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Pain Control Requirements for Percutaneous Ablation of Renal Tumors: Cryoablation versus Radiofrequency Ablation—Initial Observations¹

¹ From the Brady Urological Institute, Department of Urology (M.E.A., I.M.V., S.B.B., T.I., L.R.K., S.B.S.) and the Russell H. Morgan Department of Radiology (S.B.S.), the Johns Hopkins Medical Institutions, Baltimore, Md. Received May 10, 2004; revision requested July 21; final revision received November 23; accepted December 23. Address correspondence to S.B.S., Department of Radiology, Memorial Sloan-Kettering Cancer Center, 1275 York Ave, New York, NY 10021 (e-mail: solomons{at}mskcc.org).

	ABSTRACT

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION References

 
PURPOSE: To retrospectively compare the pain control requirements of patients undergoing computed tomography (CT)-guided percutaneous radiofrequency (RF) ablation with those of patients undergoing CT-guided percutaneous cryoablation of small (4-cm) renal tumors.

MATERIALS AND METHODS: The study was HIPAA compliant and received institutional review board exemption; informed consent was not required. Medical and procedure records of patients who underwent RF ablation and cryoablation of renal tumors from June 19, 2003, to February 28, 2004, were retrospectively reviewed for clinical data, tumor characteristics, and anesthesia information. During the study period, 10 men (mean age, 66.5 years) underwent cryoablation of 11 renal lesions, and 14 patients (11 men, four women; mean age, 68.1 years) underwent RF ablation of 15 renal tumors. Analgesic and sedative requirements during the procedure were compared. Standard anesthesia consisted of 5 mL of 1% lidocaine injected locally, and conscious sedation consisted of 50 µg of fentanyl and 1 mg of midazolam administered intravenously. The Fisher exact test and Student t test were used to compare clinical factors and drug requirements between the two groups.

RESULTS: There was no difference in terms of patient demographics, tumor diameter, or distribution of central versus noncentral lesions between the two groups. Cryoablation was associated with a significantly lower dose of fentanyl (165.0 µg [RF group] vs 75.0 µg [cryoablation group]; P < .001) and midazolam (2.9 mg [RF group] vs 1.6 mg [cryoablation group]; P = .026). In the RF group, one patient required general anesthesia, one patient required supplemental narcotics (5 mg of oxycodone) and sedatives (1 mg lorezapam), and one patient became apneic for a brief interval after receiving additional narcotics for pain during the procedure. An additional RF session was terminated early in one patient because of pain, and further medication could not be administered owing to bradycardia. No patients in the cryoablation group required any additional or alternate anesthetics.

CONCLUSION: Image-guided percutaneous cryoablation of small (4-cm) renal lesions appears to require less analgesia than RF ablation. Prospective trials with validated pain scales are needed to examine this further.

© RSNA, 2005

	INTRODUCTION

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Thermal ablative technologies, such as radiofrequency (RF) ablation and cryoablation, have been developed to provide targeted therapy for renal tumors in the hopes of improving the morbidity associated with laparoscopic and open surgery. Early clinical series employed laparoscopic techniques to deliver the energy probe in a controlled fashion and with direct vision (1,2). Optimistic preliminary outcome data and improvements in instruments, such as the development of smaller cryoablation probes, have paved the way for image-guided percutaneous ablation. In most cases, percutaneous RF ablation and cryoablation of renal lesions can be performed without general anesthesia and in an outpatient setting (3). The purpose of our study was to compare retrospectively the pain control requirements of patients undergoing computed tomography (CT)-guided percutaneous RF ablation with those of patients undergoing CT-guided percutaneous cryoablation of small (4-cm) renal tumors.

	MATERIALS AND METHODS

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Our study received institutional review board exemption, and informed consent was not required. Our study was compliant with the Health Insurance Portability and Accountability Act. Guidelines regarding the standardization of terms and reporting criteria for ablation technologies have been previously published and were used in designing this study (4).

