INTRODUCTION
Radiofrequency (RF) ablation is an alternative technique to surgical resection of lung tumors, either primary or secondary, through delivery of high-frequency alternating current using electrodes precisely placed at the target sites. Although not absolutely necessary, the vast majority of radiofrequency ablation is performed under image guidance.
DEMOGRAPHICS OF LUNG CANCER
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Lung cancer is most common cause of cancer death in both men and women, accounting for 29% of all cancer deaths in the United States.
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In the United States, 170,000 new cases of primary lung cancer are diagnosed annually.
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Despite advances in treatment technology, the overall 5-year survival rate for newly diagnosed lung cancer remains at only 15%. Survival differs with pathology, staging, and treatment.
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Non–small cell lung cancer (NSCLC)
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Stage 1(T1 N0 M0 and T2 N0 M0):
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Surgery is the treatment of choice.
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5-year survival rate at approximately 75%.
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Stage 2 (T1 N1 M0, T2 N1 M0 and T3 N0 M0):
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Surgery is the treatment of choice.
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5-year survival rate at approximately 50%.
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Stage 3 A (T1 N2 M0, T2 N2 M0, T3 N1 M0 and T3 N2 M0):
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Radiotherapy, chemotherapy, surgery, and combinations of these modalities are used for treatment.
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Overall 5-year survival does not exceed 10% to 15%.
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Stage 3 B (Any T, N3, M0 and T4, any N, M0):
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Surgery is of no benefit. Combination of chemotherapy and radiotherapy is used.
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5-year survival rate is less than 5%.
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Stage 4 (Any T, Any N, M1):
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Palliative chemotherapy is used.
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Median survival at time of diagnosis is 7.9 months.
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Small cell lung cancer (SCLC)
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Limited-stage disease: Combination chemotherapy is the cornerstone of treatment, combined with radiotherapy. Median survival of 18 to 24 months and 40% to 50% 2-year survival rate. ,
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Extensive-stage disease: Combination chemotherapy is the cornerstone of treatment. Radiotherapy alone is not beneficial.
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Lung Cancer:
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For early-stage (1 and 2) NSCLC, (15% of cases at diagnosis) surgical resection is standard therapy because it confers the best opportunity for long-term disease-free survival.
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However, only one third of these patients meet pulmonary physiologic guidelines for lobar or sublobar resection (FEV 1 > 60% predicted and carbon monoxide diffusing capacity > 60% predicted).
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For medically inoperable surgical candidates, sublobar resection, specifically wedge resection and segmentectomy, has been proposed; however, these approaches have shown high local recurrence rates of up to 50% and, therefore, are considered less favorable.
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According to the Surveillance, Epidemiology and End-Results database, 14,555 patients, or 15.7%, present with stage 1 and 2 lung cancer but are medically inoperable.
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Despite comorbid medical illness, patients who receive therapy for early-stage lung cancer enjoy better outcomes than those who do not receive therapy.
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Therefore, a sizeable proportion of patients with stage 1 and 2 lung cancer are medically inoperable and can benefit from less invasive and nonsurgical techniques, such as radiofrequency ablation.
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Metastasis:
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The lungs are the second most common organ involved by solid tumor metastases, following the lymphatic system and virtually tied with the liver. ,
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Hematogenous metastasis from nongastrointestinal tumors to the lungs is common, because the lung acts as the first capillary bed for blood-borne and lymphatic system–borne dissemination.
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Approximately 20% of patients with lung metastases and resected primary soft tissue tumors have a limited number of metastases located only in the lung. Therefore, they are potential candidates for surgical resection or metastasectomy.
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Surgical metastasectomy may result in improved disease-free survival in properly selected patients. However, with multiple surgical resections, pulmonary capacity compromise may become an important issue. A lung-sparing technique, such as radiofrequency ablation, may be an attractive option in those candidates who have limited disease, those who have controllable disease but are medically inoperable, and those who have limited recurrent or residual disease, following prior surgical resection, chemotherapy, or radiotherapy.
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PHYSICS OF RADIOFREQUENCY ABLATION
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RF ablation is a technique that requires precise placement of an electrode or electrodes into a specific location to cause local tissue destruction by controlled heating. The mechanism involves
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Application of rapid alternating electrical current with a frequency of 460 to 500 kHz in the range of radio waves. Applied electric power ranges from 10 W to 200 W with maximum current from 500 mA to 2000 mA.
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The RF electrical current is concentrated near the noninsulated tips of the electrode, and the circuit is completed by returning either to electrical grounding pads usually located on the patient’s thighs (monopolar system) or to a nearby grounding electrode (bipolar system).
