title: Implementation and evaluation of a transit dosimetry system for treatment verification.
abstract: OBJECTIVE
  To evaluate a formalism for transit dosimetry using a phantom study and prospectively evaluate the protocol on a patient population undergoing 3D conformal radiotherapy.
  
  
  METHODS
  Amorphous silicon EPIDs were calibrated for dose and used to acquire images of delivered fields. The measured EPID dose map was back-projected using the planning CT images to calculate dose at pre-specified points within the patient using commercially available software, EPIgray (DOSIsoft, France). This software compared computed back-projected dose with treatment planning system dose. A series of tests were performed on solid water phantoms (linearity, field size effects, off-axis effects). 37 patients were enrolled in the prospective study.
  
  
  RESULTS
  The EPID dose response was stable and linear with dose. For all tested field sizes the agreement was good between EPID-derived and treatment planning system dose in the central axis, with performance stability up to a measured depth of 18cm (agreement within -0.5% at 10cm depth on the central axis and within -1.4% at 2cm off-axis). 126 transit images were analysed of 37 3D-conformal patients. Patient results demonstrated the potential of EPIgray with 91% of all delivered fields achieved the initial set tolerance level of ΔD of 0±5-cGy or %ΔD of 0±5%.
  
  
  CONCLUSIONS
  The in vivo dose verification method was simple to implement, with very few commissioning measurements needed. The system required no extra dose to the patient, and importantly was able to detect patient position errors that impacted on dose delivery in two of cases.
authors.affiliation: Division of Surgery and Interventional Sciences, University College London, London, UK; Department of Radiotherapy Physics, Royal Berkshire NHS Foundation Trust, Reading, UK. Electronic address: k.ricketts@ucl.ac.uk.
authors.forename: K
authors.initials: K
authors.lastname: Ricketts
authors.affiliation: Department of Radiotherapy Physics, Royal Berkshire NHS Foundation Trust, Reading, UK.
authors.forename: C
authors.initials: C
authors.lastname: Navarro
authors.affiliation: Department of Radiotherapy Physics, Royal Berkshire NHS Foundation Trust, Reading, UK.
authors.forename: K
authors.initials: K
authors.lastname: Lane
authors.affiliation: Department of Radiotherapy Physics, Royal Berkshire NHS Foundation Trust, Reading, UK.
authors.forename: M
authors.initials: M
authors.lastname: Moran
authors.affiliation: Department of Radiotherapy Physics, Royal Berkshire NHS Foundation Trust, Reading, UK.
authors.forename: C
authors.initials: C
authors.lastname: Blowfield
authors.affiliation: Department of Radiotherapy Physics, Royal Berkshire NHS Foundation Trust, Reading, UK.
authors.forename: U
authors.initials: U
authors.lastname: Kaur
authors.affiliation: Department of Radiotherapy Physics, Royal Berkshire NHS Foundation Trust, Reading, UK.
authors.forename: G
authors.initials: G
authors.lastname: Cotten
authors.affiliation: Department of Radiotherapy Physics, Royal Berkshire NHS Foundation Trust, Reading, UK.
authors.forename: D
authors.initials: D
authors.lastname: Tomala
authors.affiliation: Department of Radiotherapy Physics, Royal Berkshire NHS Foundation Trust, Reading, UK.
authors.forename: C
authors.initials: C
authors.lastname: Lord
authors.affiliation: Department of Radiotherapy Physics, Royal Berkshire NHS Foundation Trust, Reading, UK.
authors.forename: J
authors.initials: J
authors.lastname: Jones
authors.affiliation: Department of Radiotherapy Physics, Royal Berkshire NHS Foundation Trust, Reading, UK.
authors.forename: A
authors.initials: A
authors.lastname: Adeyemi
mesh_terms: D002138:Calibration; D015331:Cohort Studies; D005189:False Positive Reactions; D006801:Humans; D021621:Imaging, Three-Dimensional; D009369:Neoplasms / Q000532:radiotherapy*; D019047:Phantoms, Imaging; D011446:Prospective Studies; D011829:Radiation Dosage; D011874:Radiometry / Q000379:methods*; D011880:Radiotherapy Planning, Computer-Assisted / Q000379:methods*; D020266:Radiotherapy, Conformal / Q000379:methods*; D015203:Reproducibility of Results; D012984:Software; D014057:Tomography, X-Ray Computed
keywords: EPIDs; Radiotherapy; Transit dosimetry; Treatment verification