title: 4D dose-position verification in radiation therapy using the RADPOS system in a deformable lung phantom.
abstract: OBJECTIVE
  A novel 4D in vivo dosimetry system (RADPOS), in conjunction with a deformable lung phantom, has been evaluated as a potential quality assurance tool for 4D radiotherapy.
  
  
  METHODS
  RADPOS detectors, which consist of a MOSFET dosimeter combined with an electromagnetic positioning probe, were placed inside the deformable lung phantom. One detector was positioned directly inside a tumor embedded in the lung phantom and another was positioned inside the lung portion of the phantom, outside the tumor. CT scans were taken with the phantom at three breathing phases, and for each phase, the detector position inside the phantom was read with the RADPOS software and compared to the position as determined from the CT data. These values were also compared to RADPOS measurements taken with the phantom on the couch of a Varian Clinac 6EX linac. The deformable phantom and the RADPOS system were also used in two radiation delivery scenarios: (1) A simulation of a free-breathing delivery and (2) a simulation of an adaptive treatment.
  
  
  RESULTS
  Compared to CT imaging, the RADPOS positional accuracy was found to be better than 2.5 mm. The radial displacement measurements taken in the CT and linac rooms agreed to within an average of (0.7 +/- 0.3) mm. Hence, the system can provide relative displacement measurements in the treatment room, consistent with measurements made in the CT room. For the free-breathing delivery, the total dose reported by RADPOS agreed to within 4% and 5% of the treatment planning doses in the tumor and the lung portion of the phantom, respectively. The RADPOS-measured dose values for the adaptive delivery were within 1.5% of the treatment plan values, which was well within the estimated experimental uncertainties.
  
  
  CONCLUSIONS
  This work has shown that the deformable lung phantom-RADPOS system can be an efficient quality assurance tool for 4D radiation therapy.
authors.affiliation: Medical Physics, The Ottawa Hospital Cancer Centre, Ottawa, Ontario K1H 8L6, Canada. acherpak@ottawahospital.on.ca
authors.forename: Amanda
authors.initials: A
authors.lastname: Cherpak
authors.forename: Monica
authors.initials: M
authors.lastname: Serban
authors.forename: Jan
authors.initials: J
authors.lastname: Seuntjens
authors.forename: Joanna E
authors.initials: JE
authors.lastname: Cygler
mesh_terms: D008168:Lung / Q000000981:diagnostic imaging / Q000503:physiopathology / Q000528:radiation effects*; D008175:Lung Neoplasms / Q000000981:diagnostic imaging / Q000503:physiopathology / Q000532:radiotherapy; D019047:Phantoms, Imaging; D011786:Quality Control; D011874:Radiometry / Q000295:instrumentation*; D011879:Radiotherapy Dosage; D020266:Radiotherapy, Conformal / Q000379:methods* / Q000592:standards; D012119:Respiration; D014057:Tomography, X-Ray Computed