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Interventional Radiology |
1 Department of Surgery, UNSW, The St. George Hospital, Sydney, Australia.
2 Present address: Department of Radiology, University Hospital Basel,
Petersgraben 4, Basel, BS 4031, Switzerland.
3 Department of Radiology, UNSW, The St. George Hospital, Sydney,
Australia.
Received November 25, 2003; accepted after revision April 19, 2004.
Address correspondence to K. Steinke.
Resection of non-small-cell primary lung cancer and metastases from several extrapulmonary primary tumors (e.g., colorectal cancer, endocrine tumors, renal cell cancer, leiomyosarcomas) has been shown to be more effective in terms of survival than medical treatment alone [3]. However, open thoracic surgery is a major invasive procedure performed with the patient under general anesthesia and is associated with considerable morbidity and the need for inpatient care [4].
Percutaneous in situ ablation of primary and secondary lung lesions using radiofrequency ablation is an evolving minimally invasive therapy. We and others have been using this therapy to attempt to improve survival rates and the quality of life of patients with unresectable lung cancer or lung metastases [5–13].
In this article, we provide an overview of the background of radiofrequency ablation, the possible techniques to be used with deployable radiofrequency electrodes, and the management and possible complications of lung ablations.
Background of Radiofrequency Ablation
From the mid 1990s [14], radiofrequency ablation has been increasingly used as a method of minimally invasive therapy for primary and secondary liver malignancies. Radiofrequency thermal ablation works by converting radiofrequency waves into heat through ionic vibration. Alternating current passing from an electrode into the surrounding tissue causes ions to vibrate in an attempt to follow the change in the direction of the rapidly alternating current. It is the ionic friction that generates the heat within the tissue and not the electrode itself. The higher the current, the more vigorous the motion of the ions and the higher the temperature reached over a certain time, eventually leading to coagulation necrosis and cell death.
The ability to efficiently and predictably create an ablation is based on the energy balance between the heat conduction of localized radiofrequency energy and the heat convection from the circulation of blood, lymph, or extra- and intracellular fluid [15].
The amount of radiofrequency heat produced is directly related to the current density dropping precipitously away from the electrodes, thus resulting in lower periphery temperatures. It can be approximated that the heat generated in a region at distance d from the electrode drops as 1 / d4.
The goal of radiofrequency ablation is to achieve local temperatures that are lethal to the targeted tissue. Generally, thermal damage to cells begins at 42°C; and once above 60°C, intracellular proteins are denatured, the lipid bilayer melts, and irreversible cell death occurs [16].
Technique
Radiofrequency Ablation Equipment
Four radiofrequency devices are currently available. A system from RITA
Medical Systems (StarBurst XL) and the one from RadioTherapeutics (RF Ablation
System) use deployable tines that expand into the tumor after an outer trocar
is positioned at the tumor edge or into the tumor. Radionics (Cool-tip RF
System) and Berchtold (Elektrotom 106 HFTT) distribute straight-needle
electrodes. The Radionics device has active (uninsulated) tips of different
sizes to create different ablation sizes. The system requires a pump that
perfuses chilled saline through the hollow ports inside the needle in a closed
system. The Elektrotom 106 HFTT system infuses normal saline to increase the
ablation area.
Each device consists of an electrical generator, needle electrode, and a grounding pad or pads.
The systems vary in the amount of generator power (50–200 W), generator cost (U.S. $12,000–30,000), size and configuration of electrodes, electrode cost (U.S. $500–1,500), use of electrode chilling, use of infusion, infusion media, parameters monitored, algorithm used, and the amount of operator input. All electrodes are currently nonreusable.
At our hospital, the model 1500X RF generator (RITA Medical Systems) used for lung ablations was chosen for two reasons. The first reason is that we prefer a system with deployable electrodes that are able to fix the tumor once deployed rather than a straight electrode that is prone to dislocate as a result of patient motion or breathing. Compared with the other deployable system (RadioTherapeutics RF Ablation System), the StarBurst XL has the larger deployment and thus with gradual deployment can produce an ablation size of from 2 to 5 cm. We also prefer the temperature-based ablation algorithm to an impedance-based ablation mode and the self-explaining data monitoring and evaluation software provided by the manufacturer of the StarBurst XL.
In this article, we describe our experiences with the deployable StarBurst XL electrode (RITA Medical Systems) that creates ablations up to 5 cm in diameter. A thermal lesion of coagulation necrosis encompasses the tumor and a 5- to 10-mm surrounding safety margin of normal healthy tissue. The ideal size of the lesion should be equal to or less than 3 cm in biggest diameter and thus treatable with a single ablation. The ideal shape of the lesion is spherical. In lesions larger than 3 cm, overlapping ablations are required.
