title: 4D in-beam positron emission tomography for verification of motion-compensated ion beam therapy.
abstract: OBJECTIVE
  Clinically safe and effective treatment of intrafractionally moving targets with scanned ion beams requires dedicated delivery techniques such as beam tracking. Apart from treatment delivery, also appropriate methods for validation of the actual tumor irradiation are highly desirable, In this contribution the feasibility of four-dimensionally (space and time) resolved, motion-compensated in-beam positron emission tomography (4DibPET) was addressed in experimental studies with scanned carbon ion beams.
  
  
  METHODS
  A polymethyl methracrylate block sinusoidally moving left-right in beam's eye view was used as target. Radiological depth changes were introduced by placing a stationary ramp-shaped absorber proximal of the moving target. Treatment delivery was compensated for motion by beam tracking. Time-resolved, motion-correlated in-beam PET data acquisition was performed during beam delivery with tracking the moving target and prolonged after beam delivery first with the activated target still in motion and, finally, with the target at rest. Motion-compensated 4DibPET imaging was implemented and the results were compared to a stationary reference irradiation of the same treatment field. Data were used to determine feasibility of 4DibPET but also to evaluate offline in comparison to in-beam PET acquisition.
  
  
  RESULTS
  4D in-beam as well as offline PET imaging was found to be feasible and offers the possibility to verify the correct functioning of beam tracking. Motion compensation of the imaged beta(+)-activity distribution allows recovery of the volumetric extension of the delivered field for direct comparison with the reference stationary condition. Observed differences in terms of lateral field extension and penumbra in the direction of motion were typically less than 1 mm for both imaging strategies in comparison to the corresponding reference distributions. However, in-beam imaging retained a better spatial correlation of the measured activity with the delivered dose.
  
  
  CONCLUSIONS
  4DibPET is a feasible and promising method to validate treatment delivery of scanned ion beams to moving targets. Further investigations will focus on more complex geometries and treatment planning studies with clinical data.
authors.affiliation: Department of Radiation Oncology and Heidelberg Ion Beam Therapy Centre (HIT), University Clinic of Heidelberg, Im Neuenheimer Feld 450, 69120 Heidelberg, Germany. katia.parodi@med.uni-heidelberg.de
authors.forename: Katia
authors.initials: K
authors.lastname: Parodi
authors.forename: Nami
authors.initials: N
authors.lastname: Saito
authors.forename: Naved
authors.initials: N
authors.lastname: Chaudhri
authors.forename: Christian
authors.initials: C
authors.lastname: Richter
authors.forename: Marco
authors.initials: M
authors.lastname: Durante
authors.forename: Wolfgang
authors.initials: W
authors.lastname: Enghardt
authors.forename: Eike
authors.initials: E
authors.lastname: Rietzel
authors.forename: Christoph
authors.initials: C
authors.lastname: Bert
mesh_terms: D002244:Carbon / Q000627:therapeutic use; D003198:Computer Simulation; D007477:Ions / Q000627:therapeutic use*; D009038:Motion; D019047:Phantoms, Imaging; D049268:Positron-Emission Tomography / Q000295:instrumentation / Q000379:methods*; D011829:Radiation Dosage; D011878:Radiotherapy / Q000379:methods*; D011879:Radiotherapy Dosage; D011880:Radiotherapy Planning, Computer-Assisted / Q000379:methods; D013997:Time Factors
chemical_list: D007477:Ions; D002244:Carbon