Patient and Tumor Characteristics
Records were reviewed for all patients who underwent percutaneous RF ablation and cryoablation of renal tumors between June 19, 2003, and February 28, 2004, at our institution. During the study period, 10 men underwent CT-guided percutaneous cryoablation of 11 renal lesions, and 14 patients (11 men, three women) underwent CT-guided percutaneous RF ablation of 15 tumors. The mean age of patients in the cryoablation and RF groups was 66.5 years (age range, 36–84 years) and 68.1 years (age range, 39–86 years), respectively. One patient in each group had two tumors that required two separate ablation sessions. None of the study patients received regular doses of analgesics prior to the procedure nor did they complain of tumor pain. All patients were discharged without a prescription for analgesics. The decision to undergo cryoablation versus RF ablation was made by the patient following discussion with the treating physician.

Tumor location was classified as central or noncentral. Central tumors were defined as tumors that extended into the renal sinus fat or came within 5 mm of the renal sinus fat. A tumor with exophytic and/or parenchymal areas was classified as central if any component of the tumor was within 5 mm of the renal sinus fat. This classification scheme is clinically relevant because central tumors pose technical challenges and may require a more conservative ablative approach owing to their proximity to major vessels and the collecting system.

Anesthesia and Analgesia
Conscious sedation was administered by a nurse at the direction of a single interventional radiologist (S.B.S.) who had more than 5 years experience with conscious sedation protocols. Initially, the patient received 50 µg of fentanyl (Fentanyl Citrate; Abbott Laboratories, North Chicago, Ill) and 1 mg of midazolam (Versed; American Pharmaceuticals Partners, Schaumburg, Ill) intravenously. These doses were then titrated by the physician (S.B.S.) to achieve an adequate comfort level throughout the procedure. In general, the drug was titrated in response to pain expressed by the patient through motion or verbal communication. Five milliliters of 1% lidocaine (Astra Zeneca, Wilmington, Del) was injected subcutaneously at the needle probe site. Electrocardiographic tracing and continuous monitoring of heart rate, oxygen saturation, and respiratory rate were performed. Blood pressure was monitored every 5 minutes.

RF Ablation Protocol
All procedures (RF ablation and cryoablation) were performed by a single interventional radiologist (S.B.S.) who had more than 5 years experience in interventional procedures. Patients were placed in the prone position on the CT gantry unit. RF ablation was performed by using a dry electrode that measured 2–5 cm in diameter, depending on the diameter of the lesion. Nine lesions were treated with an impedance-based RF ablation system (LeVeen needle; Boston Scientific, Natick, Mass), and six were treated with a temperature-based system (Starburst XL; Rita Medical Systems, Mountain View, Calif). After diagnostic biopsy was performed with an 18-gauge automated core biopsy needle (Temno needle; Allegiance Healthcare, McGaw Park, Ill), the RF ablation needle was inserted into the lesion by using CT fluoroscopy.

For impedance-based RF ablation, renal lesions were ablated by using a 15-gauge needle electrode that measured 15 cm long (LeVeen needle electrode; Boston Scientific) and an RF generator (RF2000; Boston Scientific). Ablation was performed for 15 minutes or until an increase in impedance (or "roll-off") was detected. A repeat cycle, or second phase, was then performed, which began at 70% of the wattage used to achieve roll-off during the first phase. For temperature-based RF ablation, renal lesions were ablated by using a 15-gauge needle electrode (Starburst XL, Rita Medical Systems) that was heated to a target temperature of 100°C. The diameter of the tine array began at 2 cm and was expanded to up to 4 cm, depending on the diameter of the lesion. Selection of the RF probe was based primarily on the shape of the lesion. The diameter of the lesion dictated the time of ablation at target temperature. Overlapping ablations were performed as needed. None of the RF probes traversed other organs en route to the kidney, and no major complications were noted in this set of patients. Major complications were defined as any adverse events requiring additional monitoring or follow-up treatment.