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The alternating electrical current causes agitation of ionic dipolar molecules in surrounding tissue and fluids, resulting in frictional heating that is greatest adjacent to the noninsulated portion of the electrode.
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When living tissue is heated to more than 50 ° C for at least 5 minutes, cells undergo coagulation necrosis that results in denaturation of proteins. The heat energy is then distributed radially to surrounding tissues. For ablation of tumors, tissue temperatures typically range between 60 ° C and 100 ° C, which results in rapid coagulation necrosis, enzymatic deactivation, and subsequent nearly instantaneous cell death. ,
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Control of the ablation zone and area of cell death are of utmost importance. For ablation of tumors within the lung and liver, the goal is homogeneous necrosis of the entire tumor, as well as an acceptable surrounding margin of noncancerous tissue, generally at least 1 cm. , Preservation of surrounding vital structures and organs is an additional key consideration.
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RF thermal distribution in tissue has been described by the bioheat transfer equation. The dimensions of tissue undergoing necrosis are dependent on
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Length and thickness of noninsulated active portion of the electrode.
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The longer the length of the active portion, the larger the area of ablation.
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The larger the diameter of the electrode, the larger the area of ablation.
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The ablation zone is usually elliptical, its width measuring approximately 2 to 3 cm in diameter. In order to accomplish therapeutic goal of complete tumor kill in tumors larger than 2 to 3 cm diameter, multiple overlapping spheres or cylinders of ablation are required. , As the number of overlapping ablations increase, so do the overall duration of the procedure, the likelihood of leaving islands of viable tumor, and the probability of complications.
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The highest rates of complete tumor ablation and lowest rates of complications are achieved in tumors less than 2 to 3 cm diameter.
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Power and duration of applied RF current.
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10 W to 200 W, with maximum current from 500 mA to 2000 mA.
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A minimum of 5 minutes is required to obtain cell death. Twelve minutes is optimal and usually repeated.
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Multiple electrodes and switch boxes can be used to increase the ablation zone.
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Surface temperature of RF electrode.
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Temperatures ranging from 60 ° C to 100 ° C result in rapid coagulation necrosis, enzymatic deactivation, and subsequent cell death nearly instantaneously. ,
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At temperatures in excess of 105 ° C to 115 ° C, tissue charring and carbonization, gas formation, and cavitation may occur, insulating the surrounding tissue from the effects of RF current and interfering with heat diffusion.
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Composition of surrounding tissue.
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“Heat sink” is a phenomenon of heat removal by rapidly flowing blood within an adjacent artery or vein, thereby preserving the blood vessel and an adjacent cuff of malignant perivascular tissue. Heat sink can also occur with larger bronchi.
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PATIENT SELECTION CRITERIA
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Proper patient selection is highly dependent on the anticipated goal to be achieved. Although no solid or strict criteria exist, emerging goals for ablation thus far established within the current medical literature include
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Potential for cure.
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In stage 1 and 2 NSCLC.
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In medically inoperable surgical candidates or in those with stage 1 and 2 NSCLC who refuse surgical resection.
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Limited metastases.
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Prolongation of survival.
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Pulmonary malignancy with limited disease outside the lung that can be treated with systemic therapy or locoregional methods.
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Slowly growing pulmonary malignancy, with the main bulk of tumor within the lung.
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Cytoreduction of large tumors to potentially alter the susceptibility of viable tumor tissue to chemotherapy or radiotherapy.
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Relief of symptoms.
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Pain commonly present with peripheral tumors invading the parietal pleura or chest wall, for example, mesothelioma
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Ablation of tumors adjacent to vital structures as a preemptive strike before invasion
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Cytoreduction of tumor volume, resulting in reduction of symptoms and significant improvement in the quality of life
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PROCEDURE
Preprocedure Evaluation
Once the patient is selected, evaluation before ablation includes:
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Directed patient history
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Directed physical examination with special attention to cardiopulmonary compromise
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Electrocardiogram and pulmonary function tests (PFTs), the latter particularly with lung disease and/or resection. Many patients with lung cancer are or have been smokers. PFTs are required to establish:
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Adequacy of oxygenation
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Pulmonary reserve
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Flow volume spirometry
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Medication history
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Anticoagulant and antiplatelet medications need to be tapered or stopped before the anticipated procedure.
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Before the procedure, conversion to subcutaneous or low-molecular-weight heparin can be considered, which is stopped at least 24 hours before the procedure.
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Current computed tomography (CT) scan should be available that is not older than 4 weeks. In case of primary lung cancer, current staging with positron emission tomography (PET) or mediastinoscopy is required. CT scan is used
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To ensure amenability of tumor size and number to ablation.
