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Clinical Observations |
1 Department of Radiology, Division of Abdominal Imaging and Intervention,
Brigham and Women's Hospital, 75 Francis St., Boston, MA 02115.
2 Department of Radiology, Massachusetts General Hospital, Boston, MA
02114.
Received February 6, 2007;
accepted after revision May 22, 2007.
Partially supported by grant U41RR019703 from the National Institutes of
Health.
Abstract
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CONCLUSION. Benign inflammatory nodules occur rarely after percutaneous ablation of renal tumors and may mimic tumor seeding of the applicator track.
Keywords: cryoablation • percutaneous ablation • radiofrequency ablation • renal cell carcinoma • tumor seeding
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Percutaneous ablation of renal tumors using either radiofrequency ablation or cryoablation involves nephron-sparing techniques that aim to further decrease procedure-related morbidity relative to partial nephrectomy and allow patients who are not surgical candidates to be treated. Although longer follow-up data will be required before percutaneous ablation becomes the standard of care, initial results have been promising [4–6].
Implantation ("seeding") of tumor cells within the applicator track is a potential complication of percutaneous ablation that has been reported in one patient after radiofrequency ablation of a renal cell carcinoma [7]. We report six patients who had benign imaging findings that initially suggested applicator track seeding.
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Radiofrequency Ablation Procedures
Of 310 procedures, 235 were performed using radiofrequency ablation guided
by CT (LightSpeed, GE Healthcare; n = 224) or sonography (Logic 700,
GE Healthcare; n = 11) on 205 renal tumors (mean, 3.2 cm; range,
1.0–8.9 cm) [4].
Prophylactic antibiotics were not administered before the procedure. The skin
was prepared and draped using sterile technique. For most cases (n =
205), a 200-W generator with impedance-controlled pulsed current was used with
internally cooled straight single or cluster electrodes (n = 190) or
with a switching controller allowing three straight single electrodes
(n = 15) (Cool-tip, Valleylab) with multiple overlapping 12-minute
ablations to treat the entire tumor. The remaining cases were performed using
a 150- to 200-W generator and multitined expandable electrodes with
(n =4) or without (n = 26) saline instillation (3–5
cm) (StarBurst XLi or StarBurst XL, RITA Medical Systems) and a target
temperature of 105°C, also with multiple overlapping ablations. In a
minority of cases, dextrose 5% in water (D5W) was injected with fine
(20-gauge) needles to prevent burning adjacent structures. The electrode track
was ablated to reduce the risk of hemorrhage in patients with central tumors,
coagulopathy, or evidence of bleeding during the procedure. Track ablation was
performed at operator discretion in a minority of cases with 200–300 mA
of current, allowing the temperature to reach over 50°C as the electrode
was slowly and incrementally with drawn. To avoid skin pain or burns, track
ablation was discontinued when the active tip of the electrode was within
1–2 cm of the skin. Thirty tumors required a second radiofrequency
ablation session because CT scans at 1 month showed residual tumor.
Cryoablation Procedures
Of 310 procedures, 75 were percutaneous cryoablations of 82 renal tumors
(mean, 2.6 cm; range, 1.0–6.6 cm) guided by a 0.5-T open-configuration
MRI system (Signa SP, GE Healthcare)
[5]. During seven procedures,
two tumors were targeted. Before the cryoablation procedure, the skin was
prepared and draped using sterile technique. Prophylaxis for infection was
provided with 1 g IV cefazolin sodium (Ancef, SmithKline Beecham
Pharmaceuticals) administered every 8 hours for 24 hours. A cryoablation
delivery system (CryoHit, Galil Medical) was used with 13- or 17-gauge
cryoprobes. Depending on the size of the lesion, one to seven (mean, three)
cryoprobes were placed in tandem alongside an MRI-compatible 22- to 20-gauge
biopsy needle (MRI-compatible biopsy needle, E-Z-EM). A freeze-thaw-freeze
cycle (15-minute freeze, 10-minute passive thaw, 15-minute freeze) was used.
