Nonthermal technique promising for small renal tumors

Dallas-Percutaneous irreversible electroporation (IRE) is showing promise as a novel minimally invasive approach for treating small renal tumors, according to the experience of urologists at the University of Texas Southwestern Medical Center, Dallas.

Read commentary on this study by J. Stuart Wolf, Jr., MD

“To our knowledge, ours is the first clinical study evaluating percutaneous IRE for treatment of small renal masses, and outcomes in our patients indicate the procedure is feasible and safe,” said first author Monica Morgan, MD, who was a fellow in minimally invasive surgery and endourology at the time of the study and is currently assistant instructor in urology.

“Determination of its oncologic efficacy will depend on longer follow-up in more patients,” added Dr. Morgan, who worked on the research with Jeffrey Cadeddu, MD, and colleagues.

Findings from the retrospective study including 20 consecutive patients were presented at the 2014 World Congress of Endourology and SWL in Taipei, Taiwan.

The patients underwent the procedure between April 2013 and March 2014. A total number of 21 tumors were treated, with a median size of 2.2 cm (1.2 to 3.6 cm). All patients underwent computed tomography with intravenous contrast prior to the procedure to help delineate the masses. The procedures were performed under general anesthesia with cardiac synchronization, complete neuromuscular blockade, and CT guidance using 15-cm monopolar probes (NanoKnife, Angiodynamics). Median number of probes used was four (range, two to five).

Contrast-enhanced CT was performed immediately after the procedure to evaluate the area of ablation, and follow-up with contrast-enhanced CT scanning was scheduled at 6 weeks, 6 months, and 1 year postoperatively.

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There were no intraoperative or postoperative complications. Of 19 patients seen at 6 weeks, two demonstrated contrast enhancement on their 6-week follow-up CT and therefore were considered treatment failures. Both patients were treated early in the series and subsequently underwent salvage radiofrequency ablation.

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Seventeen patients were seen at 6 months, and all seemed tumor-free, but recurrence was seen in one of eight patients at 1 year. The latter individual chose to undergo partial nephrectomy and was tumor-free with follow-up to 6 months, Dr. Morgan reported.

Technique avoids heat sink effect

IRE involves application of short pulses of DC electric current that creates irreversible pores in the cell membrane leading to apoptosis. Dr. Morgan explained that as a nonthermal technique, IRE has the potential for greater efficacy and safety than thermal ablative techniques (ie, radiofrequency and cryotherapy), as it avoids any heat sink effect that can occur when treating near large blood vessels as well as collateral tissue damage.

“Histology studies show there is a sharp demarcation around the edge of the measured target tissue; therefore, we are able to calculate and have the potential to ablate with great accuracy,” Dr. Morgan said.

However, IRE has some limitations and drawbacks relative to the other techniques. The patient must undergo a complete neuromuscular blockade during the ablation portion of the procedure, and application of pulses must be timed precisely to the myocardial refractory period to prevent induction of arrhythmias.

In addition, probe placement is technically challenging and the learning curve is steep. The probes are placed to bracket the tumor, rather than to violate it. Also, they must be completely encased in tissue to prevent arcing, in parallel to each other (<10 degrees deviation), and with the tips on the same plane.

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“Small deviations in probe placement can lead to incomplete ablation,” Dr. Morgan explained.

“Procedure times exceeded 3.5 hours in our early cases, but were reduced to about 2 hours later on in this series,” she added.

Based on characteristics of the failures and recurrence, Dr. Morgan said that it appears IRE may ablate better deeper in the kidney and less well in the periphery, perhaps because fat acts like an insulator. In addition, it may be more effective for treating smaller tumors (<3 cm).

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