title: Analysis of the influence of111In on90Y-bremsstrahlung SPECT based on Monte Carlo simulation.
abstract: OBJECTIVE
  90Y-ibritumomab tiuxetan (Zevalin) which is used for the treatment of malignant lymphomas can be used for SPECT imaging based on bremsstrahlung from90Y beta particles. However, gamma rays emitted by111In, which is administered to evaluate the indication for the treatment, contaminate the90Y bremsstrahlung images. Our objective is to investigate the influence of111In on the90Y SPECT images using Monte Carlo simulation.
  
  
  METHODS
  We used an in-house developed simulation code for the Monte Carlo simulation of electrons and photons (MCEP). Two hot spheres with diameters of 40 mm were put in an elliptical phantom. Both spheres ("sphere 1" and "sphere 2") were filled with90Y and111In mixed solutions. The activities of90Y in sphere 1 and sphere 2 were 241 and 394 kBq/mL, respectively, and the ones of111In were 8.14 and 13.3 kBq/mL, respectively. The background activity of90Y was 38.6 kBq/mL, whereas that of111In was 1.30 kBq/mL; moreover, the acquisition time was 30 min. Two energy windows were used: one is 90-190 keV included the111In photopeak; the other is 90-160 keV. To evaluate the quality of the SPECT images, the contrast recovery coefficient (CRC) and the constant noise ratio (CNR) of the SPECT images were derived.
  
  
  RESULTS
  For the energy window between 90 and 160 keV, the111In count was 74 % of the total. In that case, the CRC values were 30.1 and 30.7 % for "sphere 1" and "sphere 2", respectively, whereas the CNR values were 6.8 and 12.1, respectively. For the energy window between 90 and 190 keV, the111In count reached 85 % of the total count. The CRC and CNR values were 38.6 and 40.0 % and 10.6 and 19.4, respectively.
  
  
  CONCLUSIONS
  Our simulation study revealed that the cross talk between111In and90Y in SPECT imaging is rather serious. Even for the energy window excluding the111In photopeak, the count ratio of90Y was less than 30 % of the total. However, the influence of111In on90Y-SPECT imaging cannot be ignored, and the count ratio because of111In is important to estimate the density of90Y.
authors: Division of Medical Quantum Science, Department of Health Sciences, Graduate School of Medical Sciences, Kyushu University, 3-1-1 Maidashi, Higashi-ku, Fukuoka, 812-8582, Japan.
authors: Hiroaki
authors: H
authors: Shiba
authors: Division of Medical Quantum Science, Department of Health Sciences, Graduate School of Medical Sciences, Kyushu University, 3-1-1 Maidashi, Higashi-ku, Fukuoka, 812-8582, Japan. takahsr@hs.med.kyushu-u.ac.jp.
authors: Akihiko
authors: A
authors: Takahashi
authors: Department of Clinical Radiology, Graduate School of Medical Sciences, Kyushu University, Fukuoka, Japan.
authors: Shingo
authors: S
authors: Baba
authors: Division of Radiology, Department of Medical Technology, Kyushu University Hospital, Fukuoka, Japan.
authors: Kazuhiko
authors: K
authors: Himuro
authors: Division of Radiology, Department of Medical Technology, Kyushu University Hospital, Fukuoka, Japan.
authors: Yasuo
authors: Y
authors: Yamashita
authors: Division of Medical Quantum Science, Department of Health Sciences, Graduate School of Medical Sciences, Kyushu University, 3-1-1 Maidashi, Higashi-ku, Fukuoka, 812-8582, Japan.
authors: Masayuki
authors: M
authors: Sasaki
mesh_terms: D007205:Indium Radioisotopes; D009010:Monte Carlo Method; D019047:Phantoms, Imaging; D015899:Tomography, Emission-Computed, Single-Photon; D015021:Yttrium Radioisotopes
chemical_list: D007205:Indium Radioisotopes; D015021:Yttrium Radioisotopes
keywords: 111In; 90Y; Ibritumomab tiuxetan (Zevalin); Monte Carlo simulation; Single photon emission computed tomography (SPECT)