title: Robotic-based carbon ion therapy and patient positioning in 6 degrees of freedom: setup accuracy of two standard immobilization devices used in carbon ion therapy and IMRT.
abstract: OBJECTIVE
  To investigate repositioning accuracy in particle radiotherapy in 6 degrees of freedom (DOF) and intensity-modulated radiotherapy (IMRT, 3 DOF) for two immobilization devices (Scotchcast masks vs thermoplastic head masks) currently in use at our institution for fractionated radiation therapy in head and neck cancer patients.
  
  
  METHODS
  Position verifications in patients treated with carbon ion therapy and IMRT for head and neck malignancies were evaluated. Most patients received combined treatment regimen (IMRT plus carbon ion boost), immobilization was achieved with either Scotchcast or thermoplastic head masks. Position corrections in robotic-based carbon ion therapy allowing 6 DOF were compared to IMRT allowing corrections in 3 DOF for two standard immobilization devices. In total, 838 set-up controls of 38 patients were analyzed.
  
  
  RESULTS
  Robotic-based position correction including correction of rotations was well tolerated and without discomfort. Standard deviations of translational components were between 0.5 and 0.8 mm for Scotchcast and 0.7 and 1.3 mm for thermoplastic masks in 6 DOF and 1.2-1.4 mm and 1.0-1.1 mm in 3 DOF respectively. Mean overall displacement vectors were between 2.1 mm (Scotchcast) and 2.9 mm (thermoplastic masks) in 6 DOF and 3.9-3.0 mm in 3 DOF respectively. Displacement vectors were lower when correction in 6 DOF was allowed as opposed to 3 DOF only, which was maintained at the traditional action level of >3 mm for position correction in the pre-on-board imaging era.
  
  
  CONCLUSIONS
  Setup accuracy for both systems was within the expected range. Smaller shifts were required when 6 DOF were available for correction as opposed to 3 DOF. Where highest possible positioning accuracy is required, frequent image guidance is mandatory to achieve best possible plan delivery and maintenance of sharp gradients and optimal normal tissue sparing inherent in carbon ion therapy.
authors.affiliation: Department of Radiation Oncology, University of Heidelberg, INF 400, Heidelberg 69120, Germany. Alexandra.jensen@med.uni-heidelberg.de
authors.forename: Alexandra D
authors.initials: AD
authors.lastname: Jensen
authors.forename: Marcus
authors.initials: M
authors.lastname: Winter
authors.forename: Sabine P
authors.initials: SP
authors.lastname: Kuhn
authors.forename: Jürgen
authors.initials: J
authors.lastname: Debus
authors.forename: Olaf
authors.initials: O
authors.lastname: Nairz
authors.forename: Marc W
authors.initials: MW
authors.lastname: Münter
mesh_terms: D002244:Carbon / Q000627:therapeutic use; D006258:Head and Neck Neoplasms / Q000532:radiotherapy; D006801:Humans; D007103:Immobilization / Q000295:instrumentation* / Q000379:methods; D007477:Ions / Q000627:therapeutic use; D056888:Patient Positioning / Q000295:instrumentation* / Q000379:methods; D011878:Radiotherapy / Q000295:instrumentation* / Q000379:methods*; D011880:Radiotherapy Planning, Computer-Assisted; D050397:Radiotherapy, Intensity-Modulated / Q000295:instrumentation / Q000379:methods; D012371:Robotics / Q000295:instrumentation* / Q000379:methods
chemical_list: D007477:Ions; D002244:Carbon