A Monte Carlo study on ²²³Ra imaging for unsealed radionuclide therapy

Purpose: Radium-223 (²²³Ra), an α-emitting radionuclide, is used in unsealed radionuclide therapy for metastatic bone tumors. The demand for qualitative ²²³Ra imaging is growing to optimize dosimetry. The authors simulated ²²³Ra imaging using an in-house Monte Carlo simulation code and investigated the feasibility and utility of ²²³Ra imaging. Methods: The Monte Carlo code comprises two modules, HEXAGON and NAI. The HEXAGON code simulates the photon and electron interactions in the tissues and collimator, and the NAI code simulates the response of the NaI detector system. A 3D numeric phantom created using computed tomography images of a chest phantom was installed in the HEXAGON code. ²²³Ra accumulated in a part of the spine, and three x-rays and 19 γ rays between 80 and 450 keV were selected as the emitted photons. To evaluate the quality of the ²²³Ra imaging, the authors also simulated technetium-99m (^99mTc) imaging under the same conditions and compared the results. Results: The sensitivities of the three photopeaks were 147 counts per unit of source activity (cps MBq^-1; photopeak: 84 keV, full width of energy window: 20%), 166 cps MBq^-1 (154 keV, 15%), and 158 cps MBq^-1 (270 keV, 10%) for a low-energy general-purpose (LEGP) collimator, and those for the medium-energy general-purpose (MEGP) collimator were 33, 13, and 8.0 cps MBq^-1, respectively. In the case of ^99mTc, the sensitivity was 55 cps MBq^-1 (141 keV, 20%) for LEGP and 52 cps MBq^-1 for MEGP. The fractions of unscattered photons of the total photons reflecting the image quality were 0.09 (84 keV), 0.03 (154 keV), and 0.02 (270 keV) for the LEGP collimator and 0.41, 0.25, and 0.50 for the MEGP collimator, respectively. Conversely, this fraction was approximately 0.65 for the simulated ^99mTc imaging. The sensitivity with the LEGP collimator appeared very high. However, almost all of the counts were because of photons that penetrated or were scattered in the collimator; therefore, the proportions of unscattered photons were small. Conclusions: Their simulation study revealed that the most promising scheme for ²²³Ra imaging is an 84-keV window using an MEGP collimator. The sensitivity of the photopeaks above 100 keV is too low for ²²³Ra imaging. A comparison of the fractions of unscattered photons reveals that the sensitivity and image quality are approximately two-thirds of those for ^99mTc imaging.