Pulmonary Tumors
Bradley B. Pua
Stephen B. Solomon
Thermal ablation of pulmonary tumors is an option for local tumor control for unresectable tumors. Radiofrequency ablation (RFA), first described in the lungs in 2000, has since been proven to be both feasible and efficacious in the treatment of lung cancers (1). Ablative techniques have been used to treat primary lung cancers, metastatic disease to the lungs, and in palliation for painful chest wall masses. Thermal ablative techniques include heat-based modalities such as RFA and microwave ablation (MWA) in addition to cold-based techniques such as cryoablation.
RFA utilizes frictional energy imparted by oscillating ions within tissue to heat and treat tumors. Cells undergo coagulation necrosis when heated to more than 54°C for more than 1 second. Therapeutic RFA strives to bring tissue temperatures to the range of 60°C to 100°C. Because this technology relies on tissue and electrical conduction, RFA can be difficult to apply in lung where air is a poor conductor.
The mechanism of cell death caused by MWA is similar to RFA. With microwave, a continuously oscillating microwave field targets polar molecules (predominately water), which align with this field resulting in increased kinetic energy and tissue temperatures. This does not require electrical conduction and therefore allows microwave energy to penetrate more effectively allowing for generation of a larger volume of heat surrounding each applicator.
Cryoablation causes cell death through cell membrane disruption from both intra- and extracellular ice crystal formation and subsequent release of intracellular material. Although cellular necrosis is related to tissue type, temperatures of — 35°C to —20°C are usually required. Current cryoablation systems utilize the Joule-Thomson effect to induce these extreme temperatures. Gases (typically argon) are delivered at high pressures through an internal feed line into an expansion chamber in the cryoprobe; as the gas expands, a heat sink is created cooling the probe and surrounding tissues. As thermal conductivity is secondary to passive conduction, the probes’ cooling capacity is related to the size of the cryoprobe.
Lung Cancer
Primary lung cancer is the leading cause of cancer death in the United States. Non-small cell lung cancer (NSCLC) represents 85% of these cancers, whereas small cell represents approximately 15%. Small cell lung cancers are generally more aggressive, and patients with this subtype generally present with extensive lymph node involvement and metastatic disease. Currently, this characteristic limits ablation of this tumor subtype to salvage therapy, with the mainstay of therapy being systemic chemotherapy and radiotherapy. Patients with NSCLC, on the other hand, present much earlier, allowing these patients to be treated with local therapies such as surgery, stereotactic body radiotherapy, and ablation. Currently, surgical therapy (lobar resection) is considered as first line for treatment of early-stage lung cancer with ablation reserved for patients who are not surgical candidates. Additional indications for treatment of NSCLC may include (a) salvage (poor or no response to chemotherapy, radiation, or surgery) or (b) a single growing focus of tumor in a patient with otherwise stable metastatic disease.
Treatment of metastasis in the lungs is still a much-debated topic. Treatment of limited metastatic disease with surgical metastasectomy has been validated retrospectively with improved actuarial survival seen if complete surgical excision (R0 resection) is achieved (2). Thermal ablation is an option for this subset of patients as well, and offers this patient population, who often may face additional future metastases, a lung function preserving treatment.
Indications
Biopsy-Proven Malignancy
1. Early-stage primary lung cancer
a. Medically inoperable/refusing surgery
(1) Limited pulmonary reserve (see “Preprocedure Preparation” section)
b. Recurrence: after surgery, radiation, or chemotherapy
2. Limited metastatic disease Widely metastatic disease responding to systemic chemotherapy with a single focus that is not responding
3. Salvage therapy
a. Prior radiation: local recurrence in postradiation bed
b. Prior surgery: local recurrence in postsurgical bed
c. Symptom palliation: usually pain
Contraindications
Absolute
1. Uncorrected coagulopathy
a. Uncorrected laboratory abnormality
(1) INR >1.5
(2) Platelets less than 50,000 per µL
b. Uncorrected bleeding diathesis
2. Bacteremia or active infection
a. Post-ablated tissue can serve as a nidus for infection, which may result in abscess formation.
Relative
1. Tumors adjacent to vital organs
a. Mediastinum, large blood vessels (aorta, main pulmonary artery), esophagus, and chest wall
b. This is considered a relative contraindication; separation techniques (as described later) can be used to help create a buffer zone between tumor and neighboring vital structures.
2. Proximity to vessels larger than 3 mm
a. Although not a true contraindication, studies have suggested that proximity to vessels of this size may cause a significant enough “heat sink” effect resulting in higher rate of local tumor progression (3).
Preprocedure Preparation
1. History and physical examination with attention to
a. History of bleeding diathesis
b. Concurrent cardiopulmonary compromise, which may affect choice of sedation.
(1) Compromised pulmonary function may contraindicate surgical resection.
(a) Pulmonary function tests may be transiently affected after ablation.
(2) Ablation can be performed in the contralateral lung in a patient with prior pneumonectomy (4).
c. Pacemakers and metallic implants
(1) Although RFA and MWA in patients with pacemakers/defibrillators has been reported, it is still recommended that these devices be deactivated during ablation of pulmonary tumors (5). Deactivation with a magnet over the pacemaker or defibrillator will remove sensing activity temporarily until the magnet is removed. Cryoablation may be utilized in these instances.
(2) Metal implants, when small, can heat up due to the circuit created from the RFA probe to the grounding pad.
2. Team approach
a. Treatment of patients with primary and metastatic tumors should ideally be performed after discussion with the patient’s multidisciplinary team.
(1) Ideally, patients should be presented and discussed in a multidisciplinary setting, such as tumor board.
3. Preprocedure biopsy
a. Lesions to be treated should generally be biopsy-proven malignancy.
(1) Some prefer biopsies be performed on a separate occasion because
(a) Biopsy findings may alter management.
(b) Potential of hemorrhage during biopsy may obscure lesion to be treated, thus decreasing effectiveness. A short interval between biopsy (˜1 week) and ablation will allow these postbiopsy changes to resolve.
4. Preprocedure imaging and choice of guidance
a. Preprocedure computed tomography (CT) and/or positron emission tomography (PET)/CT should be performed to assess stage, trajectory planning, and serve as baseline for follow-up.
b. Guidance
(2) Ultrasound (US): helpful in treatment of peripheral lung lesions or chest wall masses
