Room: Exhibit Hall | Forum 4
Purpose: The combination of low-dose molecular radiotherapy (MRT) and checkpoint blockade can potentially enhance the immune response against immunologically â€œcoldâ€? tumors. To this end, we are investigating the capability of a radiolabeled alkylphosphocholine (NM600) to deliver immunomodulatory dose to the tumor microenvironment without inducing systemic immunosuppression. This work considers the dosimetric impact of prolonged â?¸â?¶Y-NM600 PET/CT imaging for theranostic treatment planning in â?¹â?°Y-NM600 MRT.
Methods: A companion dog presenting with widespread metastatic osteosarcoma was administered 178 MBq of â?¸â?¶Y-NM600, and longitudinal PET/CT scans were acquired at 2.5, 24, and 48 h post-injection. â?¸â?¶Y-NM600 PET/CT volumes were used in a Monte Carlo voxel-based dosimetry platform to calculate integral prescription doses (Gy/GBq) to ROIs by â?¹â?°Y-NM600 MRT and â?¸â?¶Y-NM600. The prolonged uptake and retention of â?¸â?¶Y-NM600 can be more precisely characterized with >48 h imaging but requires considerably more tracer activity given the 14.7 h half-life. The dosimetric impact of achieving similar 48 h â?¸â?¶Y-NM600 activity concentrations at later timepoints was assessed in the context of â?¹â?°Y-NM600 MRT delivering 2.0 Gy to the tumor.
Results: PET/CT imaging confirmed selective uptake and retention of â?¸â?¶Y-NM600 in tumors (3.4 SUV at 48 h) and hepatobiliary excretion of the tracer (4.0 SUV at 48 h). In the current protocol, â?¸â?¶Y-NM600 delivers only 5.8% of MRT to the tumor, but 14.5% of MRT to the red marrow. For prolonged imaging, tracer activity and imaging dose increases exponentially while tumor-specificity of the theranostic treatment decreases.
Conclusion: This work characterizes â?¸â?¶Y-NM600 tracer dosimetry and indicates the potential for imaging through 72-96 h. With modest compromises in image quality and scan duration at late timepoints, the requisite tracer activity for prolonged imaging can be reduced to limit any potential impact on tumor-specificity.