Author: David P. Adam, Allison Cartee, Eric C. Frey, Michael Ghaly, Bin He, Robert Francois Hobbs, Natalie Rose Kania, Ana Kiess, Ian Marsh, Bonnie N. C. President, Steven Rowe, George Sgouros, Phuoc Tran, Hao Wang 👨🔬
Affiliation: Johns Hopkins University, Radiopharmaceutical Imaging and Dosimetry, LLC (Rapid), 2The Russell H. Morgan Department of Radiology and Radiological Science, Johns Hopkins University, Department of Radiation Oncology and Molecular Radiation Sciences, Johns Hopkins Medicine, Department of Radiation Oncology and Molecular Radiation Sciences, Johns Hopkins University, Johns Hopkins Medical Institute, Department of Radiation Oncology, University of Maryland School of Medicine, Department of Oncology, Johns Hopkins University, School of Medicine, Department of Radiology, University of North Carolina, Chapel Hill 🌍
Purpose: Advanced treatment paradigms combining SPECT/CT imageable, alpha emitting, radiopharmaceutical therapy (RPT) and stereotactic body radiotherapy (SBRT) can provide the potential of dose escalation due to synergistic cell kill at the expense of the corresponding orthogonal normal tissue toxicities from each modality. This work investigates retrospective combined dosimetry for an SBRT+ Ra-223 RPT treatment paradigm for five patients with oligiometastatic prostate cancer enrolled in a Phase II clinical trial with the application of Ra-223 SPECT/CT imaging-based dosimetry.
Methods: Novel acquisition and reconstruction methods facilitated Ra-223 SPECT/CT images. Serial SPECT/CT patient scans were performed at approximately 2-, 24-, and 48-hours post-injection of standard of care (SOC) Ra-223. SOC photon SBRT plans and dosimetry were performed using the RayStation treatment planning system (TPS). SPECT/CT images were registered in 3D Slicer to perform dose-rate calculations and integration; Ra-223 dosimetry was performed assuming local energy deposition to water. Ra-223 dosimetry was imported into the TPS and registered to sum the dosimetry. A Gy-RBE factor of 5 was utilized to characterize the alpha dose deposition biological effect.
Results: Activity concentrations ranged between 185-334 Bq/ml for the T5 vertebra contour of study patient 5. There are assumed to be large uncertainties associated with Ra-223 voxelized activity distributions and resultant dosimetry, therefore mean absorbed dose was the preferred quantity. For the T5 lesion, 0.08 Gy/MBq, 0.42 Gy/cycle, a cumulative 2.52 Gy, and 12.6 RBE-Gy was calculated from the Ra-223. The SBRT treatment plan had a mean T5 dose of 32.87 Gy, resulting in a combined mean 45.4 RBE-Gy for T5.
Conclusion: This work demonstrates the potential of SPECT imaging of alpha emitting compounds to perform clinical dosimetry for combined SBRT+RPT paradigms. Incorporating RPT dose during the optimization of EBRT plans can further improve the quality of treatment plans and provide physicians with additional flexibility in treatment decisions.