Quantitative SPECT/CT imaging of Ra224

Introduction: Quantitative SPECT/CT imaging of Ra224 Authors: S. Grønningsæter¹, C. Stokke^1,2 and LT. Mikalsen^1,31Division of Radiology and Nuclear Medicine, Oslo University Hospital, Oslo, Norway²Department of Physics, University of Oslo, Oslo, Norway³ Department of Life Sciences and Health, Oslo Metropolitan University, Oslo, Norway

Introduction: Ra-224 bound to calcium-carbonate particles is currently being investigated in two clinical phase 1 trials as a therapeutic agent for patients with intraperitoneal carcinomatosis originating from ovarian and colorectal cancer. While the radionuclide’s therapeutic properties are based on emission of alpha-particles, there are also a substantial number of photons originating from several steps of the decay chain, especially by the daughter Pb-212. There could therefore be a potential for theragnostic SPECT/CT imaging. If successful quantification can be performed, it will allow dosimetry studies, resulting in a greater control of how the activity is distributed in the body. The aim of the current study was to investigate the feasibility of quantitative Ra-224 SPECT/CT imaging.Methods: A hollow cylindrical phantom of 6283 ml was filled with a homogenous mixture of Ra-224-RaCl with an activity concentration of 1,08 kBq/ml and used to estimate a calibration factor for Ra224 by repeated scans. The activity recovery for spheres was evaluated using a NEMA IEC PET Body Phantom, without the lung insert. The phantom’s spheres were filled with Ra-224 with a concentration of 49,3 kBq/ml and scanned repeatedly over 14 days. Ra224 has a half life of 3.6 days. A Siemens Symbia Intevo Bold SPECT/CT was used with medium energy collimators (ME), a 20 % energy window at 240 keV, dual 5 % scatter windows, 60 views and 30 minutes acquisition time. The reconstruction was performed with Flash3d in the Tomo Reconstruction workflow using Syngo MI Applications. A 12 mm Gaussian filter was used both as a post filter and on the scatter window. The reconstruction used CT for attenuation correction. Seven different iterative levels varying from 10-900 iterations × subsets were investigated.

Results: In the homogeneous phantom we obtained a calibration factor of 4,6 ± 0,3 cnts/hour per kBq/ml for the current clinical reconstruction with 30×2 iterations×subsets. This collapsed when the activity concentration fell below 0,1 kBq/ml (0,7 MBq). In the body phantom, we obtained decent results across a range of iterative settings from 30-120 updates. Pooling the data from three repetitions at each of three time points (day 4 - 7 - 12), the range of recovery coefficients for of these reconstructions at increasing sphere sizes was (3-8 %), (6-19 %), (10-28 %), (25-61 %), (62-99 %) and (133-135 %), respectively. The largest sphere had converged at 30 updates (135 %), but not at 15 (85 %), while the second largest increased throughout the specified range.The most precise result in the two largest spheres was obtained at 30×1 for the largest sphere (CoV = 5.0 %) and at 30x4 iterations for the second largest (CoV = 5.2 %). The coefficient of variations for the current clinical reconstruction (30×2) were 76 %, 42 %, 47 %, 34 %, 7 % and 6 %, for increasing sphere sizes. The most accurate results were obtained with 30×30 iterations×subsets with recovery coefficients of 101 % and 127 % in the two largest spheres, but with poor precision (CoV = 13 %).Conclusion: Quantification of Ra-224 shows promising

Results: The current clinical reconstruction is in the region with the best compromise between accuracy and precision and is therefore a reasonable choice for further studies.