Thermal ablation for the treatment of primary and secondary pulmonary malignancies

Abstract

In the review article entitled "The Role of Percutaneous Image-Guided Thermal Ablation for the Treatment of Pulmonary Malignancies" (1) recently published in the American Journal of Roentgenology (AJR) by Mouli and colleagues, the authors reviewed thermal ablation techniques [radiofrequency ablation (RFA), microwave ablation (MWA), and cryoablation] for the treatment of pulmonary malignancies [non-small cell lung cancer (NSCLC) and metastasis] with respect to treatment mechanism, local efficacy, imaging modalities used for guidance, treatment response evaluation, and clinical outcomes. The authors also reviewed the comparative studies of thermal ablations with surgery and stereotactic beam radiotherapy (SBRT).Ablation is a good candidate for treating pulmonary malignancies because lungs have heat and electrical insulating characteristics, which enable larger volumes of tissue to be ablated using thermal energy than is possible for other body tissues (2). Because thermal energy delivery is limited by the heat-sink effect of adjacent blood vessels and airways, the presence of vessels or bronchi greater than 3 mm in diameter within the ablation zone are predictors of incomplete local treatment (3). Ablation zones must exceed tumor dimensions to obtain adequate margins, and in practice, sufficient RFA-induced ground-glass opacity (GGO) indicates complete ablation (4). MWA permit larger ablation zones than RFA because delivers energy simultaneous using multiple probes, and thus, provides larger tumor ablation volumes and faster ablation times. Cryoablation uses compressed argon gas to generate subzero temperatures, but unlike heat-based ablation, cryoablation does not create GGO; instead, it creates iceballs.CT is the preferred imaging modality for guidance during thermal ablation. It provides excellent contrast between tumors and normal lung parenchyma, and multi-planar CT images enable accurate and rapid probe placement. CT is also the modality of choice for postablation follow-up evaluations (Figure 1). Immediately after ablation therapy, targeted lesions are replaced by dense opacity surrounded by GGO (5). GGO margins of <3 mm have been associated with local treatment failure. During the early post-ablation period (<2 months), central dense opacity and surrounding GGO serves as the new "baseline", and any increase in lesion size during follow-up should be considered local progression. However, morphologic evolution occurs in ablation zones and its extent depends on the thermal ablation methods used, for example, RFAtreated lesions show a relatively slow rate of involution, with a 40% decrease in size at 15 months after treatment, whereas cryoablated lesions show more rapid involution on follow-up CT images (6). PET/CT may be helpful for differentiating morphologic evolution and local recurrence, but early PET/CT within 3 months of ablation can be confounded by inflammatory changes. Editorial