During freezing, probe-tip temperatures reached a nadir of approximately
–130°C. If the ice ball did not encompass the tumor entirely and
include a 5-mm margin of tissue beyond the tumor, additional cryoprobes were
placed. To prevent freezing adjacent structures, normal saline was injected
using fine (20- to 22-gauge) needles before five of 75 cryoablation
procedures. During 18 of 75 procedures, external manual compression was used
to displace bowel loops
[10].
Follow-Up Imaging
All patients underwent CT or MRI before ablation, with and without IV
contrast material; these results formed a baseline comparison for subsequent
imaging. CT (LightSpeed [GE Healthcare] or Somatom Volume Zoom, Somatom
Sensation 16, or Somatom Sensation 64 [Siemens Medical Solutions]) was
performed with 2.5- to 5-mm collimation at 220–240 mA and 140 kVp. MRI
(Signa, GE Healthcare) included the following sequences: transverse
T2-weighted imaging with fast spin-echo (TRrange/TE,
3,267–7,000/100; echo-train length, 12; section thickness, 4 mm; gap, 1
mm; field of view, 30–36 cm), breath-hold fast-recovery fast spin-echo
(1,200–2,996/91–94; echo-train length, 17–22; section
thickness, 5 mm; gap, 1 mm; field of view, 32–40 cm), and/or single-shot
fast spin-echo (17,240–53,380/184–190; section thickness, 5 mm;
gap, 1 mm; field of view, 32–40). We also used transverse T1-weighted
imaging with a spin-echo sequence (500–800/14; section thickness
4–5 mm; gap, 1 mm; field of view, 30–36 cm) or a spoiled
gradientr-ecalled echo (GRE) sequence (300–400/2.2, 4.7; dual echo; flip
angle, 90°; section thickness, 5 mm; gap, 1 mm; field of view, 32–40
cm) and transverse fat-suppressed T1-weighted dynamic imaging with a spoiled
GRE sequence (260–435/4.2; flip angle, 75°; section thickness,
4–6 mm; gap, 1 mm; field of view, 34–40 cm) or a 3D
fast-acquisition multiple-excitation spoiled GRE sequence
(5.2–7.3/1.5–2.2; flip angle, 10°; section thickness, 2.5 mm
[effective]; gap, 0 mm; field of view, 32–40 cm) before and after IV
gadolinium administration (gadopentetate dimeglumine; Magnevist, Berlex).
After ablation, CT or MRI using the same protocol as the preprocedural examination was performed at 24 hours and/or 1 month and then every 3 months for the first year and every 6–12 months thereafter. Most patients who underwent radiofrequency ablation were followed up with CT, and most patients who underwent cryoablation were followed up with MRI. There were some patients treated with radiofrequency ablation who were followed up with MRI instead of CT because of renal insufficiency. Some patients treated with cryoablation were followed up with CT if CT was requested for another indication; one patient had a cardiac pacemaker implanted in the interval, and therefore, could no longer be followed up with MRI. The mean length of follow-up was 2.6 years for radiofrequency ablation procedures and 1.8 years for cryoablation procedures.
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Applicator track nodules occurred after four (1.7%) of 235 radiofrequency ablation procedures and two (2.7%) of 75 cryoablation procedures. Nodules occurring after radiofrequency ablation were ring-enhancing (n =2; mean size, 1.4 cm) or ill-defined enhancing regions of soft tissue (n = 2; mean size, 4.1 cm) (Fig. 1A, 1B, 1C, 1D). These findings were first visible at 3, 6, 7, and 52 months and were adjacent to the ablated renal tumor within the perinephric space alone (n = 3) or within the perinephric space and abdominal wall (n = 1, Fig. 1A, 1B, 1C, 1D).
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Among patients who developed an applicator track nodule, self-limited hematuria was observed after one radiofrequency ablation, and a small perinephric hematoma developed after another radiofrequency ablation that persisted 3 months after the procedure. The hematoma was in the same area as the applicator track nodule that appeared 3 months later. A small perinephric hematoma was visible at 24 hours after one cryoablation procedure and resolved by 1 month. This hematoma was not in the same area as the applicator track nodule. No major complications as defined by Goldberg et al. [11] occurred after procedures associated with an applicator track nodule.
