title: Dosimetric impact of the interplay effect during stereotactic lung radiation therapy delivery using flattening filter-free beams and volumetric modulated arc therapy.
abstract: OBJECTIVE
  We investigated the dosimetric impact of the interplay effect during RapidArc stereotactic body radiation therapy for lung tumors using flattening filter-free (FFF) beams with different dose rates.
  
  
  METHODS
  Seven tumors with motion ≤20 mm, treated with 10-MV FFF RapidArc, were analyzed. A programmable phantom with sinusoidal longitudinal motion (30-mm diameter "tumor" insert; period = 5 s; individualized amplitude from planning 4-dimensional computed tomography) was used for dynamic dose measurements. Measurements were made with GafChromic EBT III films. Plans delivered the prescribed dose to 95% of the planning target volume, created by a 5-mm expansion of the internal target volume. They comprised 2 arcs and maximum dose rates of 400 and 2400 MU/min. For 2400 MU/min plans, measurements were repeated at 3 different initial breathing phases to model interplay over 2 to 3 fractions. For 3 cases, 2 extra plans were created using 1 full rotational arc (with contralateral lung avoidance sector) and 1 partial arc of 224° to 244°. Dynamic and convolved static measurements were compared by use of gamma analysis of 3% dose difference and 1 mm distance-to-agreement.
  
  
  RESULTS
  For 2-arc 2400 MU/min plans, maximum dose deviation of 9.4% was found in a single arc; 7.4% for 2 arcs (single fraction) and <5% and 3% when measurements made at 2 and 3 different initial breathing phases were combined, simulating 2 or 3 fractions. For all 7 cases, >99% of the area within the region of interest passed the gamma criteria when all 3 measurements with different initial phases were combined. Single-fraction single-arc plans showed higher dose deviations, which diminished when dose distributions were summed over 2 fractions. All 400 MU/min plans showed good agreement in a single fraction measurement.
  
  
  CONCLUSIONS
  Under phantom conditions, single-arc and single-fraction 2400 MU/min FFF RapidArc lung stereotactic body radiation therapy is susceptible to interplay. Two arcs and ≥2 fractions reduced the effect to a level that appeared unlikely to be clinically significant.
authors.affiliation: Department of Radiation Oncology, VU University Medical Center, Amsterdam, The Netherlands. w.verbakel@vumc.nl
authors.forename: Chin Loon
authors.initials: CL
authors.lastname: Ong
authors.forename: Max
authors.initials: M
authors.lastname: Dahele
authors.forename: Ben J
authors.initials: BJ
authors.lastname: Slotman
authors.forename: Wilko F A R
authors.initials: WF
authors.lastname: Verbakel
mesh_terms: D003131:Combined Modality Therapy / Q000379:methods; D019583:Dose Fractionation, Radiation; D056973:Four-Dimensional Computed Tomography; D006801:Humans; D008175:Lung Neoplasms / Q000000981:diagnostic imaging / Q000473:pathology / Q000628:therapy*; D009068:Movement; D058958:Organs at Risk; D019047:Phantoms, Imaging; D016634:Radiosurgery / Q000379:methods*; D011880:Radiotherapy Planning, Computer-Assisted / Q000379:methods; D050397:Radiotherapy, Intensity-Modulated / Q000379:methods*; D012119:Respiration; D013997:Time Factors