Radiology: Use of marker coils halves CT fluoro time during RF ablation

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Placement of marking coils during biopsy of small renal neoplasms aided localization at subsequent radiofrequency (RF) ablation and was associated with shorter fluoroscopy time, according to a study published online March 8 in Radiology.

Several factors provide impetus for the use of marking coils during biopsy of small renal masses. Improved CT and MR resolution have led to an increase in the detection of small incidental renal masses. Meanwhile, RF ablation has emerged as a nonsurgical option for patients who are poor surgical candidates.

“As experience with renal RF ablation techniques increases and its use becomes more widespread, improved techniques to increase the safety, efficacy, and reliability of treatment are important,” Jason A. Pietryga, MD, diagnostic radiologist with Rhode Island Hospital-Lifespan in Providence, and colleagues wrote. The authors noted that many patients in this population have abnormal renal function, thus reducing use of contrast is a safety consideration. However, visualization of the masses can be difficult owing to the smaller size of the masses, patient obesity and lesion location.

Pietryga and colleagues devised a technique of placing a 5 mm radiopaque marking coil during biopsy to help localize poorly visualized solid renal masses and target the tumor during subsequent RF ablation. The researchers conducted a retrospective study to evaluate the technique.

The study population was comprised of 23 patients who had a marking coil placed at biopsy and 23 patients matched for tumor characteristics who underwent the procedure without a coil. The researchers compared fluoroscopy times and technical success rates for both groups. Initial coil placement and subsequent ablation took place from April 2008 to September 2010.

The decision to place a radiopaque coil was based on the difficulty in localizing the tumor at prebiopsy imaging and during the biopsy. Coils were placed via contrast-enhanced CT guidance with localization confirmed by fluoroscopic image. During the ablative procedure, the operator performed unenhanced CT to localize the mass relative to the coil and used fluoroscopic guidance to target the coil and tumor.

After analyzing the data, Pietryga and colleagues reported mean fluoroscopy time during RF ablation was significantly shorter for patients in the coil group (28 seconds) than in the control group (66 seconds).

The authors also found mean fluoroscopy time during biopsy and ablation was nearly equivalent in the coil group at 27 and 28 seconds, respectively, despite the expectation that CT fluoroscopy time would be greater during ablation due to the intermittent verification of positioning and repositioning of the RF applicator. However, in the control group, fluoroscopy time was 32 seconds at biopsy and 75 seconds at ablation.

The researchers defined technical success as the lack of enhancement (more than 20 Hounsfield units) in or adjacent to the ablation bed at follow-up imaging. Technical success in the coil group was 100 percent, based on average follow up of 10 months. In the control group, 20 of 22 patients who underwent imaging had no residual disease at average follow up of 13.1 months.

Pietryga et al acknowledged the limitations of the study: its retrospective nature, small patient population and lack of long-term follow-up of clinical outcomes.

They wrote, “Our early results are promising, and we have started placing coils at biopsy of all poorly visualized renal neoplasms in which subsequent RF ablation is planned,” and emphasized the safety of the method as well as its assistance in localization with decreased fluoroscopy time and equivalent outcomes.