Bilateral metastases are treated, but for safety reasons, only one lung at a time should be ablated. The procedure can be repeated, to treat either new or recurrent tumor.
The SunBurst XL has nine active electrodes (five with thermocouples) and an active trocar tip. The electrodes are well distributed, for a space-filling globe design with one electrode straight along the axis of the probe, four curved electrodes around the "equator," and the remaining four curved electrodes along the "northern hemisphere" [14]. The thermocouples are located in the tip of the straight electrode plus in four of the electrodes around the equator. These thermo-couples are used to provide temperature feedback for monitoring, ablation control, and tissue temperature after ablation. Data from the generator—power, impedance, temperature, time, and so on—can be collected and displayed graphically on a PC.
Patient Selection
Patients are selected for the procedure by agreement of a joint tumor
board. Patients chosen for radiofrequency ablation treatment are judged not
suitable for surgery because the site and distribution of their lung tumors
(metastases) or limited cardiorespiratory function disqualifies them as
candidates for surgery. Bilateral metastases are treated, but for safety
reasons only one lung at a time should be ablated. In lesions larger then 3
cm, overlapping ablations are required. The procedure can be repeated to treat
either new or recurrent tumor.
Tumors that exhibit preferential spread to the lungs as the only site of metastasis include sarcoma, renal cell cancer, and head and neck tumors. These metastases are more appropriate to treat with radiofrequency ablation than tumors that metastasize to multiple organ sites, such as breast cancer and melanomas. Colorectal carcinoma lies between the two groups: its most common site of metastasis is the liver, but the lung is the most common site of extraabdominal disease spread. Our experience with more than 30 patients with colorectal carcinoma who had lung metastases ablated with radiofrequency and who had follow-up of at least 1 year indicates that a significant proportion of patients have evidence of successful local control at 1 year [11].
Procedure
Informed consent is obtained from all patients. The patient is positioned
on the CT table according to the location of the lesion or lesions. Because
the ablation procedure takes at least 15 min per lesion at the target
temperature, the patient should be positioned so that he or she is as
comfortable as possible, preferably supine or prone. Patient blood pressure,
pulse rate, and blood oxygen saturation are monitored and recorded via a pulse
oximeter.
The patient is made into an electrical circuit by placing two grounding pads on the back muscles or one on each thigh with the longer part facing the coagulation site, therefore providing a large leading edge of the pad to lessen the risk of skin burns. In procedures for which a large amount of radiofrequency energy is delivered to the patient, repeated checking of the grounding pad temperature is advisable to avoid skin burns [15].
The tissue down to the pleura is anesthetized with local anesthetic. Sedation or analgesia generally consists of conscious sedation—we use a combination of midazolam and pethidine—that can be given and increased on demand. Articles about liver radiofrequency ablation and the data from a worldwide survey on lung radiofrequency ablation [17] advocate analog sedation as the sedation of choice in the routine setting. We have used conscious sedation, managed by a qualified nurse with radiologist supervision, with no adverse event to date. Occasional agitation despite (or due to) profound sedation is sometimes observed, but none of our patients has required anesthetist intervention. Not being dependent on the anesthetist service allows greater flexibility in the management of radiofrequency procedures and the CT room and is cost-effective. We presume that general anesthesia could be necessary in procedures that are foreseeable as long or as painful—that is, in cases in which there is broad pleural contact of the tumor or the patient is likely to be agitated and move.
Because space is limited in the CT gantry, it is possible to use only 10- and 15-cm-long probes; the 25-cm probes are used mostly for laparoscopic treatments. The electrode is positioned under CT guidance, preferably CT fluoroscopy. Guidance without fluoroscopy is more complicated and thus less precise because of motion caused by patient breathing. Open ablations after thoracotomy are used for experimental purposes in animal models but have no clinical importance.
With the use of CT fluoroscopy, the radiation exposure for the patient is decreased because electrode placement into the tumor is faster with real-time visual control than with repeated helical CT control [18]. Clinicians performing the procedure can minimize their radiation exposure by using a tool, such as an artery clamp, to grasp and move the device during scanning.
Similar to selecting an access site for diagnostic biopsies, one should chose the access site least likely to harm crossing vessels or nerves, thus minimizing pleural crossing and allowing a minimum depth of the electrode inside the thoracic cage to reduce displacement from patient breathing or motion or the weight of the generator cable.
