18F-FDG Small-Animal PET for Monitoring the Therapeutic Effect of CT-Guided Radiofrequency Ablation on Implanted VX2 Lung Tumors in Rabbits

The primary goals of this study were to investigate the behavior of normal lung tissues after radiofrequency ablation (RFA) and to determine the suitability of 18 F-FDG PET, using a dedicated small-animal scanner, for monitoring the early therapeutic effects of RFA on VX2 lung tumors (VX2s) in rabbits. Methods: Fourteen Japanese white rabbits with normal lungs underwent RFA, followed by 18 F-FDG PET at 1 d and at 1, 2, 4, and 8 wk. In addition, 7 rabbits with untreated VX2s underwent 18 F-FDG PET, and 13 rabbits with RFA-treated VX2s underwent 18 F-FDG PET at 1 d (n = 7) or 1 wk (n = 6) after the treatment. Results: After RFA of normal lungs, ring-shaped accumulations of 18 F-FDG, which coincided with inflammation caused by ablation, were observed. The mean early- (40-60 min after injection) and delayed (100-120 min)-phase ablated lesion-to-muscle ratios were, respectively, 2.9 ± 1.0 and 3.3 ± 0.8 (1 d), 4.1 ± 0.6 and 5.2 ± 0.9 (1 wk), 4.1 ± 1.0 and 5.3 ± 1.5 (2 wk), 3.1 ± 0.5 and 3.6 ± 1.1 (4 wk), and 1.8 ± 0.1 and 2.3 ± 0.1 (8 wk). At 4 and 8 wk, the uptake was less than that at 1 and 2 wk (P < 0.05). VX2s showed mean tumor-to-muscle ratios of 6.6 ± 2.1 and 8.6 ± 3.3 at the early and delayed phases, respectively. For ablated tumors, the respective ratios were 0.8 ±0.4 and 1.1 ± 0.7 (1 d) and 1.2 ± 0.5 and 1.5 ± 0.7 (1 wk). These values were significantly lower than those for nonablated tumors (P < 0.001). Histopathologic examination confirmed the absence of viable tumors. 18 F-FDG accumulation around ablated tumors reflected thermally damaged normal tissues and was significantly lower than that of control VX2s (P < 0.01). Conclusion: Our data suggest that 18 F-FDG PET is promising for evaluating the therapeutic response of lung malignancies to RFA: Accumulation of 18 F-FDG in surrounding normal tissues appears to be time dependent, and the data suggest that, clinically, 18 F-FDG PET should be performed 4 wk or more after RFA. Delayed-phase images seem to better distinguish tumor from inflammation than do early-phase images.