Double duty: Split-dose FDG PET/CT guides and confirms tumor ablation

A two-step method of F-18 FDG dosing during guided PET/CT tumor ablation procedures has been found to improve post-treatment monitoring without compromising on localization and also provides an opportunity to reintervene while patients are still on the table, according to a study published online April 5 in Radiology.

Edmund Ronan Ryan, from the department of radiology and division of interventional radiology at Memorial Sloan-Kettering Cancer Center in New York City, and colleagues performed split-dose F-18 FDG PET/CT guidance and post-procedural treatment assessment for a range of ablation methods. The technique provided better clearance of a relatively lower guidance dose followed by a stronger dose of FDG to pick up any remaining metabolic activity from residual tumors.

“Split-dose FDG PET/CT may be a useful tool to provide both guidance and endpoint evaluation, allowing an op­portunity for repeat intervention if necessary,” wrote Ryan et al.

A group of 23 patients with 29 active tumors underwent minimally invasive percutaneous ablation of their tumors with split-dose F-18 FDG fused PET/CT for localization of tumors and immediate follow-up evaluation of treatment. Most tumors were of the liver, four tumors were of the lung and one each in the thigh and adrenal gland. The majority of tumors, 18 total, were deemed metastatic colorectal adenocarcinoma, and the remaining were of a range of metastatic tumors, with one categorized as primary hepatocellular carcinoma. Typical treatment of choice was radiofrequency ablation for 17 tumors, and other methods included cryoablation for one tumor, irreversible electroporation for five and microwave ablation for the remaining six.

Initial guidance of ablative instruments was carried out using fused PET/CT for 26 lesions and a combination of ultrasound and PET/CT was indicated for three lesions using a technique of repeated breath-hold CT images fused to initial PET data. Split dose F-18 FDG methodology included a localization dose of 4-mCi of FDG followed by a post-procedure dose of 8-mCi for same-site treatment assessment. 

“Contrast material–enhanced CT, US, and MR imaging have all been used to assess and confirm ablation of the tumor and margin. However, while contrast-enhanced imaging tech­niques can depict the morphologic and perfusion changes caused by ablation, these techniques lack the metabolic in­formation provided with PET imaging. FDG PET has been demonstrated to be a useful tool to assess the effective­ness of ablation by detecting the pres­ence of residual or recurrent viable tu­mor tissue at follow-up at various time periods after the ablation, although inflammation can confound evaluation at early time points.”

Immediate follow-up showed one instance of residual FDG activity, which led to immediate biopsy and additional intervention. Additional postprocedure follow-up at a later date showed tumor recurrence in two of the 29 original tumors. This was attributed to proximity to sensitive anatomy.

“One recurrence followed abla­tion of a 4.4-cm lesion close to a ma­jor bile duct that was ablated by using irreversible electroporation to reduce the risk of bile duct injury,” wrote the authors. “The second recurrence followed radiofrequency ab­lation of a caudate lobe lesion adjacent to the main portal vein.”

Median uptake from localization dose to PET scan was 78 minutes and 35 minutes for postprocedure evaluation. Average span of time between injections was 261 minutes. First dose was effectively cleared, leaving about 0.65 mCi remaining at the point of redose. Localization imaging was found to be noisier, but had no adverse effect on localization and researchers estimated a two-thirds reduction in radiation dose for imaging operators.

“All ablations were successfully completed according to the preprocedure plan for each patient,” concluded the authors. “In 28 of 29 lesions, no re­sidual PET activity was identified at the immediate postablation PET acquisi­tion. Contrast-enhanced CT performed at the end of ablation also confirmed technical success.”

Repeat studies will need to be conducted to replicate these findings before guidelines could change to reflect this new dosing protocol.