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Oncologic Efficacy of CT-Guided Percutaneous Radiofrequency Ablation of Renal Cell Carcinomas

Abstract and Introduction

Abstract

Objective: A single institution's experience with CT-guided percutaneous radiofrequency ablation of biopsy-proven renal cell carcinomas (RCCs) was studied to determine the disease-free survival and complication rate.
Materials and Methods: Data from 125 RCCs in 104 patients treated with curative intent was reviewed. Radiofrequency ablation treatments were performed using conscious sedation and local anesthesia. Patients were followed with contrast-enhanced CT or MRI. Tumor control was defined as the absence of contrast enhancement in the tumor on CT or MRI.
Results: Tumor size ranged from 0.6 to 8.8 cm (mean, 2.7 cm; SD, 1.5 cm). Of the 125 treated tumors, 116 (93%) were completely ablated (109 in a single ablation session, seven after a second ablation session) with a mean follow-up interval of 13.8 months. All 95 RCCs smaller than 3.7 cm were completely ablated, and 21 (70%) of 30 larger tumors were completely ablated, with nine showing evidence of residual viable tumor on follow-up scans. Tumor size smaller than 3.7 cm was significantly associated with achieving complete tumor eradication (p <>p <>Conclusion: CT-guided percutaneous radiofrequency ablation is a safe method to treat small RCCs. This study indicates that radiofrequency ablation can reliably eradicate RCCs smaller than 3.7 cm. Treatment of larger RCCs will result in an increased risk of residual RCC.

Introduction

The number of renal cell carcinomas (RCCs) diagnosed annually is increasing largely as a result of an increased rate of incidental detection of renal neoplasms.[1,2] This increase is accounted for by an increase in use of cross-sectional abdominal imaging studies, including CT, sonography, and MRI, for the evaluation of patients with suspected abdominal disorders.[1,2] The true incidence of RCC may also be increasing independently in addition to the increased incidental detection with cross-sectional imaging.[3]

The standard therapy for RCC has been radical nephrectomy.[4] Nephron-sparing approaches have proven to have equivalent oncologic outcomes in select (tumor diameter less than 4 cm) renal tumors.[5,6] A dilemma is created when patients are unwilling or unable to undergo surgical tumor resection or they are prone to recurrent renal surgery due to a hereditary predisposition to develop multifocal RCC (von Hippel-Lindau disease and hereditary papillary RCC). Radiofrequency ablation has been used for the treatment of solid organ tumors, primarily hepatic tumors, since the 1990s,[7-9] and CT-guided percutaneous radiofrequency ablation has been shown to be a promising alternative treatment option for problematic patients with RCCs.[10-14]

Radiofrequency ablation is generally an outpatient procedure that offers the advantage of being a minimally invasive procedure that does not require general or regional anesthetic and results in a short convalescence with minimal morbidity.[10-13] First introduced in 1997 for the treatment of renal neoplasms,[15] this technique has had encouraging early results.[10-17] However, previous studies have been small series; have reported on treatment of renal masses, many of which were not proven RCC; and follow-up intervals were short.[10-17] The current study was designed to analyze the results of CT-guided percutaneous radiofrequency ablation on a consecutive series of patients with biopsy-confirmed RCC who were treated at one institution from May 2000 through June 2006.

Materials and Methods

This retrospective study was approved by the institutional review board at our institution. The medical records for all patients with biopsy-proven RCC treated with percutaneous radiofrequency ablation at this institution between May 2000 and June 2006 were reviewed. From that group of 117 patients, patients who had yet to undergo follow-up imaging (10 patients) and those with stage III or IV disease who had been treated for cytoreduction or symptomatic relief only (three patients), were excluded from the study. Inclusion criteria required that patients had no imaging evidence of metastatic disease or local spread to the renal vein on pretreatment CT or MRI scans and had at least one imaging follow-up with CT or MRI before and after IV contrast material injection after radiofrequency ablation. This resulted in a cohort of 125 RCCs treated in 104 patients. This group of 125 RCCs constitutes the study group reported in this article.

We reviewed the clinical and pathologic information and radiographic studies for these 104 patients. Before radiofrequency ablation treatment, all study patients were evaluated first by a urologic surgeon and considered to be at substantially increased risk for developing complications during and after renal surgery due to comorbidities or syndromes with multifocal RCC. Pretreatment evaluation in all patients included abdominal imaging with CT or MRI before and after IV contrast administration, chest imaging with a chest radiograph or CT scan, and laboratory studies including CBC, serum creatinine, blood urea nitrogen, liver function test, and coagulation studies. Two biopsies were performed separate from the initial ablation procedure, one percutaneously at another hospital and one during laparotomy for bowel surgery. All other biopsies were completed immediately before the ablation procedure using CT guidance with a cytotechnologist on site. At least one tumor in each patient was confirmed as RCC by biopsy. In the 10 (10%) patients with multiple tumors, only one tumor was biopsied, but the imaging characteristics of the other tumors were identical (Figure 1, Figure 2, Figure 3 and Figure 4) to the biopsy-proven RCC and these tumors were diagnosed as RCCs.





All radiofrequency ablation procedures were performed by a single radiologist. Radiofrequency ablation treatment of all tumors in this study was completed with a system using a 200-W generator and saline-cooled treatment probes (Cool-Tip, ValleyLab). We have previously reported the percutaneous radio-frequency ablation technique adhered to in this series[11] (Figure 5, Figure 6, Figure 7, Figure 8, Figure 9 and Figure 10). Three lengths of active electrode tips were used: 2.5-cm electrode tips with three electrode cluster arrays and single probes with 2.0- and 3.0-cm electrode tips. The tip length was selected based on tumor size. The largest possible electrode tips were selected without being greater than 1 cm longer than the diameter of the neoplasm. For neoplasms larger than 2 cm, the cluster electrode was used. Overlapping ablations were performed based on tumor size and shape, with the intent to destroy the entire mass.


