title: Technical Note: Motion-perturbation method applied to dosimetry of dynamic MLC target tracking--A proof-of-concept.
abstract: OBJECTIVE
  Previous studies show that dose to a moving target can be estimated using 4D measurement-guided dose reconstruction based on a process called virtual motion simulation, or VMS. A potential extension of VMS is to estimate dose during dynamic multileaf collimator (MLC)-tracking treatments. The authors introduce a modified VMS method and quantify its performance as proof-of-concept for tracking applications.
  
  
  METHODS
  Direct measurements with a moving biplanar diode array were used to verify accuracy of the VMS dose estimates. A tracking environment for variably sized circular MLC apertures was simulated by sending preprogrammed control points to the MLC while simultaneously moving the accelerator treatment table. Sensitivity of the method to simulated tracking latency (0-700 ms) was also studied. Potential applicability of VMS to fast changing beam apertures was evaluated by modeling, based on the demonstrated dependence of the cumulative dose on the temporal dose gradient.
  
  
  RESULTS
  When physical and virtual latencies were matched, the agreement rates (2% global/2 mm gamma) between the VMS and the biplanar dosimeter were above 96%. When compared to their own reference dose (0 induced latency), the agreement rates for VMS and biplanar array track closely up to 200 ms of induced latency with 10% low-dose cutoff threshold and 300 ms with 50% cutoff. Time-resolved measurements suggest that even in the modulated beams, the error in the cumulative dose introduced by the 200 ms VMS time resolution is not likely to exceed 0.5%.
  
  
  CONCLUSIONS
  Based on current results and prior benchmarks of VMS accuracy, the authors postulate that this approach should be applicable to any MLC-tracking treatments where leaf speeds do not exceed those of the current Varian accelerators.
authors.affiliation: Department of Radiation Oncology, Moffitt Cancer Center, Tampa, Florida 33612.
authors.forename: Vladimir
authors.initials: V
authors.lastname: Feygelman
authors.affiliation: Department of Radiation Oncology, Moffitt Cancer Center, Tampa, Florida 33612.
authors.forename: Brian
authors.initials: B
authors.lastname: Tonner
authors.affiliation: Department of Physics, University of South Florida, Tampa, Florida 33612.
authors.forename: Cassandra
authors.initials: C
authors.lastname: Stambaugh
authors.affiliation: Department of Radiation Oncology, Moffitt Cancer Center, Tampa, Florida 33612.
authors.forename: Dylan
authors.initials: D
authors.lastname: Hunt
authors.affiliation: Department of Radiation Oncology, Moffitt Cancer Center, Tampa, Florida 33612.
authors.forename: Geoffrey
authors.initials: G
authors.lastname: Zhang
authors.affiliation: Department of Radiation Oncology, Moffitt Cancer Center, Tampa, Florida 33612.
authors.forename: Eduardo
authors.initials: E
authors.lastname: Moros
authors.affiliation: Canis Lupus LLC, Merrimac, Wisconsin 53561.
authors.forename: Benjamin E
authors.initials: BE
authors.lastname: Nelms
mesh_terms: D000465:Algorithms; D016477:Artifacts; D003198:Computer Simulation; D006801:Humans; D008954:Models, Biological; D009369:Neoplasms / Q000532:radiotherapy*; D056888:Patient Positioning / Q000379:methods*; D010865:Pilot Projects; D011874:Radiometry / Q000379:methods*; D011879:Radiotherapy Dosage; D011880:Radiotherapy Planning, Computer-Assisted / Q000379:methods*; D011881:Radiotherapy, Computer-Assisted / Q000379:methods*; D015203:Reproducibility of Results; D012680:Sensitivity and Specificity