Once the needle tip is in the right place, the tines are deployed to a 2-cm array and the probe is connected to the generator. One should be alert of the risk of "push back" in that instead of the electrodes being moved forward from the device's trocar and into the tumor, the trocar is pushed backward to expose the electrodes proximal to the tumor and not penetrating the tumor, as intended.
The protocol we use recommends a power setting at 50 W, adjustable to 150 W, and a temperature setting at 90°C. These values have been assessed in animal experiments and are recommended by the distributor. The aim is gradual heating that leads to coagulation necrosis. We attempt to avoid charring with subsequent loss of heat dissipation by starting with a low power setting.
Intermittent radiologic confirmation of the position of the probe should be performed, especially after changing deployment size or if the patient has moved significantly.
When the average temperature has been maintained for the required duration, the generator switches off automatically. The clinician then retracts the electrodes into the trocar. To reduce the risk of tumor seeding while retracting the probe and lessen the risk of hemorrhage while retracting the probe, the clinician performs track ablation by turning down the output while slowly drawing the needle out.
Patient Follow-Up
We perform follow-up chest CT at 1 month and then at 3-month intervals
after ablation within the first year followed by 6-month intervals thereafter.
The size of the ablated lesions and their contrast uptake are assessed. Our
experiences have shown an initial increase in lesion size compared with the
pretreatment lesion because the ablation area encompasses not only the lesion
but also a surrounding safety margin. This size increase is followed by a
gradual shrinking [12].
Residual scars may be permanent (Figs.
1A,
1B,
1C,
1D,
1E, and
1F). The 1-month follow-up CT
measurement is obtained as a postradiofrequency reference measurement; any
additional increase in size or in attenuation is suggestive of recurrence.
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PET is also a good and sensitive procedure for follow-up [19], providing good discrimination between residual granulation tissue and recurrent tumor, but access to PET at our institution is limited by availability and cost.
Technical Complications
The complications that we describe in this section resulted from our experiences and were not derived from the literature that describes lung radiofrequency ablation. The combination of a hard tumor in soft lung sometimes can make tumor penetration difficult. If a hard tumor bounces away from the electrode, it is advisable to attempt to deploy the tines by 2–3 mm and then spike the lesion with the leading tip of the straight electrode (Figs. 2A, 2B, and 2C) before another approach at transfixing the tumor from another access site. Repositioning and multiple attempts to transfix the tumor often lead to moderate to large pneumothoraces (Figs. 3A and 3B). We have placed a pleural catheter (Pleurocath, Plastimed) with the patient sedated in three patients when a pneumothorax was noted during the radiofrequency procedure and have successfully continued the ablation.
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A major problem is desiccated tissue that adheres to the electrode and
forms an electrically insulating coating resulting in charring that can
prevent the easy withdrawal of the electrodes into the trocar. Charring can
prevent the intended dissipation of the heat away from the electrodes,
resulting in a sudden rise in the measured system impedance (> 500 
)
and a corresponding decrease in the delivered power. The increase in impedance
can result in the generator automatically switching off
[16]. Pausing the
radiofrequency for 30–60 sec to allow conductive body fluids to
rehydrate the tissue or retracting and redeploying the electrodes to clean the
adherent charred tissue from the electrodes may allow an electrical connection
to be reestablished. Forceful withdrawal of the device can cause tissue injury
along the probe track [9].
Clinical Complications
The reported clinical complications again reflect our experiences, which are based on radiofrequency treatment of 46 patients (111 lung tumors) to date without any periprocedural deaths and with a modest complication rate. The pneumothorax rate for our patients was 28%, and a third of these patients required a chest tube. Intrapulmonary bleeding with or without hemoptysis can occur. The rate of intraparenchyma hemorrhage was 6% for our patients.
Cavitations occurred in 20% of the lesions treated and occurred especially if the ablated area exceeded the size of the initial lesion by more than 200% [12]. This difference between lesion size and ablated area occurs regularly in lesions less than 2 cm in the largest diameter. The intended ablation should encompass not only the tumor but also a 1-cm surrounding safety margin of normal healthy tissue, thus leading to diameters three times larger than the tumor. Because none of our patients experienced superinfection of these cavitations, this condition can be considered a side effect rather than a complication of radiofrequency ablation. Cavitations usually resolve over the first 2–4 weeks.
