Analysis of the influence of 111In on 90Y-bremsstrahlung SPECT based on Monte Carlo simulation

Objective 90 Y-ibritumomab tiuxetan (Zevalin) which is used for the treatment of malignant lymphomas can be used for SPECT imaging based on bremsstrahlung from 90 Y beta particles. However, gamma rays emitted by 111 In, which is administered to evaluate the indication for the treatment, contaminate the 90 Y bremsstrahlung images. Our objective is to investigate the influence of 111 In on the 90 Y 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 with 90 Y and 111 In mixed solutions. The activities of 90 Y in sphere 1 and sphere 2 were 241 and 394 kBq/mL, respectively, and the ones of 111 In were 8.14 and 13.3 kBq/mL, respectively. The background activity of 90 Y was 38.6 kBq/mL, whereas that of 111 In was 1.30 kBq/mL; moreover, the acquisition time was 30 min. Two energy windows were used: one is 90–190 keV included the 111 In 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, the 111 In 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, the 111 In 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 between 111 In and 90 Y in SPECT imaging is rather serious. Even for the energy window excluding the 111 In photopeak, the count ratio of 90 Y was less than 30 % of the total. However, the influence of 111 In on 90 Y-SPECT imaging cannot be ignored, and the count ratio because of 111 In is important to estimate the density of 90 Y.