A Phase II Clinical Trial of Interactive MR-Guided Interstitial Radiofrequency Thermal Ablation of Primary Kidney Tumors: Preliminary Results

Purpose:

To perform the first Phase II clinical trial designed to evaluate the efficacy and toxicity of interactive MR-guided radiofrequency interstitial thermal ablation (RF-ITA) of primary renal tumors.

Materials and Methods:

10 patients with peripheral renal cell carcinoma and contraindications to surgery were treated by percutaneous RF-ITA entirely guided and monitored on 0.2T MRI. Using a 200 Watt radiofrequency ablation system and custom-fabricated MR-compatible 17-gauge, 2-3-cm cool-tip electrodes, pulsed RF current was applied for single/multiple cycle(s) of 12-15 minutes until the entire tumor was replaced by enlarging low signal zone on intermittently acquired T2 and/or STIR images during the procedure. MR scans of the kidneys were acquired prior to, immediately and 2 weeks after ablation, and then every 3 months for one year and every 6 months thereafter.

Results:

Treated tumors ranged from approximately 1 to 17 ml in volume. Successful RF-electrode insertion/repositioning into the renal mass was achieved in all cases under direct MR fluoroscopic guidance. A total of 30 ablation cycles were conducted at 21 electrode positions in the 10 procedures with complete ablation achieved in all cases by the end of the procedure. Apart from 2 small self-limited perirenal hematomas, no intra/post-procedural complications were observed. No tumor recurrence occurred over a mean follow-up period of 18.5 ± 6 months.

Conclusion:

This investigation reports the early findings of application of interactive MR-guided RF-ITA for treatment of primary renal tumors. Although results are preliminary, the high success rate in achieving complete ablation and the absence of tumor recurrence are encouraging.

Figure 1:Illustration of the typical paradigm used for MRI-guided and monitored RF-ITA of renal cell carcinoma.

(A) The interventional MRI suite set-up during thermal ablation procedures. The open MRI system configuration facilitates proper access to the patient during the procedure. The in-room RF-shielded LCD monitor (straight arrow) equipped with a computer mouse and foot pedal (not shown) along with the ability to control fast gradient echo sequences from the scanner-side, all facilitate interactive near-real-time navigation of the RF electrode into the targeted tumor in a safe, time-efficient manner. The RF generator (curved arrow) is also operated at scanner-side.

(B) Once the electrode is positioned within the targeted renal tumor, the radiofrequency generator is switched on to start the ablation procedure while pumping iced water through special channels within the RF-electrode shaft to prevent charring at the electrode / tumor interface that would stop further RF deposition and interfere with adequate tumor destruction. The development of the thermal lesion is monitored through intermittent MR imaging (FSE T2 and / or FSE STIR) during the ablation session.

Figure 2: MR images obtained intermittently during RF-ITA of an exophytic anterior lower pole right-sided clear-cell carcinoma in a 75-year-old male patient.

(A) Transverse FSE T2-weighted image (TR/TE/NSA/ETL = 3465/105/4/17) following MR image-guided insertion of electrode (arrowheads) but before ablation demonstrates intermediate-signal anterior exophytic tumor (arrows) involving the lower pole of the right kidney (K).

(B) Transverse FSE T2-weighted image (TR/TE/NSA/ETL = 3465/105/4/17) following 2 ablations cycles lasting 15 and 6 minutes at the same electrode location demonstrate complete thermal damage of the tumor at this transverse level as indicated by the development of uniformly low signal (arrows) around the needle electrode (arrowheads).

(C) When evaluated on coronal FSE T2-weighted image (TR/TE/NSA/ETL = 1856/105/4/17) acquired immediately following transverse images shown in part b, most of the tumor is treated as indicated by the development of hypointensity (straight arrow) around the needle electrode (arrowhead), but a residual untreated portion of the tumor is detected by the intermediate signal crescent (curved arrow) seen capping the thermal lesion near the junction of the tumor with the kidney (k). Transverse images also documented untreated zone in a superoposterior location.

(D) Coronal FISP image (TR/TE/NSA/FA = 17.8/8.1/3/90º) demonstrates the RF-electrode position (arrowhead) after interactive repositioning supero-posteriorly into residual tumor tissue (straight arrows). Guidance into residual tumor was based on localization from T2-weighted images (figure part C), and was confirmed with additional FSE T2-weighted images prior to next ablation cycle, since the treated tumor (curved arrow) does not appear hypointense on FISP images. When PSIF images are used for guidance, as was routinely done later in the trial, the thermal lesion could be defined on guidance images in addition to the confirmatory FSE T2-weighted study. A 3rd ablation cycle was performed at this location for 12 additional minutes.

(E) Coronal FSE T2-weighted image (TR/TE/NSA/ETL = 1898/105/4/17) after third RF application revealing complete replacement of the tumor by hypointense thermally damaged tissue (asterisk) surrounded by a faint hyperintense rim of reactive tissue changes (arrowheads).

(F) Transverse FSE T2-weighted image (TR/TE/NSA/ETL = 1898/105/4/17) after third RF application shows hypointense thermal lesion completely replacing superior portion of tumor (asterisk), and documents adequate treatment of a margin of normal kidney (arrowheads).