After the procedure, most patients experience some pain, usually pleuritic in type, that is typically treated with nonopioid analgesics. Although fewer than 5% of patients require stronger pain medication, it is advisable to be liberal with analgesia to allow deep respiration and prevent possible superinfection of the treated, suboptimally aerated portion of the lung.
Fever—a slightly raised temperature (< 39°C)—is observed in nearly all patients for 1 week after ablation. Patients may also develop pneumonia or an abscess may form, but there usually are symptoms in addition to the raised temperature, such as chills and severe coughing. Prophylactic periprocedural medication with an antibiotic–fungostatic combination (as used before endoscopic or surgical procedures) may be of value but, to our knowledge, has not yet been studied formally.
Two of our patients have reported expectorating small pieces of desiccated lung tissue. This could result from the ablated tissue gaining access to a bronchus.
Sympathetic pleural effusion to an amount that does not require tapping is often seen after ablation on an upright chest radiograph with the lateral costophrenic angle obliterated. Symptomatic effusions requiring tapping occur in less than 5% of the procedures and are often hemorrhagic.
One of our patients experienced pleural tumor seeding with new malignant effusion 6 months after radiofrequency ablation to a single colorectal pulmonary metastasis adjacent to the pleura. The patient had a lung biopsy before radiofrequency ablation. A causative relationship to the radiofrequency ablation procedure, which was terminated with track ablation, is not proven but clearly is possible.
Discussion
The long-term results and prognostic analysis based on 5,206 cases in the International Registry of Lung Metastases showed that complete lung resection is the principal prognostic index of survival [20]. The actuarial survival rate after complete metastasectomy is reported as 36% at 5 years, 26% at 10 years, and 22% at 15 years [21].
Jaklitsch et al. [22] reported 56 patients who had sequential metastasectomies, with a minimum of two and a maximum of six separate lung resections. Their conclusion was that multiple attempts to reestablish intrathoracic control of metastatic disease are justified in carefully selected patients, but the magnitude of benefit decreases with each subsequent attempt. Radiofrequency ablation, which is less invasive, has less morbidity and mortality and spares more healthy tissue than resection, appears to be a promising option for patients with relapse after metastasectomy.
Mineo et al. [23] showed that the type of resection did not significantly affect survival when 85 patients had pulmonary metastasectomy by conventional resections with either diathermy dissection or stapler suture lines, lobectomy, or laser ablation. Minimally invasive surgery, especially by laser device, is recommended for less morbidity.
Radiofrequency ablation is an established therapy in patients with primary and secondary liver tumors [23–26] but has not been widely used to date in the lung. The advantages of radiofrequency ablation over other minimally invasive treatments such as cryoablation, both intraoperatively [27] and percutaneously [28], include a smaller diameter probe; more precise placement due to the smaller tip size; and a reduction in major side effects such as intraoperative hemorrhage, renal insufficiency, temperature injury to adjacent organs, and coagulopathies [29].
Experience with laser ablation in the human lung is so far restricted to endobronchial therapy [30].
Goldberg et al. [31] investigated the feasibility and safety of percutaneous radiofrequency ablations of lung in rabbits using a percutaneous coaxial needle technique with an 18-gauge radiofrequency probe. The same group showed that radiofrequency ablation of VX2 tumor nodules with a diameter of 6–12 mm was successful in the rabbit lung [32].
Miao et al. [33] recently reported on radiofrequency ablation of VX2 tumors implanted in rabbit lungs and evaluated therapeutic efficacy by survival rate and findings from MRI, postmortem microangiography, and histology. The control group that underwent a sham operation died within 3 months. In the treated group, tumor eradication was achieved in 75% (9/12), of which four rabbits survived free of disease longer than 3 months and five rabbits were free of viable tumor when sacrificed. One rabbit had incomplete tumor ablation, and two rabbits had local tumor relapse.
The successful percutaneous radiofrequency ablation in a canine lung tumor model has been reported lately [34]. The first attempts to percutaneously radiofrequency ablate primary and secondary lung tumors in humans are promising.
Recent publications on CT-guided percutaneous pulmonary radiofrequency ablation support the claim that this minimally invasive treatment technique is evolving into an accepted therapeutic procedure for patients with primary and secondary lung malignancies [6–13]. Data on long-term results after pulmonary radiofrequency ablation of primary and secondary tumors are still required.
Conclusion
Radiofrequency ablation has an expanding role in the treatment of inoperable malignancies primary and secondary to the lung. Further work is needed to assess the patient group most likely to benefit from treatment with this minimally invasive ablation technique.
References
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