CT-guided thermal ablation of lung tumors

General description of procedure, equipment, technique

CT-guided thermal ablation of lung tumors has emerged during the past ten years as a method to be considered in the local treatment of lung cancer and in management of patients with limited pulmonary metastatic disease.

Three techniques are currently available for thermal ablation in the chest, each of which is based on percutaneous placement of needles into lung tumors: radiofrequency ablation (RFA), cryoablation, and microwave ablation. Technique-specific equipment and needle probes are required for each. Tumor destruction is based on either heating of the mass to achieve coagulation necrosis (RFA and microwave ablation) or freezing and thawing to produce ice crystal formation and cell death (cryoablation).

Indications and patient selection

Thermal ablation of lung tumors is utilized primarily in two patient populations: patients with Stage I lung cancer who are not candidates for tumor resection (Figure 1A) (Figure 1B) (Figure 1C) (Figure 1D) and patients with a limited number of pulmonary metastases (typically, fewer than four) in whom the primary tumor is locally controlled and who have no evidence of other distant, metastatic disease (Figure 2A) (Figure 2B).

Figure 1A.

Figure 1B.

Figure 1C.

Figure 1D.

Figure 2A.

Figure 2B.

Colorectal cancer, melanoma, germ cell neoplasms, and breast cancer are the most common primary tumors for which treatment of limited, pulmonary metastatic disease may provide a survival benefit. Occasionally, palliative thermal ablation is used to treat symptomatic malignancy in the chest. An example is a patient with localized chest wall invasion in whom thermal ablation may relieve pain and improve quality of life.

Contraindications

Patients who cannot tolerate CT-guided needle placement for thermal ablation of lung tumors are not candidates for this therapy. Examples of such patients include patients who have significant bleeding diatheses, patients who are unable to maintain a recumbent position for the procedure, and patients who are unable to hold their breath or cooperate (e.g., because of cognitive difficulties) for safe needle placement and ablation.

In addition, specific characteristics of the lesion may preclude successful ablation: Tumors larger than 3cm in diameter are associated with high rates of local recurrence. Tumors adjacent to the trachea, large central bronchi, esophagus, heart, or mediastinal vessels are associated with significant risk of injury to these structures. In addition, large vessels produce a “heat sink” effect that is due to rapidly flowing blood, resulting in decreased effectiveness of thermal ablation in the portion of the tumor adjacent to the vessel.

Chest wall or juxtapleural tumors may be best managed using cryoblation, as RFA or microwave ablation of peripheral lesions may induce chest pain.

Details of how the procedure is performed

All thermal ablation techniques of the lung involve CT-guided placement of probes into the tumor to be treated in a manner similar to that employed for CT-guided, percutaneous lung biopsy (Figure 1B) (Figure 1C) (Figure 2B). Conscious sedation or general anesthesia may be used, and the patient is monitored throughout the procedure.

A generator unit delivers treatment via the CT-placed needle into the tumor and surrounding tissue. With RFA the generator produces an alternating electrical current, with cryoablation the administration of argon or nitrogen gas is employed, and with microwave-based techniques energy deposition into tissues occurs via a microwave antenna incorporated as part of the needle.

Ablation times are typically ten to thirty minutes, depending on the modality used, the size and number of tumors treated, and the number of probes or needles employed.

Interpretation of results

The technical adequacy of the treatment can be assessed either during the procedure using CT scans or proper intra-lesional needle placement, or on immediate post-procedure scans.

A halo of ground glass opacity surrounding the treated lesion and adjacent lung noted on a CT scan performed immediately following RFA or microwave ablation suggests successful treatment (Figure 1D). A visible decrease in attenuation noted on CT images obtained during cryoablation reflects formation of an ice ball encompassing the tumor.

Some operators obtain immediate, post-ablation, contrast-enhanced CT images to look for areas of persistent tumor enhancement that suggest incomplete tumor ablation. Additional treatment may then be directed toward these regions.

Performance characteristics of the procedure (applies only to diagnostic procedures)

N/A

Outcomes (applies only to therapeutic procedures)

Limited data are available on patient outcomes from the use of thermal ablation of lung tumors. Most published studies have focused on technical success and short-term complications. However, several case series suggest that thermal ablation can be successful in achieving local tumor control and improved survival in patients with Stage I non-small-cell lung cancer and in those with limited pulmonary metastases from non-pulmonary malignancies.

For lesions less than 3cm in diameter, local tumor progression-free rates for RFA approach 50 percent at three years. In one of the largest series evaluating RFA for patients with Stage I lung cancer, one-, three-, and five- year survival rates were 78 percent, 36 percent, and 27 percent, respectively. In selected RFA-treated patients with lung metastases, five-year survival rates of 45-50 percent have been demonstrated. Estimated recurrence rates for lung tumors treated with microwave ablation is 44% at 3 years.

The experience with cryotherapy is not as extensive as that with RFA, but the technique appears to be similarly effective in achieving local tumor control and in affording improved survival benefits.

Alternative and/or additional procedures to consider

In a patient with small (T1a) non-small-cell lung cancer who is not a surgical candidate, stereotactic body radiotherapy should be considered as an alternative treatment. The technique involves application of one to five high-fraction radiation doses to the tumor. Preliminary results suggest similar local control rates and survival benefits compared to those with thermal ablation.

Patients who develop local recurrence of treated lesions or who develop additional limited sites of metastatic lung disease following initial treatment may be considered for repeat treatment with thermal ablation or stereotactic radiotherapy.

Complications and their management

The most common complications of the procedure are the same as those of image-guided transthoracic needle biopsy of the lung: pneumothorax, bleeding, and pain, among others.

Pneumothorax occurs in approximately 30 percent of procedures. Large (i.e., > 30%), enlarging on repeat imaging, and symptomatic pneumothoraces are treated with small-bore catheter insertion using image-guidance; chest tube insertion rates approximate 10 percent. Small, asymptomatic pneumothorax can be generally observed while the patient is administered 100% oxygen for nitrogen washout.

Hemoptysis occurs in about 10 percent of procedures. Hemorrhage is more commonly seen following cryoablation of lung tumors and is usually managed conservatively.

Pain most often occurs with treatment of peripheral lesions that result in tissue infarction and pleurisy. Management commonly employs narcotic analgesics and/or non-steroidal anti-inflammatories.

Pulmonary infection is most typically seen in patients with underlying COPD; it occurs in approximately 30 percent of treated patients. Oral antibiotics may be useful in patients who have signs and symptoms of infection following ablation.

Rarely, damage to adjacent structures, including large airways or the esophagus, has been reported. Care must be taken when treating lesions in these areas.

Self-limited, small pleural effusions, likely resulting from pleural irritation or peripheral infarction of subpleural tumors, are common.

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