Cryoablation Protocol
Patient positioning for cryoablation was identical to that for RF ablation. All patients were treated with 2.4-mm probes (Cryocare; Endocare, Irvine, Calif). After an 18-gauge needle was used to perform an automated biopsy, the cryoprobe was inserted into the lesion. The probe possesses a sharp tip that is inserted directly into the tumor by using free-hand placement and CT fluoroscopy guidance. More than one probe was used, depending on the diameter of the lesion. Two 10-minute freeze cycles, which were followed by an active or passive thaw, were performed per lesion. Spiral CT was used intermittently to monitor the ice ball. None of the cryoablation probes traversed other organs en route to the kidney, and no major complications were noted in this set of patients.

Data Collection
Patient records were reviewed by two of the authors (M.E.A. and I.M.V.). Data regarding tumor diameter, location, ablation modality, number of probes used, number and completeness of ablation sessions, and any adverse events were recorded. Additionally, data regarding the amount of fentanyl and midazolam that were used during the procedures were tabulated. Supplemental analgesic and sedative use was also noted. Clinical information regarding comorbidities was reviewed, and the American Society of Anesthesiologists (ASA) score was used to assess the patients' overall health status (Table 1). ASA scores were generated by an experienced urologic surgeon (S.B.B.) on the basis of clinical information retrieved from the patients' charts. This physician was blinded to the ablation modality used.

	RESULTS

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Group Characteristics
A summary of the results is presented in Table 2. There was no difference between the two groups in terms of mean patient age (68.1 years in RF group vs 66.5 years in cryoablation group, P = .86), mean tumor diameter (2.0 cm in RF group vs 2.3 cm in cryoablation group, P = .34), and distribution of ASA scores (P = .67). No significant differences existed in the location or side of the tumors treated between the two groups. The RF group included three women, whereas the cryoablation group did not include any women. The mean number of probes used per cryoablation session was 2.1. By comparison, only one electrode was used during all RF ablations. Notably, there was no difference in analgesic requirements between those patients who underwent cryoablation with one probe (n = 5) and those who underwent cryoablation with two or more probes (n = 6). Additionally, there was no difference in analgesic requirements when comparing the two different RF ablation systems (impedance vs temperature-based systems).

	DISCUSSION

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Percutaneous thermal ablative alternatives, such as RF ablation and cryoablation, are particularly attractive because they circumvent many of the issues that limit current surgical approaches. Su et al (3) recently reported on a cohort of patients who were at high risk for surgery and who safely underwent percutaneous RF ablation. Such patients—namely, the elderly and those with clinically important comorbidities—would otherwise be followed up expectantly (ie, conservatively with observation and close follow-up) or be forced to undergo dangerous surgical procedures. Pavlovich et al (6) presented the initial results of percutaneous RF ablation of patients with hereditary renal tumors. These patients typically acquire multiple small low-grade tumors throughout their life. Percutaneous ablation may help preserve renal function while treating tumors in these patients, without the morbidity of repetitive surgical procedures. Additionally, percutaneous thermal ablation may be performed on an outpatient basis without any incisions.

The recent availability of smaller cryoprobes has increased the options available for percutaneous ablation. Previously, only RF systems were available for percutaneous therapy, despite the more extensive literature examining cryoablation of renal lesions. As oncologic data accumulate for these approaches, issues such as analgesia will become important in guiding patients toward the appropriate therapy. Given the equal effectiveness of both procedures, patients and physicians are likely to prefer the least painful ablative procedure. Furthermore, a less painful procedure is more likely to be completed. As mentioned in the Results section, one procedure in the RF group had to be stopped prematurely because of pain, and another procedure resulted in oversedation to provide adequate pain control. The former procedure was terminated after the patient exhibited excessive motion secondary to pain, despite the generous titration of medications.

