title: Frameless stereotactic body radiotherapy for lung cancer using four-dimensional cone beam CT guidance.
abstract: OBJECTIVE
  To quantify the localization accuracy and intrafraction stability of lung cancer patients treated with frameless, four-dimensional (4D) cone beam computed tomography (CBCT)-guided stereotactic body radiotherapy (SBRT) and to calculate and validate planning target volume (PTV) margins to account for the residual geometric uncertainties.
  
  
  METHODS
  Sixty-five patients with small peripheral lung tumors were treated with SBRT without a body frame to 54 Gy in three fractions. For each fraction, three 4D-CBCT scans were acquired: before treatment to measure and correct the time-weighted mean tumor position, after correction to validate the correction applied, and after treatment to estimate the intrafraction stability. Patient-specific PTV margins were computed and subsequently validated using Monte Carlo error simulations.
  
  
  RESULTS
  Systematic tumor localization inaccuracies (1 SD) were 0.8, 0.8, and 0.9 mm for the left-right, craniocaudal, and anteroposterior direction, respectively. Random localization inaccuracies were 1.1, 1.1, and 1.4 mm. Baseline variations were 1.8, 2.9, and 3.0 mm (systematic) and 1.1, 1.5, and 2.0 mm (random), indicating the importance of image guidance. Intrafraction stability of the target was 1.2, 1.2, and 1.8 mm (systematic) and 1.3, 1.5, and 1.8 mm (random). Monte Carlo error simulations showed that patient-specific PTV margins (5.8-10.5 mm) were adequate for 94% of the evaluated cases (2-28 mm peak-to-peak breathing amplitude).
  
  
  CONCLUSIONS
  Frameless SBRT can be safely administered using 4D-CBCT guidance. Even with considerable breathing motion, the PTV margins can safely be kept small, allowing patients with larger tumors to benefit from the advantages of SBRT. In case bony anatomy would be used as a surrogate for tumor position, considerably larger PTV margins would be required.
authors.affiliation: Department of Radiation Oncology, The Netherlands Cancer Institute-Antoni van Leeuwenhoek Hospital, Amsterdam, The Netherlands. j.sonke@nki.nl
authors.forename: Jan-Jakob
authors.initials: JJ
authors.lastname: Sonke
authors.forename: Maddalena
authors.initials: M
authors.lastname: Rossi
authors.forename: Jochem
authors.initials: J
authors.lastname: Wolthaus
authors.forename: Marcel
authors.initials: M
authors.lastname: van Herk
authors.forename: Eugene
authors.initials: E
authors.lastname: Damen
authors.forename: Jose
authors.initials: J
authors.lastname: Belderbos
mesh_terms: D000328:Adult; D000368:Aged; D000369:Aged, 80 and over; D000465:Algorithms; 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; D006801:Humans; D008175:Lung Neoplasms / Q000000981:diagnostic imaging / Q000473:pathology / Q000601:surgery*; D008297:Male; D008875:Middle Aged; D009010:Monte Carlo Method; D009068:Movement; D016634:Radiosurgery; D011880:Radiotherapy Planning, Computer-Assisted / Q000379:methods*; D012119:Respiration; D012189:Retrospective Studies; D047368:Tumor Burden