History
A 67-year-old woman with metastatic colorectal carcinoma with previous history of left lateral hepatic segmentectomy for metastases presented with recurrence in the liver at the resection margin. In view of a solitary liver metastasis amenable to percutaneous ablation, the patient was admitted for image-guided microwave ablation.
Findings
Microwave ablation was planned to include Fludeoxyglucose F18 (18F FDG) PET/CT metabolic imaging guidance with second injection of 18F FDG to assess perfusion of peritumoral liver tissue to confirm complete tumor ablation with an adequate margin. On the day of ablation therapy, the patient was injected with 8 mCi (296 MBq) of 18F FDG about 1 hour prior to ablation therapy. The patient was brought into the Advanced Multimodality Image Guided Operating (AMIGO)1 suite equipped with the Biograph™ mCT (configured with CT Fluoroscopy). The patient was placed under general anesthesia. An initial planning PET/CT was performed with the patient supine in the treatment position. Both CT and PET acquisitions were acquired covering a single bed position over the liver during suspended respiration under general anesthesia. The CT and PET images were both acquired during a single 30-second breath-hold and were obtained under suspended ventilation in order to eliminate respiratory motion and ensure optimal image registration.
The fused PET/CT images were used to plan the location of insertion of the microwave ablation probe, along with its direction and distance to the tumor. A single microwave probe was inserted following a small skin incision and advanced to the planned depth based on the PET/CT. After confirmation of the correct positioning of the microwave probe within the tumor by another PET/CT (similar to the initial one using another 30-second suspension of ventilation to eliminate respiratory motion), the microwave ablation was performed. Immediately after ablation, a contrast enhanced CT followed by PET acquisition using suspended ventilation for 30 seconds was performed to determine the extent of the ablation zone and assess the amount of residual hypermetabolic tumor beyond the ablation zone.
As shown on Figure 3, the ablated tumor retains pre-procedurally administered 18F FDG on the post ablation PET/CT performed with administration of CT contrast. The tumor uptake of 18F FDG in relation to the hypoenhancing ablation zone seen on the fused images confirms that the ablation zone is inadequate at the supero-medial aspect of the tumor. Based on the delineation of the post ablation PET/CT, an overlapping microwave ablation was further performed in order to extend the ablation of the supero-medial margin and include the entire metastatic tumor.
Following the second microwave ablation procedure, a PET perfusion study was performed to demonstrate the perfusion of the unablated liver surrounding the ablation zone. A second dose of 3 mCi (111 MBq) of 18F FDG was injected and five minutes later a single bed position PET/CT acquisition was performed during a 60-second breath-hold with suspended ventilation in order to eliminate respiratory motion.
The trapped 18F FDG within the tumor after ablation enabled persistent visualization of the entire tumor. The second injection of 18F FDG demonstrated perfusion of the normal liver tissue following the completion of the microwave ablation with increased contrast between well-perfused normal liver tissue and non-perfused ablation zone thereby clearly demonstrating the ablation zone margins and confirming that the entire metastatic tumor has been covered by the ablation zone. The tumor shows 18F FDG from the pre-procedural injection trapped within it and PET/CT images acquired without respiratory motion is clearly able to define the margin of the tumor and that of the hypoperfused ablation zone and confirm that the ablation zone completely covers the entire tumor thereby confirming a successful ablation procedure without evidence of residual tumor.
Comments
Microwave ablation achieves tumor cell killing by heating of tissues ultimately leading to coagulation necrosis. Uptake of 18F FDG within the tumor does not dissipate following microwave ablation as evident from the persistent 18F FDG uptake within the tumor immediately after the microwave ablation, which helps determine the extent of coverage of the ablation zone following microwave ablation and presence of hypermetabolic tumor beyond the ablation zone. As demonstrated by the PET/CT images acquired immediately after the initial microwave ablation, a substantial amount of tumor laid beyond the margins of the initial ablation and a subsequent ablation was required to ensure complete tumor coverage. However, to better visualize the extent of microwave ablation, a second 18F FDG injection and a subsequent breath-hold PET/CT acquisition immediately following the injection was required to demonstrate the perfusion of the normal liver tissue around the microwave ablation zone which does not have any perfusion. The increased perfusion of the normal liver and the complete absence of perfusion in the ablation zone creates a sharp contrast in 18F FDG uptake which helps visually ascertain the exact margin of the ablation zone and clearly determine if the tumor (which shows 18F FDG uptake trapped within it from the pre-procedural injection) has been adequately covered by the ablation zone or not. In this case, the second ablation procedure guided by PET/CT was adequate to cover the entire tumor.
Conclusion
18F FDG PET/CT acquired with breath-hold created by suspension of ventilation during anesthesia can help guide microwave ablation procedures and help achieve complete ablation of tumor. In this case example, a second injection of 18F FDG improves visualization of ablation margins by demonstrating perfusion of normal liver and highlighting the contrast between normally perfused liver and non-perfused ablation zone margins. The use of a widely available isotope, such as 18F FDG, to demonstrate liver perfusion in this way can further expand the use of PET/CT guided ablation procedures
