LiteratureWatch July-December 2001

Comparison of renal ablation with monopolar radiofrequency and hypertonic-saline-augmented bipolar radiofrequency: in vitro and in vivo experimental studies

OBJECTIVE

We sought to determine whether hypertonic-saline (HS)-augmented bipolar radiofrequency ablation has advantages over monopolar radiofrequency ablation for creating larger areas of coagulation necrosis in the kidney.

MATERIALS AND METHODS

Using a 200-W generator and bipolar perfused-cooled electrodes or a monopolar cooled-tip electrode, we performed 14 radiofrequency ablations in explanted bovine kidneys. Radiofrequency was applied in standard monopolar (n = 7) or bipolar (n = 7) modes at 100 W for 10 min. In the bipolar mode, the perfused-cooled electrodes were placed at interelectrode distances of 3 cm, and a 6% sodium chloride solution was instilled into tissue at a rate of 2 mL/min through the electrodes. For in vivo experiments, either monopolar (n = 7) or HS-augmented bipolar (n = 7) radiofrequency ablation was performed in the lower pole of canine kidneys. Three days after the procedure, contrast-enhanced CT scans were obtained to evaluate the volumes of the ablation regions, and the kidneys were harvested for gross measurements. Technical parameters such as changes in impedance and current during radiofrequency ablation and dimensions of the thermal ablation zones were compared between the two groups.

RESULTS

In ex vivo and in vivo experiments, the frequency of the pulsed radiofrequency application caused by rises in impedance was higher in the monopolar mode than in the bipolar mode during the application of radiofrequency energy. The in vivo study showed that the bipolar radiofrequency ablation allowed larger mean current flows than the monopolar radiofrequency ablation (i.e., mean +/- SD, 1,654 +/- 144 mA vs 967 +/- 597 mA) (p < 0.05). Ex vivo studies revealed that the volumes of bipolar radiofrequency-induced ablation regions were substantially larger than those of monopolar radiofrequency-induced ablation regions (26.1 +/- 10.5 cm(3) vs 10.2 +/- 4.2 cm(3)). In vivo studies showed bipolar radiofrequency ablation achieved larger coagulation necrosis than monopolar radiofrequency (3.2 +/- 0.3 cm vs 2.4 +/-0.4 cm) (p < 0.05). This was confirmed by the measured volume of nonenhancing area on contrast-enhanced CT (20.4 +/- 6.4 cm(3) vs 13.5 +/- 6.0 cm(3)).

CONCLUSION

HS-augmented bipolar radiofrequency ablation using perfused-cooled electrodes shows better performance in creating coagulation necrosis than monopolar radiofrequency ablation in the kidney of an animal model.

Cryoablation of Renal and Prostate Tumors

During the past decade, cryoablation has been applied to benign and malignant conditions within the prostate and kidney. The essence of cryosurgery lies in producing temperatures low enough to cause necrosis in target tissues while avoiding lethal conditions in healthy peripheral tissues. It works by two main mechanisms: (1) at the cellular level via solute damage and intracellular ice formation; and (2) at the vascular level as a result of thrombosis and subsequent coagulative and ischemic necrosis. Investigation of cryoablation for renal tumors began in 1964, and by the 1990s, attention was turning to its use as a means of treating renal tumors. Modern renal cryosurgery is applied using minimally invasive techniques. Cryotherapy was first applied in the prostate in 1966 and soon thereafter was used to treat prostate cancer. Today, prostate cryosurgical techniques employ ultrasound monitoring and urethral warming to minimize urethral and rectal complications.

Cryotherapy and radiofrequency ablation: pathophysiologic basis and laboratory studies

PURPOSE OF REVIEW

There is increasing interest in minimally invasive alternatives to surgery, especially as the natural history of small renal masses appears in the majority to be that of very slow growth. Cryoablation and radiofrequency ablation are two energy-based therapies that can be applied in a minimally invasive manner. We will review the recent clinical and laboratory studies that have formed the scientific foundation of the current clinical protocols and how these protocols may change in light of recent observations.

RECENT FINDINGS

Although there is literature supporting enhanced cell death with the use of a passive thaw in cryoablation, recent data suggest that the use of an active thaw is no different. The active thaw process will effectively cryoablate renal tissue as well as significantly reduce overall operative time. There is lack of uniformity in the effectiveness of radiofrequency ablation for renal masses. It has been concluded that hematoxylin and eosin staining is inadequate for assessment of cell viability after radiofrequency ablation and thus, nicotinamide adenine dinucleotide staining should be included in the histological assessment of tissue.

SUMMARY

Cryoablation is the most studied modality and its ability to both directly and indirectly damage cells is generally understood. Clinical experience will further refine knowledge about optimal freezing temperature and freeze-thaw cycles. The coagulation necrosis of radiofrequency ablation is an effective means of destroying cancerous tissue but targeting this energy has been difficult and treatment failures have occurred.