title: Assessing the dosimetric impact of real-time prostate motion during volumetric modulated arc therapy.
abstract: OBJECTIVE
  To develop a method for dose reconstruction by incorporating the interplay effect between aperture modulation and target motion, and to assess the dosimetric impact of real-time prostate motion during volumetric modulated arc therapy (VMAT).
  
  
  METHODS
  Clinical VMAT plans were delivered with the TrueBeam linac for 8 patients with prostate cancer. The real-time target motion during dose delivery was determined based on the 2-dimensional fiducial localization using an onboard electronic portal imaging device. The target shift in each image was correlated with the control point with the same gantry angle in the VMAT plan. An in-house-developed Monte Carlo simulation tool was used to calculate the 3-dimensional dose distribution for each control point individually, taking into account the corresponding real-time target motion (assuming a nondeformable target with no rotation). The delivered target dose was then estimated by accumulating the dose from all control points in the plan. On the basis of this information, dose-volume histograms and 3-dimensional dose distributions were calculated to assess their degradation from the planned dose caused by target motion. Thirty-two prostate motion trajectories were analyzed.
  
  
  RESULTS
  The minimum dose to 0.03 cm(3) of the gross tumor volume (D0.03cc) was only slightly degraded after taking motion into account, with a minimum value of 94.1% of the planned dose among all patients and fractions. However, the gross tumor volume receiving prescription dose (V100%) could be largely affected by motion, dropping below 60% in 1 trajectory. We did not observe a correlation between motion magnitude and dose degradation.
  
  
  CONCLUSIONS
  Prostate motion degrades the delivered dose to the target in an unpredictable way, although its effect is reduced over multiple fractions, and for most patients the degradation is small. Patients with greater prostate motion or those treated with stereotactic body radiation therapy would benefit from real-time prostate tracking to reduce the margin.
authors.affiliation: Department of Radiation Oncology, Stanford University, Stanford, California; Division of Radiation Physics, Department of Oncology, Clínica Universidad de Navarra, Pamplona, Spain. Electronic address: jdazcona@stanford.edu.
authors.forename: Juan Diego
authors.initials: JD
authors.lastname: Azcona
authors.affiliation: Department of Radiation Oncology, Stanford University, Stanford, California.
authors.forename: Lei
authors.initials: L
authors.lastname: Xing
authors.affiliation: Department of Radiation Oncology, Stanford University, Stanford, California.
authors.forename: Xin
authors.initials: X
authors.lastname: Chen
authors.affiliation: Department of Radiation Oncology, Stanford University, Stanford, California.
authors.forename: Karl
authors.initials: K
authors.lastname: Bush
authors.affiliation: Department of Radiation Oncology, Stanford University, Stanford, California.
authors.forename: Ruijiang
authors.initials: R
authors.lastname: Li
mesh_terms: D019583:Dose Fractionation, Radiation; D004307:Dose-Response Relationship, Radiation; D006801:Humans; D021621:Imaging, Three-Dimensional; D008297:Male; D009010:Monte Carlo Method; D009068:Movement; D010315:Particle Accelerators; D019047:Phantoms, Imaging; D011467:Prostate / Q000528:radiation effects; D011471:Prostatic Neoplasms / Q000532:radiotherapy*; D011874:Radiometry / Q000379:methods*; D011879:Radiotherapy Dosage; D011880:Radiotherapy Planning, Computer-Assisted / Q000295:instrumentation / Q000379:methods; D050397:Radiotherapy, Intensity-Modulated / Q000295:instrumentation* / Q000379:methods*; D013997:Time Factors