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Evaluate a safe access route
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Comorbid disease in the lung
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Location and relationship of tumor to vital structures
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Presence of pacemaker or implantable cardioverter defibrillator
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RF current can cause potential device malfunction.
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Possibility of coagulation necrosis at myocardial implantation site seems invalid, likely from massive heat sink effect of cardiac ventricular blood flow.
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Patients dependent on pacemaker for small percentage of time can have temporary deactivation with temporary cardiac passing available on standby.
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Patients with constant dependence on pacemaker may benefit from alternative modalities, like cryoablation, microwave and laser thermal ablation.
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Histopathologic diagnosis of the lesions should be obtained before the ablation. Biopsy can be performed before the ablation as a separate session or immediately before the ablation as a concurrent session. The latter is less preferred because
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Procedure time may be prolonged.
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Presence of complications from biopsy procedure can delay or postpone the ablation.
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Postbiopsy hemorrhage along the needle tract may obscure the tumor margins and detection of post ablation zone of ground-glass opacity (GGO) less accurate.
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Anesthesia
RF ablation can be painful, especially in close proximity to the parietal pleura, and controlled breathing with a stationary targeted tumor is desirable. The procedure can be performed under general anesthesia or deep or moderate conscious sedation. No consensus exists among authors, and each operator’s decision is dependent on many variables including comorbid disease, tumor location, and risk for potential complications.
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General anesthesia
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Advantages
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Greater degree of patient comfort
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Better control of the airways
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Presence of an expert in pulmonary and cardiac management in case of complications
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Disadvantages
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Higher cost
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Logistical challenges requiring second participating service
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Longer procedure times related to anesthesia setup and availability
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Increased risk for pneumothorax owing to positive pressure ventilation
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Risks specific to general anesthesia
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Deep conscious sedation
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Advantages
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Lower cost than general anesthesia
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Shorter procedure times than with anesthesia
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Disadvantages
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Potential for less patient comfort and procedural pain
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Preparation for Procedure
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Consent for the procedure and other interventions, if needed, should be obtained.
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Coagulation cascade, including prothrombin time, partial thromboplastin time, International Normalized Ratio, and bleeding time, needs to be checked on the day of procedure, especially if the patient is on anticoagulation, or within 7 days.
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Prophylactic antibiotics are given routinely by some interventionists 1 hour before the procedure and in postprocedural period, the argument being that devitalized tissue at the end of ablation is a potential nidus for superinfection. However, the benefits have yet to be proven.
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For the lung, CT is the most widely used modality for directed and precise electrode placement. Volumetric acquisition of imaging data allows for computer reconstruction of three-dimensional volume data sets for pre-, intra-, and postprocedure analysis. Multiplanar reformatted display of images both parallel to (long axis views) and perpendicular to (short axis views) the axis of the RF electrodes can provide valuable information regarding depth of device penetration, position of an electrode shaft or tines within or adjacent to tumor, the configuration of multiple devices, and the relationship to essential, nontargeted structures. The principal limitation of CT is the lack of real-time imaging feedback for interactive advancement of devices into the targeted tumor. Some operators have found CT fluoroscopy to be a useful adjunct for device placement ( Fig. 27-1A and B ).
Figure 27-1
A 60-year-old female with biopsy-proven right lower lobe metastatic uterine sarcoma. A, Axial noncontrast computed tomography image at the level of the right atrium shows the soft tissue metastasis adjacent to the right atrium ( arrowheads ). Owing to its firm consistency, multiple attempts to place an electrode through its center were unsuccessful. B, Multiplanar reformat (MPR) with sagittal reconstruction demonstrates two electrodes placed at the superior and inferior surface of the target lesion, pinching the lesion in between ( arrowheads ). The conferred ablation zone geometry is certain to encompass the tumor and provide an acceptable margin.
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Patient positioning on the CT table warrants attention.
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In general, the shortest distance from skin to the tumor is optimal.
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When equivocal, posterior approach with the patient in prone position is favorable to supine patient position, because posterior ribs are less subject to respiratory variation.
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In general, access should pass over the superior margin of the rib to minimize the probability of intercostal artery injury, should avoid traversing bullae or interlobar fissures, and should avoid injury of essential non-target structures.
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Care should be taken when positioning the upper extremities to avoid injury to the brachial plexus.
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Off-center position of the patient on the CT table may be required to provide sufficient clearance between the chest wall and the inner surface of the CT gantry in order to accommodate the RF electrode. The CT table should be as low as possible to maximize gantry clearance.
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Dispersive grounding pads with large surface areas must be applied to the patient’s body, preferably perpendicular to the RF electrode path and not located over bony prominences. The grounding pads should be in good contact with the skin, and if necessary, the skin must be shaved. Grounding pads are usually placed over thighs. When more than one electrode is used, more than two grounding pads and usually at least two pads can be placed to prevent cutaneous pad burns.
