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Abstract and Introduction
Abstract
Objective: This retrospective study was performed to assess the feasibility, safety, and short-term outcome of percutaneous cryoablation of large solid renal tumors.
Materials and Methods: We reviewed 40 percutaneous cryoablation procedures performed on 40 patients with renal tumors 3 cm in diameter or larger. All patients underwent cryoablation with CT monitoring. Technical success was defined by extension of the ice ball beyond the tumor margin and postablation imaging findings of no contrast enhancement in the area encompassing the original tumor. Complications meeting grade 3 of the National Cancer Institute Common Terminology Criteria for Adverse Events were recorded.
Results: Mean ± SD tumor diameter was 4.2 ± 1.1 cm (range, 3.0-7.2 cm). Technical success was achieved in 38 (95%) of 40 cryoablation procedures. There was one grade 3 adverse event (3% rate of significant complications). Follow-up images obtained 3 months or longer (mean, 9 ± 6 months; range, 3-22 months) after ablation were available for 26 (65%) of the 40 patients. No local tumor recurrence or tumor progression was found.
Conclusion: Percutaneous cryoablation of renal tumors measuring 3 cm or larger is technically feasible and relatively safe. Short-term follow-up results are encouraging, although long-term follow-up is necessary to assess true treatment efficacy.
Introduction
Percutaneous management of solid renal tumors with radiofrequency ablation and cryoablation has been established as a technically feasible treatment in selected patients.[1-4] Allowing for relatively short-term follow-up, these percutaneous techniques are effective in tumor management. The success rate is 90-100% for radiofrequency ablation[1-4] and 92-100% for cryoablation.[3,5,6]
A common theme in the radiofrequency ablation literature is difficulty managing larger tumors, typically defined as larger than 3 cm in diameter. Such large tumors often necessitate additional radiofrequency ablation sessions, and technical success rates are lower than those reported for smaller tumors.[2,7,8] Although the published findings on percutaneous cryoablation are limited, authors[3,5,6] have tended to treat patients with smaller tumors, usually less than 5 cm, even though cryoablation technology allows simultaneous operation of several cryoprobes to generate large confluent ice balls for tumor treatment.
Given that 80% of surgically resected renal tumors are larger than 3 cm,[9] it is clear that percutaneous ablation techniques must evolve to allow management of these larger tumors if we are to offer this alternative to a greater number of appropriate patients. Such patients are typically considered at high risk for surgery because of previous renal resection or advanced comorbid medical conditions. For this reason, we reviewed our experience in the percutaneous cryoablative management of renal tumors measuring 3 cm or more in diameter.
Materials and Methods
Patients
Approval for this retrospective study was obtained from our institutional review board, and the study was compliant with the Health Insurance Portability and Accountability Act. Informed consent was waived by the institutional review board. A retrospective review of the cases of patients who underwent cryoablation of a renal mass from March 2003 through August 2006 yielded 68 patients. Forty (59%) of these patients had tumors 3 cm or larger in greatest dimension and constitute our study population. The mean age was 76 years (range, 47-93 years). There were 29 men and 11 women.
All patients underwent formal consultation in our department of urology. On the basis of results of critical discussion between the urologist and radiologist, patients were considered for cryoablation. Indications for percutaneous ablation included previous renal surgery in seven (17%) of the cases and comorbid medical conditions in 28 (70%). Five (13%) of the patients wanted to avoid major kidney resection and chose ablation as an alternative to surgery. In all 40 cases, cryoablation was chosen over radiofrequency ablation because of tumor size. Eleven patients had additional concerns, including proximity of renal tumors to the colon (3/40, 7%), renal collecting system (5/40, 13%), and adrenal gland (3/40, 7%).
Tumor Characteristics
All patients underwent cross-sectional imaging that showed a solid renal mass. Maximum tumor size was determined on the basis of the largest measurement obtained with the technique that best depicted the renal tumor. Tumors were classified as exophytic, intraparenchymal, or central, depending on their position relative to the renal parenchyma. Any tumor that extended into the renal sinus fat was classified as a central tumor. Exophytic tumors were those in which 50% or more of the circumference was outside the renal capsule. Intraparenchymal tumors were those in which less than 50% of the tumor circumference was outside the capsule. The location of the tumor epicenter was recorded as upper, middle, or lower part of the kidney and as anterior, lateral, or posterior aspect of the kidney.
