title: Evaluation of parameters influencing S values in mouse dosimetry.
abstract: Clinical radionuclide therapy studies are commonly preceded by studies with small animals. Reliable evaluation of therapeutic efficacy must be based on accurate dosimetry. This study was performed to evaluate the influence of the mass of organs, the shape of organs, and the distances between organs on S values for mice.
  
  
  METHODS
  A voxel-based version of a geometric model of a mouse was developed for input in our Monte Carlo program based on EGS4. Simulations were made for each source organ separately to resolve the S values for each organ. For verification purposes, S values were calculated for spheres of different masses and compared with the S values in the MIRDOSE3.1 software and with the S values on the Radiation Dose Assessment Resource Web site. The variation in the mass of the organs was determined from dissected mice. The influence of the shape of an organ was investigated by successive elongation of a sphere into spheroids with a constant mass. The right kidney was moved in the phantom of the mouse to evaluate the effect of organ distances on S values. The absorbed fractions for the mouse model presented here were compared with the results from some previously published models. The radionuclides used were (90)Y, (131)I, (111)In, and (99m)Tc.
  
  
  RESULTS
  The results showed that the organ mass for one animal can differ by up to 33% from the mean mass. If linear interpolation from S value tables is used to obtain an S value for the specific mass of an organ, then the S value can differ by up to 80% from its true value. The corresponding deviation obtained by scaling according to mass is 20%. The shape of an organ was found to be the least important parameter for the S value. The cross-absorbed S value is strongly dependent on the geometry and the emitted radiation. For example, a 9.2-mm movement of the kidney can cause the S value from the liver to the right kidney to decrease to 0.05% of its original value for (90)Y.
  
  
  CONCLUSIONS
  We conclude that the mass and the shape of organs and their locations relative to each other have considerable effects on mouse dosimetry.
authors.affiliation: Department of Medical Radiation Physics, The Jubileum Institute, Lund University, SE-221 85 Lund, Sweden. Cecilia.Hindorf@radfys.lu.se
authors.forename: Cecilia
authors.initials: C
authors.lastname: Hindorf
authors.forename: Michael
authors.initials: M
authors.lastname: Ljungberg
authors.forename: Sven-Erik
authors.initials: SE
authors.lastname: Strand
mesh_terms: D000465:Algorithms; D000818:Animals; D001822:Body Burden; D003198:Computer Simulation; D018499:Linear Energy Transfer / Q000502:physiology*; D051379:Mice; D008954:Models, Biological; D009010:Monte Carlo Method; D009928:Organ Specificity; D011874:Radiometry / Q000379:methods*; D011879:Radiotherapy Dosage; D012062:Relative Biological Effectiveness; D015203:Reproducibility of Results; D012680:Sensitivity and Specificity; D014915:Whole-Body Counting / Q000379:methods*