title: Study of the impact of tissue density heterogeneities on 3-dimensional abdominal dosimetry: comparison between dose kernel convolution and direct Monte Carlo methods.
abstract: Dose kernel convolution (DK) methods have been proposed to speed up absorbed dose calculations in molecular radionuclide therapy. Our aim was to evaluate the impact of tissue density heterogeneities (TDH) on dosimetry when using a DK method and to propose a simple density-correction method.
  
  
  METHODS
  This study has been conducted on 3 clinical cases: case 1, non-Hodgkin lymphoma treated with (131)I-tositumomab; case 2, a neuroendocrine tumor treatment simulated with (177)Lu-peptides; and case 3, hepatocellular carcinoma treated with (90)Y-microspheres. Absorbed dose calculations were performed using a direct Monte Carlo approach accounting for TDH (3D-RD), and a DK approach (VoxelDose, or VD). For each individual voxel, the VD absorbed dose, D(VD), calculated assuming uniform density, was corrected for density, giving D(VDd). The average 3D-RD absorbed dose values, D(3DRD), were compared with D(VD) and D(VDd), using the relative difference Δ(VD/3DRD). At the voxel level, density-binned Δ(VD/3DRD) and Δ(VDd/3DRD) were plotted against ρ and fitted with a linear regression.
  
  
  RESULTS
  The D(VD) calculations showed a good agreement with D(3DRD). Δ(VD/3DRD) was less than 3.5%, except for the tumor of case 1 (5.9%) and the renal cortex of case 2 (5.6%). At the voxel level, the Δ(VD/3DRD) range was 0%-14% for cases 1 and 2, and -3% to 7% for case 3. All 3 cases showed a linear relationship between voxel bin-averaged Δ(VD/3DRD) and density, ρ: case 1 (Δ = -0.56ρ + 0.62, R(2) = 0.93), case 2 (Δ = -0.91ρ + 0.96, R(2) = 0.99), and case 3 (Δ = -0.69ρ + 0.72, R(2) = 0.91). The density correction improved the agreement of the DK method with the Monte Carlo approach (Δ(VDd/3DRD) < 1.1%), but with a lesser extent for the tumor of case 1 (3.1%). At the voxel level, the Δ(VDd/3DRD) range decreased for the 3 clinical cases (case 1, -1% to 4%; case 2, -0.5% to 1.5%, and -1.5% to 2%). No more linear regression existed for cases 2 and 3, contrary to case 1 (Δ = 0.41ρ - 0.38, R(2) = 0.88) although the slope in case 1 was less pronounced.
  
  
  CONCLUSIONS
  This study shows a small influence of TDH in the abdominal region for 3 representative clinical cases. A simple density-correction method was proposed and improved the comparison in the absorbed dose calculations when using our voxel S value implementation.
authors.affiliation: Department of Nuclear Medicine, Beaujon Hospital, Assistance Publique-Hôpitaux de Paris APHP, Clichy, France. arnaud.dieudonne@bjn.aphp.fr
authors.forename: Arnaud
authors.initials: A
authors.lastname: Dieudonné
authors.forename: Robert F
authors.initials: RF
authors.lastname: Hobbs
authors.forename: Rachida
authors.initials: R
authors.lastname: Lebtahi
authors.forename: Fabien
authors.initials: F
authors.lastname: Maurel
authors.forename: Sébastien
authors.initials: S
authors.lastname: Baechler
authors.forename: Richard L
authors.initials: RL
authors.lastname: Wahl
authors.forename: Ariane
authors.initials: A
authors.lastname: Boubaker
authors.forename: Dominique
authors.initials: D
authors.lastname: Le Guludec
authors.forename: Georges
authors.initials: G
authors.lastname: Sgouros
authors.forename: Isabelle
authors.initials: I
authors.lastname: Gardin
mesh_terms: D000911:Antibodies, Monoclonal; D006528:Carcinoma, Hepatocellular / Q000175:diagnosis / Q000000981:diagnostic imaging*; D006801:Humans; D021621:Imaging, Three-Dimensional; D007457:Iodine Radioisotopes; D008113:Liver Neoplasms / Q000175:diagnosis / Q000000981:diagnostic imaging*; D008187:Lutetium; D008228:Lymphoma, Non-Hodgkin / Q000175:diagnosis / Q000000981:diagnostic imaging*; D008863:Microspheres; D015233:Models, Statistical; D009010:Monte Carlo Method; D018358:Neuroendocrine Tumors / Q000175:diagnosis / Q000000981:diagnostic imaging*; D011868:Radioisotopes; D011874:Radiometry / Q000379:methods*; D011877:Radionuclide Imaging; D011879:Radiotherapy Dosage; D015021:Yttrium Radioisotopes
chemical_list: D000911:Antibodies, Monoclonal; D007457:Iodine Radioisotopes; D011868:Radioisotopes; D015021:Yttrium Radioisotopes; D008187:Lutetium; C119496:tositumomab I-131