In this series, we did not require tissue confirmation of renal cell carcinoma before ablation because findings at percutaneous biopsy can be unreliable in excluding this neoplasm.[10,11] In addition, we and others[12] believe that the absence of malignant cells in a biopsy specimen cannot be used alone to conclude a renal mass is benign. Ninety percent of renal tumors larger than 3 cm are malignant.[9]
Treatment Procedure
Ablation was performed by four experienced interventional radiologists with 5, 6, 5, and 25 years of experience. Oral informed consent was obtained. We performed cryoablation under general anesthesia. Although others[3] have had success with conscious sedation, we find that general anesthesia allows greater control of respiration during cryoprobe placement and maximizes patient tolerance of the procedure.
The cryoablation system (Endocare) used allows independent and simultaneous operation of up to eight cryoprobes. The cryoprobe (Perc-24, Endocare) is a sealed 2.4-mm diameter (13-gauge) needle that, according to the manufacturer, generates an ice ball up to 3.7 cm in diameter and up to 5.7 cm along the probe shaft. Rapid expansion of argon gas in a sealed cryoprobe with a distal uninsulated portion results in rapid freezing of tissue, and the temperature can reach -100°C within seconds.[13] The diameter of the ice ball is controlled by the rate of gas delivery to the probe and the duration of freezing. Thawing is achieved by replacing the argon gas with helium gas.
Before the cryoablation procedure, images were critically reviewed for anticipated probe placement. Allowing for a 3-cm effective short-axis diameter of the ice ball, we planned 1.0- to 1.5-cm probe-shaft spacing within the tumor, overall probe positioning being defined by the geometric features of the tumor and expected ice ball size.
Cryoprobes were placed into the tumor before biopsy because bleeding after biopsy can obscure the tumor and make cryoprobe placement difficult or impossible. In the group of patients selected, two or more sterile cryoprobes were introduced through a skin nick by one or more of the participating radiologists. Sonography was used to guide cryoprobe placement. We used an Acuson Sequoia sonography system (Siemens Medical Solutions), typically with a 4- or 6-MHz transducer, although we occasionally used other transducers, depending on tumor location and conspicuity. CT was used to verify placement of the cryoprobes. Additional probes were placed if CT findings suggested the tumor might not have been completely covered with initial probe placement. After confirmation of the accuracy of the cryoprobe positions and before freezing, biopsy of the targeted renal mass usually was performed. We used an 18-gauge biopsy device (Bard Monopty, CR Bard) to obtain one or two cores of tissue. Biopsy was not performed on some patients because the tumor was obscured by the cryoprobes.
CT monitoring during ablation was performed with one of two systems (GE HiSpeed CT/i system, GE Healthcare; Somatom Sensation Open 40-MDCT system, Siemens Medical Solutions). CT has been shown accurate in monitoring of ice ball size and location and for prediction of cell death.[14,15] Cryoprobe positions were confirmed with 2.5- to 5.0-mm slice thickness with standard CT technique (120 kV peak and approximately 240 mA). Each lesion was subjected to a single treatment cycle of freezing, thawing, and freezing again.
During the freezing portions of the cycle, limited unenhanced CT images were obtained approximately every 2 minutes at body window and level settings (width, 400; level, 40 H) and 2.5- to 5.0-mm collimation for monitoring of the growth of the ice ball. Duration of freezing was based on growth of the ice ball relative to the tumor. Reconstructed images were generated depending on the proximity of critical structures (e.g., ureter, bowel, and adrenal gland). Because complete cell death occurs approximately 3 mm inside the edge of the ice ball,[16,17] the goal was to extend the ice ball 5 mm beyond the tumor margin during both freezing portions of the cycle. The procedure was complete when the ice ball extended beyond the tumor margin during the second freeze. After the second freeze cycle, cryoprobes were actively warmed with helium gas and withdrawn when the temperature was greater than 20°C.
Technical success was defined as extension of the ice ball beyond the tumor margin and acquisition of postablation images showing no contrast enhancement in the area encompassing the original tumor. Lesion location and size, number of cryoprobes, duration of freeze and thaw periods of each cycle, maximum and minimum ice ball diameters, and serum creatinine levels before and after cryoablation were recorded for each patient.