Intraprocedural Technique
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The Initial CT is performed, and the site of entry is marked using radiopaque grid or radiopaque marker like a lead ball.
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Once the skin entry site is confirmed, it is prepared with antiseptic solution and draped with sterile towels.
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After cutaneous anesthesia is achieved, a coaxial or Chiba needle is directed to the pleural surface, where liberal local anesthetic is deposited to achieve pleural anesthesia.
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Several techniques have been used to successfully place the RF electrode into the targeted tumor:
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One option initially places a 21- or 22-gauge, 10- to 15-cm length localizing needle into the tumor. The thinner localizing needle is believed to be associated with a lower risk of pneumothorax and hemorrhage than the larger bore electrode should multiple passes be necessary to obtain optimal device position. Once the small-gauge localizing needle is situated satisfactorily within the tumor, it can be used as a guide for advancement of RF electrode. Because the localizing needle is not electrically insulated, it must be removed before switching on the RF current.
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Alternatively, some operators advance a coaxial cannula or RF electrode directly, with or without CT fluoroscopy guidance.
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Electrode selection depends on operator preference. Three electrodes have been most widely used, each electrode unique in its design and ablation zone geometry
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The LeVeen electrode (Boston Scientific; Watertown, MA) employs tines within the shaft of the electrode. Once in position, the tines are deployed laterally, then arching backward, forming an umbrella-like array ( Fig. 27-2A ). The LeVeen electrode is optimally deployed when its tines arise from the center of the target tumor and extend immediately beyond the peripheral edges of the tumor. The resultant ablation zone appears as a horizontal oval.
Figure 27-2
The most commonly used radiofrequency ablation electrodes. A, The LeVeen electrode (Boston Scientific; Watertown, MA) employs an expandable multiarray, in which all tines are initially contained within the electrode shaft. Once the tip of the shaft is in position, the tines are deployed, arcing horizontally and backward, forming its umbrella-like array. B, The Starburst XL electrode (Angiodynamics; Queensbury, NY) also employs an expandable multiarray, in which its tines course forward from the shaft tip. Once deployed, the completed array is reminiscent of the stems in a bouquet of flowers. C, The internally cooled single-needle electrode or triple-cluster electrode (Covidien; Boulder, CO) uses an active tip of varying length that determines the size of the ablation zone.
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The tines of the Starburst and Side-Deployment (SDE and Talon) type electrodes (Angiodynamics; Queensbury, NY) course forward from either the tip (Starburst) or the side of the electrode (SDE) and laterally, similar to the stems in a bouquet of flowers (see Fig. 27-2B ). With this design, the tines are deployed from the electrode shaft at the near aspect of the tumor and then advanced until they reach the far aspect of the tumor. The ablation zone achieved is a vertical oval, oriented to the shaft of the electrode.
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The straight internally cooled single-electrode or triple-cluster electrodes (Covidien; Boulder, CO) are placed such that the active exposed tip is situated in the deep portion of the tumor and extends 0.5 to 1.0 cm into nontumorous lung (see Fig. 27-2C ). The resultant ablation zone is a vertical oval.
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Once the electrode or electrodes are in position, RF ablation is started. The goal is to achieve homogenous coagulation necrosis of the entire tumor, as well as an adjacent 0.5- to 1.0-cm margin of nontumorous lung.
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End point parameters.
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Each device vendor has provided end point parameters that presume adequate ablation. Repeat ablations can be performed, if these endpoints are not achieved.
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The LeVeen system increases the electrical power (Watts) delivered in a stepwise fashion at prescribed intervals until the impedance (Ohms) rises precipitously, a phenomenon frequently termed roll-off.
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The angiodynamics generator provides full power at 250 W. When tissue temperature exceeds target temperature, usually 90 °C, as detected by thermocouples located in alternate tines, the tines are advanced until the desired array diameter is achieved. Following manufacturer recommendations, ablation is continued at target temperature until specified ablation end times are achieved.
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The Cool-tip system gradually increases electrical power until an impedance rise of 20 O over baseline is detected, at which time power delivery is reduced. The heating cycle is terminated after an ablation time of 12 minutes. The tissue temperature, as measured by an embedded thermocouple, should exceed 70 ° C immediately after power termination and remain above 60 ° C for at least 3 minutes.
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Irrespective of the electrode used, imaging end points can be helpful to determine adequate ablation, specifically the appearance of GGO on CT encompassing the treated tumor in its entirety, measuring at least 0.5 to 1.0 cm in width ( Fig. 27-3A and B ).
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