Fusion imaging proves useful for RFA follow-up

PET/CT provides reliable and accurate information on treatment response and disease recurrence for patients undergoing radiofrequency ablation treatment of primary and metastatic liver tumors, according to a series of studies by German researchers.

While it's too early to jump to definitive conclusions, particularly considering the sample sizes of the studies, PET/CT sensitivity proved slightly superior to MRI and might have other advantages compared with standard imaging, according to coauthor Dr. Patrick Veit from the department of diagnostic and interventional radiology and neuroradiology at the University Hospital of Essen.

"PET/CT might be the best way right now to assess patients after RFA," Veit said.

The positive remarks are based on study results that Veit's team presented at the European Congress of Radiology in Vienna.

The rim-like enhancement of a lesion's margins after ablation usually compromise anatomic imaging's ability to anticipate disease recurrence. In their first study, Veit and colleagues compared PET/CT with MRI to determine which was more accurate at detecting recurrence after RFA. Researchers enrolled 20 patients with 28 primary and metastatic liver lesions who underwent MRI and PET/CT before and 24 hours after ablation, with follow-up at one, three, six, and 12 months.

Although they learned that very small areas of residual tumor may be difficult to detect with either modality, they found PET/CT more accurate in detecting local tumor recurrence after RFA. PET/CT identified nine out of 10 recurrences, whereas MRI detected only six of these lesions. PET/CT and MRI's sensitivity, specificity, and accuracy rates were 82% / 69.3%, 100% / 100%, and 91% / 89.7 %, respectively.

The post-RFA rim-like contrast enhancement phenomenon may also blur the lines between residual tumor and regenerative tissue on anatomic imaging and increase tracer uptake on FDG-PET. The team's second study compared the appearance of normal liver tissue after RFA on FDG-PET/CT and contrast-enhanced CT and contrast-enhanced MRI. The investigators induced two RFA lesions, 3 cm in diameter each, in 21 fully-anesthetized, healthy miniature pigs. They performed post-interventional follow-up with all three modalities at one, three, and 10 days; and at one, two, three, and six months post-intervention.

They found FDG-PET and PET/CT were superior to anatomic imaging when assessing residual tumor. There was a caveat, though. Functional imaging has to be performed within one day of intervention. Peripheral hyperperfusion and tissue regeneration may limit visualization on follow-up at a later time on either functional or anatomic data sets.

Currently, all patients undergoing RFA at Essen's hospital receive PET/CT before and 24 hours after the procedure. One of the advantages is being able to see some metastases earlier and rely only on anatomic imaging RFA guidance instead of performing PET and then another CT for the intervention, Veit said.

One considerable drawback, however, is that fusion imaging takes longer than CT-only evaluation. It is also more expensive than CT or ultrasound, and at least as expensive as MRI. Researchers did not evaluate fusion imaging's cost-effectiveness in these studies.

Patients and clinicians should know that PET/CT provides more accuracy for post-RFA surveillance. However, PET and CT should be performed at full diagnostic capacity, meaning full FDG dose and IV contrast, respectively, Veit said.

"For some clinical questions, a suboptimal low-dose CT without contrast might be enough. When we are looking for small lesions, full diagnostic CT and PET data are indispensable," he said.

For more information from the Diagnostic Imaging archives:

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East and West shake hands over liver RFA

MR juggles obstacles, advantages in liver RFA guidance

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