Real-time iterative monitoring of radiofrequency ablation tumor therapy with¹⁵O-water PET imaging

A method that provides real-time image-based monitoring of solid tumor therapy to ensure complete tumor eradication during image-guided interventional therapy would be a valuable tool. The short, 2-min half-life of ¹⁵O makes it possible to perform repeated PET imaging at 20-min intervals at multiple time points before and after image-guided therapy. In this study, ¹⁵O-water PET was evaluated as a tool to provide real-time feedback and iterative image guidance to rapidly monitor the intratumoral coverage of radiofrequency (RF) ablation therapy. Methods: Tumor RF ablation therapy was performed on head and neck squamous cell carcinoma (SCC) xenograft tumors (length, ∼23 mm) in 6 nude rats. The tumor in each animal was ablated with RF (1-cm active size ablation catheter, 70°C for 5 min) twice in 2 separate tumor regions with a 20-min separation. The ¹⁵O-water PET images were acquired before RF ablation and after the first RF and second RF ablations using a small-animal PET scanner. In each PET session, approximately 100 MBq of ¹⁵O-water in 1.0 mL of saline were injected intravenously into each animal. List-mode PET images were acquired for 7 min starting 20 s before injection. PET images were reconstructed by 2-dimensional ordered-subset expectation maximization into single-frame images and dynamic images at 10 s/frame. PET images were displayed and analyzed with software. Results: Pre-RF ablation images demonstrate that ¹⁵O-water accumulates in tumors with ¹⁵O activity reaching peak levels immediately after administration. After RF ablation, the ablated region had almost zero activity, whereas the unablated tumor tissue continued to have a high ¹⁵O-water accumulation. Using image feedback, the RF probe was repositioned to a tumor region with residual ¹⁵O-water uptake and then ablated. The second RF ablation in this new region of the tumor resulted in additional ablation of the solid tumor, with a corresponding decrease in activity on the ¹⁵O-water PET image. Conclusion: ¹⁵O-water PET clearly demonstrated the ablated tumor region, whereas the unablated tumor continued to show high ¹⁵O-water accumulation. ¹⁵O-water imaging shows promise as a tool for on-site, real-time monitoring of image-guided interventional cancer therapy.