The impedance control setting was used on the radiofrequency ablation generator. Ablations were continued for 8 minutes unless the generator sensed a rapid rise in tissue impedance twice and automatically switched to energy pulsing. Increased impedance indicates tissue carbonization and boiling, which result in decreased energy deposition. When this occurred, ablation was continued until the next even minute after a minimum of 6 minutes. This technique, which was used to treat all 125 RCCs, was developed on the basis of our observations and experience treating liver tumors with this radiofrequency ablation device. This technique is based on the fact that rapid rises in impedance indicate high temperatures in surrounding tissues, and once achieved two or more times, the volume of tissue ablated with continued radiofrequency ablation in the same site has been nearly maximized. Tissue temperature was recorded after each ablation and, if below 50°C, another ablation in the same location was performed. A CT scan of the kidney was obtained immediately after the ablation session to assess tumor destruction. If the serum creatinine was 1.5 mg/dL or less, contrast-enhanced CT was performed. Recognition of continued contrast enhancement within the neoplasm was interpreted as incomplete tumor ablation, and an additional tumor ablation was performed with the intent of destroying the remaining viable tumor.

Patients were given conscious sedation using a combination of midazolam and fentanyl administered IV with continuous monitoring of vital signs including pulse rate, blood pressure, blood oxygen saturation, and capnography. Local anesthesia at the puncture site was achieved with subcutaneous injection of 5-10 mL of a solution of 1% lidocaine. All patients were continuously monitored by a nurse throughout the radiofrequency ablation procedure.

A small number of patients in this study had pacemakers. These patients were not excluded. Immediately before treatment, patients with pacemakers were seen by a cardiologist, and a doughnut-shaped magnet was placed on the skin overlying the pacemaker to deactivate the sensing feature of the pacemaker. After the radiofrequency ablation session, patients with pacemakers were reevaluated by a cardiologist, who interrogated and reset the pacemakers.

Immediately after the completion of the radiofrequency ablation procedure, patients were transferred to a short-stay area in the hospital where their vital signs were monitored hourly; antiemetics and analgesics were administered on an as-needed basis in this area. A CBC was obtained 5 hours after completion of the radiofrequency ablation procedure. Patients were discharged if they had no sign of complications with stable vital signs, no evidence of active bleeding (stable blood pressure, pulse, hematocrit, and hemoglobin level), and no pain requiring IV analgesics.

Patients were followed up with CT or MRI before and after injection of IV contrast material 1-3 months after the procedure (initial tumor control). At regular intervals thereafter patients were reevaluated with CT or MRI before and after injection of IV contrast material in addition to a complete serum metabolic panel and a chest radiograph. For all scans obtained for follow-up, an electronic region of interest was used to interrogate each treated tumor on the scans before and after IV contrast material injection to assess for evidence of tumor enhancement. When scans obtained after IV contrast injection showed lack of contrast enhancement (<>




Patients with recurrent disease were assessed as candidates for further percutaneous radiofrequency ablation therapy. If feasible, an additional radiofrequency ablation session was completed. Beginning in late 2005, percutaneous cryoablation was used to treat some patients with residual disease after radiofrequency ablation. This subset of patients is described further in the Results section of this article.

Patient records and scans made during the procedure and throughout the follow-up period were reviewed and all complications attributable to the radiofrequency ablation procedure were recorded.

Analysis of the data included review of pretreatment, treatment, and follow-up scans for each treated tumor. The diameter of each treated tumor was measured using electronic calipers on the scan slice showing the greatest diameter. To determine if tumor position affected radiofrequency ablation results, the tumor position was categorized using two different tumor features from the pretreatment scans. Location of the tumor was classified by dividing the cephalocaudal length of the kidney into thirds: upper, middle and lower. Each third was then divided into four quarters: posteromedial, posterolateral, anteromedial, and anterolateral. The position of each tumor was categorized based on one of these 12 areas using the segment containing the largest volume of each tumor. Second, tumor location was categorized as being exophytic, parenchymal, central, or mixed, based on a previously described classification system.[18] If a tumor was completely surrounded by renal parenchyma with no contact with the perirenal fat, it was classified as a parenchymal tumor. Tumors that grew into the renal sinus but not beyond the renal capsule were classified as central tumors. Tumors with greater than 25% of their circumference contacting perirenal fat were classified as exophytic. Tumors that contacted both the renal sinus and the perirenal fat were classified as mixed tumors.

The effects of location, type, and size of tumors on disease-free survival time were assessed by a Cox regression analysis. Survival time was compared across poles (lower, middle, and upper), positions (medial vs lateral, anterior vs posterior), and tumor types (exophytic vs other). The association between the size of tumors and survival time was also assessed. In the Cox regression analysis, the residual tumors detected on the first follow-up scan were considered to have zero disease-free times.

Furthermore, the association between the likelihood of complications and variables named above was also assessed. Fisher's exact test was used to investigate whether the likelihood of complications differed across poles, positions, and tumor locations. A logistic regression analysis was used to assess the association between the size of tumors and the likelihood of complications. All the statistical analyses were performed with the SAS package, version 9.1 (SAS Institute), on a PC.