Follow-up Imaging
Immediately after treatment and 3-6 months, 12 months, 18 months, 24 months, and 36 months after cryoablation, CT examinations were performed with one of five MDCT scanners (LightSpeed Ultra or LightSpeed 16, GE Healthcare; Sensation 16, Sensation 40, or Sensation 64, Siemens Medical Solutions). Examinations were performed without and with 140 mL of IV iohexol (Omnipaque 300, Amersham Health) and included arterial (45 seconds), nephrographic (90 seconds), and excretory (300 seconds) renal phases with 2.0- to 2.5-mm slice thickness and interval, 120 kVp, 195-350 mA, and 0.5-second rotation time.
For patients with allergies to iodinated contrast medium or with renal insufficiency, contrast-enhanced MRI examinations were performed within 48 hours of ablation with a twin-speed 1.5-T system (Signa Excite, GE Healthcare). The renal MRI examination consisted of a three-plane localizer sequence performed with single-shot fast spin-echo or a fast spoiled gradient-echo technique. This sequence was followed by a coronal single-shot fast spin-echo sequence (TE, 80; bandwidth, 83 kHz; flip angle, 110°; matrix size, 256 x 256; number of excitations, 0.5; field of view, 44 cm; slice thickness, 5 mm; interslice gap, 1 mm).
Axial in-phase and out-of-phase spoiled gradient-echo images were obtained and included the adrenal glands and kidneys (TR/TE, 100-200/2.1 and 4.2; flip angle, 70°; bandwidth, 32 kHz; matrix size, 256 x 192; number of excitations, 1; slice thickness, 6 mm; interslice gap, 1 mm). Respiration-triggered fast spin-echo T2-weighted images were obtained (TR equal to two R-R intervals; TE, 85; echo-train length, 12; bandwidth, 32 kHz; matrix size, 256 x 224; number of excitations, 2; slice thickness, 5-6; interslice gap, 1 mm). Dynamic fat-saturated 3D fast spoiled gradient-echo images were obtained before and after contrast administration (3.4/1.6; number of excitations, 0.75; bandwidth, 83 kHz; flip angle, 15°; matrix size, 256 x 224; section thickness, 3-4 mm; number of sections acquired, 50-60 with zero filling; in-plane matrix size, 512 x 512; 50% overlapping sections along z-axis).
Parallel imaging with an acceleration factor of 1.8 in the phase-encoding direction was performed with a proprietary array spatial sensitivity-encoding technique (Asset, GE Healthcare). An axial low-resolution fast spoiled gradient-echo calibration scan preceded the parallel image acquisitions. Gadodiamide (Omniscan, Amersham Health) was injected IV at a rate of 3 mL/s (final concentration, 0.1 mmol/kg). A 2-mL test bolus of contrast material was injected before acquisition of contrast-enhanced 3D images to determine the appropriate scan delay to achieve an optimal set of arterial phase images.
A second set of contrast-enhanced images was acquired approximately 10 seconds after the arterial phase images. A third set of images was acquired approximately 2 minutes after the second set was complete. Last, an axial fat-saturated 2D spoiled gradient-echo sequence (100-200/2.6; flip angle, 70°; bandwidth, 32 kHz; matrix, 256 x 192; phase, 0.75; field of view, 32-40 cm; slice thickness, 6 mm; interslice gap, 1 mm) was acquired. Field of view for all axial sequences was adjusted according to patient size and typically ranged from 34 to 44 cm and had a phase field of view of 0.70-1.0.
Follow-up CT and MRI images were examined by three reviewers to determine the extent of ablation, technical success, and presence of complications. Local tumor progression was defined as any tumor that showed intralesional enhancement or serial increase in size compared with images from the follow-up examination immediately after ablation. Biopsy was not performed as part of routine follow-up.
Complications
Clinically important complications were defined according to the National Cancer Institute Common Terminology Criteria for Adverse Events, version 3.0.[18] These criteria are supported by the Cancer Therapy Evaluation Program of the National Cancer Institute. Grade 3 (severe adverse event) or greater complications were recorded.
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