Feasibility of Thorium-227/Radium-223 Gamma-Camera Imaging During Radionuclide Therapy.

Thorium-227 (Th) is a long-lived (T = 18.7 d) α-emitter that has emerged as candidate for radioimmunotherapy. Imaging of patients treated with thorium-227 conjugates is challenging due to the low activity administered and to photon emissions with low yields. In addition, the radioactive daughter radium-223 (Ra) have photon emissions in the same energy range as Th. The long half-life of Ra (T = 11.4 d) and the possibility of redistribution motivates efforts to separate Th and Ra. The aim of this study was to investigate the feasibility of imaging of patients treated with Th-labeled-monoclonal antibody (mAb) and to determine acquisition and image processing parameters to enable discrimination between Th and Ra. Imaging was performed with a GE Discovery 670 NM/CT γ-camera. Radionuclide separation with different energy windows (EW) and collimators was studied in images of vials with either Th or Ra. Phantom acquisitions with clinically relevant activities were performed to assess image quality and the usefulness of background subtraction and spatial filtering. Two patients treated with Th-labeled-mAb were imaged. Imaging of vials showed that Ra can be distinguished from Th using multiple energy windows. Medium- and high-energy collimators showed similar performance of sensitivity and spatial resolution, whereas the low-energy collimator had higher sensitivity but poor resolution due to collimator penetration. Visually, the image quality was improved with background subtraction and spatial filtering. The patient images exhibited the expected image quality and a possibility to separate Th and Ra. γ-Camera imaging of patients treated with Th-mAb is feasible and Ra can be distinguished from Th. Image quality is substantially improved using background subtraction and a spatial smoothing filter. Acquisition settings recommended for planar images are: high-energy general purpose or medium-energy general purpose collimator, 40 min acquisition time and energy windows: (1) 70-100 keV (Th and Ra); (2) 215-260 keV (Th); (3) 260-290 keV (Ra); (4) 350-420 keV (Ra).