, Philadelphia, Pennsylvania.
authors.forename: John
authors.initials: J
authors.lastname: Byun
authors.affiliation: Department of Biostatistics and Epidemiology, University of Pennsylvania Perelman School of Medicine, Philadelphia, Pennsylvania.
authors.forename: Fei
authors.initials: F
authors.lastname: Wan
authors.affiliation: Department of Radiation Oncology, Hospital of the University of Pennsylvania, Philadelphia, Pennsylvania.
authors.forename: Smith
authors.initials: S
authors.lastname: Apisarnthanarax
authors.affiliation: Department of Radiation Oncology, Hospital of the University of Pennsylvania, Philadelphia, Pennsylvania.
authors.forename: John
authors.initials: J
authors.lastname: Christodouleas
authors.affiliation: Department of Radiation Oncology, Hospital of the University of Pennsylvania, Philadelphia, Pennsylvania.
authors.forename: Nathan
authors.initials: N
authors.lastname: Anderson
authors.affiliation: Department of Radiation Oncology, Hospital of the University of Pennsylvania, Philadelphia, Pennsylvania.
authors.forename: Charles B
authors.initials: CB
authors.lastname: Simone
authors.affiliation: Department of Radiation Oncology, Hospital of the University of Pennsylvania, Philadelphia, Pennsylvania.
authors.forename: Boon-Keng
authors.initials: BK
authors.lastname: Teo
authors.affiliation: Department of Radiation Oncology, Hospital of the University of Pennsylvania, Philadelphia, Pennsylvania.
authors.forename: Ramesh
authors.initials: R
authors.lastname: Rengan