title: Energy Dependence of Measured CT Numbers on Substituted Materials Used for CT Number Calibration of Radiotherapy Treatment Planning Systems.
abstract: BACKGROUND
  For accurate dose calculations, it is necessary to provide a correct relationship between the CT numbers and electron density in radiotherapy treatment planning systems (TPSs). The purpose of this study was to investigate the energy dependence of measured CT numbers on substituted materials used for CT number calibration of radiotherapy TPSs and the resulting errors in the treatment planning calculation doses.
  
  
  METHODS
  In this study, we designed a cylindrical water phantom with different materials used as tissue equivalent materials for the simulation of tissues and obtaining the related CT numbers. For evaluating the effect of CT number variations of substituted materials due to energy changing of scanner (kVp) on the dose calculation of TPS, the slices of the scanned phantom at three kVp's were imported into the desired TPSs (MIRS and CorePLAN). Dose calculations were performed on two TPSs.
  
  
  RESULTS
  The mean absolute percentage differences between the CT numbers of CT scanner and two treatment planning systems for all the samples were 3.22%±2.57% for CorePLAN and 2.88%±2.11% for MIRS. It was also found that the maximum absolute percentage difference between all of the calculated doses from each photon beam of linac (6 and 15 MV) at three kVp's was less than 1.2%.
  
  
  CONCLUSIONS
  The present study revealed that, for the materials with effective low atomic number, the mean CT number increased with increasing energy, which was opposite for the materials with an effective high atomic number. We concluded that the tissue substitute materials had a different behavior in the energy ranges from 80 to 130 kVp. So, it is necessary to consider the energy dependence of the substitute materials used for the measurement or calibration of CT number for radiotherapy treatment planning systems.
authors.affiliation: Department of Medical Physics and Imaging, Urmia University of Medical Sciences, Urmia, Iran.
authors.forename: Reza
authors.initials: R
authors.lastname: Mahmoudi
authors.affiliation: Solid Tumor Research center, Urmia University of Medical Sciences, Urmia, Iran.
authors.forename: Nasrollah
authors.initials: N
authors.lastname: Jabbari
authors.affiliation: Radiotherapy Center of the Omid Hospital, Urmia, Iran.|Radiotherapy Center of Parto, Urmia, Iran.
authors.forename: Mehdi
authors.initials: M
authors.lastname: Aghdasi
authors.affiliation: Department of Biostatistics and Epidemiology, Urmia University of Medical Sciences, Urmia, Iran.
authors.forename: Hamid Reza
authors.initials: HR
authors.lastname: Khalkhali
mesh_terms: D002138:Calibration; D006801:Humans; D019047:Phantoms, Imaging; D010934:Planning Techniques; D011879:Radiotherapy Dosage; D061089:Radiotherapy, Image-Guided / Q000295:instrumentation* / Q000379:methods*; D014057:Tomography, X-Ray Computed / Q000295:instrumentation* / Q000379:methods