The data presented in this study imply that cryoablation of renal lesions is better tolerated than RF ablation. The patient populations, although small, are well matched. A limitation of this study is that it was not performed prospectively by using validated pain scales. While we acknowledge this, the clinical difference in pain perceptions between the two approaches was dramatic to us. This is borne out by the statistically significant difference in drug administration during these procedures. The number of patients in the RF group who could not be cared for successfully with the standard conscious sedation protocol further underscores this assertion. This is despite the fact that patients undergoing cryoablation had more probes inserted than their RF counterparts.

The phenomenon observed in this study has potential scientific explanations. Results of classic electrophysiologic experiments have confirmed that cold temperature blocks nerve conduction (7–9). In 1955, Douglas and Malcolm (9) studied cat nerves in situ by recording action potentials during neurostimulation. A thermode was used to cool a length of the nerve between the point of stimulation and the point of recording. Nerve conduction was reduced with progressive cooling for all nerve fibers tested, including afferent unmyelinated C and myelinated A fibers, which are the same fibers involved in pain sensation. Interestingly, in their study of small afferent nerve fibers, Douglas and Malcolm noted a threshold temperature at which nerve conduction was eliminated (9). As the temperature was subsequently raised, the action potentials reappeared, thereby indicating that this was a reversible process. Although no temperature below 0°C was tested, this experiment elegantly established that cooling afferent nerves can decrease or even eliminate conduction.

In 1976, Lloyd and colleagues (10) introduced the concept of cryoanalgesia when they reported on 64 patients treated with cryoablation for intractable pain of various causes. In their study, a probe was applied to peripheral sensory nerves and achieved temperatures as low as –60°C. More recently, other investigators have confirmed the efficacy of cryoanalgesia in various clinical scenarios (11–13). In 2003, Zhou et al (14) measured somatosensory evoked potentials in rabbit sciatic nerves after applying cold to the nerve to produce injury. Unlike authors of earlier studies, Zhou et al used a freezing probe tip that was able to produce temperatures as low as –180°C. In this study, the somatosensory evoked potentials disappeared with the induction of injury at temperatures below –60°C but returned about 40 days later (14).

In addition to the electrophysiologic effects of cold temperature on nerve conduction, there are other potential mechanisms for the decrease in pain sensation with cryoablation. Vasoconstriction of blood vessels from cooling may minimize the resulting edema and reduce the release of pain-producing substances from damaged tissue.

The current study represents our initial observations regarding the intraprocedural analgesic requirements of RF ablation and cryoablation. Technique effectiveness can only be evaluated with longer follow-up, which represents a real limitation of this study. Should one modality prove to be inferior oncologically, then the issue of intraprocedural pain becomes less important. In addition, this study did not attempt to evaluate postprocedural pain, which is undoubtedly a relevant issue.

In conclusion, RF ablation and cryoablation represent two options for the percutaneous treatment of small renal tumors. In this setting, image-guided percutaneous cryoablation appears to require less analgesia than RF ablation. Prospective trials with validated pain scales are needed to examine this issue and determine the clinical efficacy of percutaneous ablative therapies.

	FOOTNOTES

 

Abbreviations: ASA = American Society of Anesthesiologists • RF = radiofrequency

Authors stated no financial relationship to disclose.

Author contributions: Guarantor of integrity of entire study, S.B.S.; study concepts, S.B.S., M.E.A., L.R.K.; study design, M.E.A., I.M.V., T.I., S.B.B.; literature research, I.M.V., M.E.A., T.I.; clinical studies, S.B.S., L.R.K.; data acquisition, M.E.A., S.B.B., I.M.V.; data analysis/interpretation, M.E.A., I.M.V., S.B.S.; statistical analysis, M.E.A.; manuscript preparation, M.E.A., S.B.S., L.R.K., I.M.V.; manuscript definition of intellectual content, M.E.A., S.B.S., S.B.B., L.R.K.; manuscript editing, M.E.A., T.I., I.M.V.; manuscript revision/review, T.I., S.B.S., M.E.A., L.R.K.; manuscript final version approval, S.B.S., M.E.A., L.R.K., S.B.B.

	References

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