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Thursday, April 13, 2017

From the SIR Residents and Fellows Section

Teaching Topic: Determining efficacy of RF ablation for primary or metastatic lung cancer based on tumor characteristics and environment.

Qiuxia Yang, MD, Han Qi, MD, Rong Zhang, MD, Chao Wan, MD, Ze Song, MD, Liang Zhang, MD, and Weijun Fan, MD. Risk Factors for Local Progression after Percutaneous Radiofrequency Ablation of Lung Tumors: Evaluation Based on a Review of 147 Tumors. 2017. 28 (4): 481-9.

Click here for abstract

Tumor-related factors that influence local tumor progression (LTP) include, size, site, orientation, organ, histology, and biology. The decision to treat primary or metastatic lung cancer with surgery or minimally invasive RF ablation is generally based on size. Larger tumors > 3 cm are considered to be more amenable to surgery. For smaller tumors or those close to 3 cm, the authors retrospectively reviewed factors influencing the complete ablative rate (CAR) and LTP. 147 tumors with an average maximum diameter of 1.8 cm +/- 1.2 receiving a single ablation and followed > 6 months were evaluated in 93 patients. 26% of patients had primary lung cancer and 74% had metastatic disease.

Clinical Pearls

What is the mechanism of action of radiofrequency ablation?

The Bovie knife is the first well-known medical device to incorporate RF ablation, which functions as cautery (pulsed current) and cutting (continuous current). Radiofrequency refers to the 3 Hz – 300 GHz range of the electromagnetic spectrum, which can cause thermal ablation of tissue. As with the Bovie knife, RF ablation devices operate in a closed electrical circuit, with the cathode (RF electrode or probe) transmitting energy through the patient’s tissue toward an anode (dispersing pads). Coagulative necrosis is achieved as dipole molecules (mostly water) next to the electrode remain aligned to the direction of the current and are forced to vibrate as rapidly as the applied current. Frictional energy released by adjacent molecules is deposited in surrounding tissue, resulting in local temperature increase. As the energy is dispersed by the pads, tissue damage is limited to the area surrounding the electrode tip. Application of too high of a generator power too quickly can cause tissue to desiccate or “char,” which acts as insulation and limits further extension of ablation.

What is meant by the “heat sink” effect?

Induced coagulation necrosis = (energy deposited x local tissue interactions) – heat loss. “Heat sink” is a cooling effect, which limits the effect of all thermal ablation methods. Flowing blood in vessels 3 mm or larger adjacent to a target lesion limit temperature variation in that area. This is potential cause for residual, unablated tissue and local tumor progression. Other studies have also postulated that tumors with proximal vascular or bronchial extension might lead to larger microscopic extension beyond the edge of the tumor. In this study, contact between tumor and blood vessels was determined as an independent risk factor for incomplete ablation in this study, and the CAR was significantly reduced.

What is considered effective tumor ablation?

A slow method of energy deposition, as to avoid desiccation with quick temperature rises, is used to heat tissue to 50-100 degrees Celsius for 4-6 minutes. Data extrapolation from surgery has established a general guideline to achieve an ablation margin 0.5 – 1.0 cm of ablated normal tissue, which is thought to account for microscopic tumor extension beyond the visualized confines of the lesion. The authors determined a complete ablative margin in this study to correspond to ground-glass opacities completely encircling the ablated lesion. In practice, it is difficult to distinguish inner necrosis areas from outer peripheral hemorrhage and inflammatory reaction on CT images and as a result, the shortest distance of overall ablative margin was measured and recorded in this study. Tumors can be monitored for local progression by determining contrast enhancement at the ablation margins on follow-up CT studies, as was done in this retrospective review. In the first three months following ablation, an enhancement zone that is peripheral, concentric, symmetric, and uniform and has smooth inner margins can be considered to correspond to reactive hyperemia, inflammation, or granulation at the marginal parenchyma. For tumors that are PET-avid prior to ablation, follow up PET studies may also be used. 

Questions to Consider

Which factors related to the efficacy of complete tumor ablation in this study?

The authors retrospectively investigated variables of tumor related factors before ablation including, tumor type, tumor size, morphology (smooth margins, lobulated and/or spiculated), and contact with blood vessels. For tumors < 3 cm, the CAR was 68.55%. For tumors > 3 cm, the CAR was 17.39%. CAR of tumors with a smooth margin was significantly greater than CAR of tumors with lobulated and/or spiculated edges. CAR of tumors with no surrounding blood vessels was 75%, which was remarkably higher than CAR of tumors with blood vessel contact. In tumors with complete ablative margin, the CAR was 74.77%, whereas it was 16.67% for tumors with incomplete ablative margin. Further, in tumors with complete ablative margins, completely ablated tumors had significantly larger ablative margin than incompletely ablated tumors (5.04 mm +/- 2.29 vs 3.71 mm +/- 2.51). Multivariate analysis showed that incomplete ablative margin and an ablative margin of 1–4 mm were independent risk factors for incomplete RF ablation of lung tumor.

Which factors related to local tumor progression (LTP)?

LTP after RF ablation is not rare, and progression at the RF ablation site is associated with poor overall survival in published reports. In this study, the LTP rate was 52% for primary lung cancers and 36.9% for lung metastases. This was higher than rates reported in the previous studies mentioned in the article, which were calculated for a period of several years and included the condition that patients received repeated lung tumor RF ablations. Of the 58 tumors with incomplete ablation, 56 tumors developed local progression at the edge of ablated lesions. 52 tumors locally recurred at the site of incomplete or shortest ablative margin. The site of shortest ablative margin was usually located at the tumor edge of contact with the blood vessels, in front of the ablation electrode, with the maximum radius perpendicular to the ablation electrode, and with protrusion of lobulated and/ or spiculated area of tumors.

Additional Citations:
Hong K, Georgiades C. Radiofrequency Ablation: Mechanism of Action and Devices. J Vasc Interv Radiol. 2010. 21(8): S179-S186

Post Author:
Rajat Chand, MD
Diagnostic Radiology Resident, PGY-2
John H. Stroger Hospital of Cook County

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