title: Effect of respiratory motion on internal radiation dosimetry.
abstract: OBJECTIVE
  Estimation of the radiation dose to internal organs is essential for the assessment of radiation risks and benefits to patients undergoing diagnostic and therapeutic nuclear medicine procedures including PET. Respiratory motion induces notable internal organ displacement, which influences the absorbed dose for external exposure to radiation. However, to their knowledge, the effect of respiratory motion on internal radiation dosimetry has never been reported before.
  
  
  METHODS
  Thirteen computational models representing the adult male at different respiratory phases corresponding to the normal respiratory cycle were generated from the 4D dynamic XCAT phantom. Monte Carlo calculations were performed using the mcnp transport code to estimate the specific absorbed fractions (SAFs) of monoenergetic photons/electrons, the S-values of common positron-emitting radionuclides (C-11, N-13, O-15, F-18, Cu-64, Ga-68, Rb-82, Y-86, and I-124), and the absorbed dose of (18)F-fluorodeoxyglucose ((18)F-FDG) in 28 target regions for both the static (average of dynamic frames) and dynamic phantoms.
  
  
  RESULTS
  The self-absorbed dose for most organs/tissues is only slightly influenced by respiratory motion. However, for the lung, the self-absorbed SAF is about 11.5% higher at the peak exhale phase than the peak inhale phase for photon energies above 50 keV. The cross-absorbed dose is obviously affected by respiratory motion for many combinations of source-target pairs. The cross-absorbed S-values for the heart contents irradiating the lung are about 7.5% higher in the peak exhale phase than the peak inhale phase for different positron-emitting radionuclides. For (18)F-FDG, organ absorbed doses are less influenced by respiratory motion.
  
  
  CONCLUSIONS
  Respiration-induced volume variations of the lungs and the repositioning of internal organs affect the self-absorbed dose of the lungs and cross-absorbed dose between organs in internal radiation dosimetry. The dynamic anatomical model provides more accurate internal radiation dosimetry estimates for the lungs and abdominal organs based on realistic modeling of respiratory motion. This work also contributes to a better understanding of model-induced uncertainties in internal radiation dosimetry.
authors.affiliation: Division of Nuclear Medicine and Molecular Imaging, Geneva University Hospital, Geneva 4 CH-1211, Switzerland.
authors.forename: Tianwu
authors.initials: T
authors.lastname: Xie
authors.affiliation: Division of Nuclear Medicine and Molecular Imaging, Geneva University Hospital, Geneva 4 CH-1211, Switzerland; Geneva Neuroscience Center, Geneva University, Geneva CH-1205, Switzerland; and Department of Nuclear Medicine and Molecular Imaging, University of Groningen, University Medical Center Groningen, Groningen 9700 RB, The Netherlands.
authors.forename: Habib
authors.initials: H
authors.lastname: Zaidi
mesh_terms: D000328:Adult; D003198:Computer Simulation; D004583:Electrons; D019788:Fluorodeoxyglucose F18; D006801:Humans; D021621:Imaging, Three-Dimensional; D008297:Male; D009010:Monte Carlo Method; D009068:Movement; D017785:Photons; D049268:Positron-Emission Tomography / Q000379:methods*; D011829:Radiation Dosage; D011868:Radioisotopes / Q000737:chemistry; D011874:Radiometry / Q000295:instrumentation* / Q000379:methods*; D012119:Respiration
chemical_list: D011868:Radioisotopes; D019788:Fluorodeoxyglucose F18