Four nodules (three after radiofrequency ablation, one after cryoablation) were biopsied percutaneously under CT guidance. Of these, one was also surgically resected. At pathology, three nodules showed signs of a chronic, reactive inflammatory process, such as perivascular lymphoplasmacytic infiltrate, histiocytes, granulation tissue, and fibrosis. A nodule after radiofrequency ablation that occurred after a persistent hematoma consisted of marked acute inflammation and granulation tissue consistent with an organizing abscess. None of the nodules showed evidence of malignancy.
Of the six nodules, two were no longer visible 4 months after detection. Three either substantially decreased in size or no longer enhanced within 3 to 6 months. The remaining nodule, which was found on biopsy to be consistent with an organizing abscess, decreased in size at 3 months and therefore was not treated with antibiotics; it has remained stable at 12 months.
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The appearance of the inflammatory nodules provides some insight into their origin. Nodules after cryoablation were located in the subcutaneous tissues peripheral to the abdominal wall and were associated with a tram-tracking pattern of enhancement along the path of the cryoprobe. Although the distal 4 cm of each cryoprobe reaches temperatures as low as –130°C, the rest of the probe's shaft reaches temperatures as low as –35°C. It is likely that the tissues adjacent to the shaft were affected to some degree. A tubular pattern of tissue injury may have occurred along the probes leading to the nodules observed in this study. This pattern led to a tram-tracking appearance of enhancement when the imaging plane was in the same plane as the probe track. In other cases, tissue injury appeared as ring-enhancing nodules when the imaging plane was perpendicular to the probe track.
Inflammatory nodules after percutaneous radiofrequency ablation were less defined, not associated with a tram-tracking appearance, and located adjacent to the ablated tumor in the perinephric region or abdominal wall musculature along the electrode track. One nodule may have been due to ablation of the electrode track, which was occasionally performed at the operator's discretion on electrode removal to prevent track seeding and bleeding at the end of the procedure. Another nodule was ultimately diagnosed as an organizing abscess that developed from a postprocedure hematoma. Although the finding persisted for several months, no infectious organism was isolated. Infections after radiofrequency ablation of renal tumors are rare, prompting many to perform radiofrequency ablation without antibiotics [4, 7, 13]. Indeed, prophylactic antibiotics were used only before cryoablation procedures in our series. Nevertheless, subclinical infection is one possible cause of applicator track nodules. Therefore, antibiotics may help prevent the appearance of application track nodules, although antibiotics before cryoablation did not prevent occurrence entirely.
Because needle track seeding cases are so rare after renal tumor ablation, it is difficult to suggest how to discriminate them from the benign inflammatory nodules we encountered. Benign inflammatory nodules may become visible from several months to years after the ablation. Three of the six applicator track nodules appeared more than 6 months after the ablation. In fact, one was first detected on MRI 52 months after radiofrequency ablation. Therefore, one cannot rely solely on delayed onset to diagnose cases of tumor track seeding.
The frequency of nodules after cryoablation (2.7%) was greater than the frequency after radiofrequency ablation (1.7%). However, the finding is rare; determination of the true incidence of this finding would require more cases. Also, most patients were followed up with MRI after cryoablation and CT after radiofrequency ablation. MRI may be more sensitive in detecting changes in soft tissue because of its greater soft-tissue contrast. A study using the same postprocedure imaging technique would be needed to determine if there is a true difference in the incidence of inflammatory nodules after the two ablation methods. Other confounding variables such as size and number of applicators preclude drawing any definitive conclusion regarding differences in the incidence of this finding between radiofrequency ablation and cryoablation.
Among patients who developed applicator track nodules, percutaneous biopsy was performed before tumor ablation and along similar tracks as the applicators. Therefore, the possibility that the inflammatory nodules were caused by the biopsy procedure cannot be excluded. However, percutaneous tumor ablation is more likely to lead to tissue injury and inflammation than percutaneous biopsy.
Limitations of this study include its retrospective design and observer subjectivity. During review of the images, we included only cases with findings that were suggestive of tumor. Indeed, four of the six applicator track nodules detected in the study had been biopsied, suggesting that nodules were likely to arouse suspicion among other practicing radiologists.
In conclusion, patients may develop new benign enhancing nodules in or around the applicator track that mimic tumor seeding. These nodules should be biopsied and not diagnosed presumptively as tumor seeding.
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