Residency · Residency · Interventional Radiology

Percutaneous Thermal Ablation: Radiofrequency and Microwave

Principles of Thermal Ablation

Percutaneous image-guided destruction of tumor tissue using heat. Goal: achieve coagulative necrosis of the tumor with an adequate ablation margin (typically 5-10 mm circumferential). Performed under CT, ultrasound, or MRI guidance. Minimally invasive alternative to surgical resection for selected liver and other solid organ tumors.

Radiofrequency Ablation (RFA)

Physics

Alternating electrical current (375-500 kHz) delivered through an electrode into the tissue. Ions in the tissue oscillate in response to the alternating current, generating frictional heat. Tissue heating is greatest immediately around the electrode; heat conducts outward. Cell death occurs at temperatures >60 degrees C (instantaneous coagulation necrosis). At >100 degrees C, tissue desiccation and charring occur, which INCREASE impedance and LIMIT further energy delivery.

Equipment

Single electrode: straight needle with an active tip (1-3 cm exposed tip). Expandable electrodes: multiple tines deploy from the needle tip to create a larger ablation zone (LeVeen, StarBurst). Cooled electrodes: internal saline cooling reduces charring at the electrode surface, allowing higher energy delivery and larger ablation zones (Cool-tip). Grounding pads: placed on the patient's thighs to complete the electrical circuit.

Ablation Zone

Typical maximum ablation zone: 3-5 cm diameter (limited by heat-sink effect and charring). Spherical or ovoid shape depending on electrode design. Heat-sink effect: blood flow in adjacent vessels (>3 mm) carries heat away from the ablation zone, reducing effectiveness near large vessels. This is the principal limitation of RFA.

Microwave Ablation (MWA)

Physics

Electromagnetic energy at microwave frequencies (915 MHz or 2.45 GHz) delivered through an antenna. Water molecules in the tissue rotate rapidly in response to the oscillating electromagnetic field, generating heat through dielectric hysteresis. Does NOT depend on tissue impedance or electrical conductivity. No grounding pads needed (antenna is the entire energy delivery system).

Advantages Over RFA

Higher intra-tumoral temperatures (>150 degrees C achievable). Faster ablation times (minutes vs. 12-20 minutes for RFA). Larger ablation zones (up to 5-6 cm with newer systems). Less susceptible to heat-sink effect (higher temperatures overcome convective heat loss). No charring limitation (does not rely on tissue impedance). Multiple antennas simultaneously: can create large, confluent ablation zones.

Equipment

Multiple commercial systems: Emprint (Medtronic), NeuWave (Ethicon/J&J), Solero (AngioDynamics), Certus 140 (NeuWave). Antenna designs: straight, triaxial, thermally insulated shaft to prevent skin burns. Available in various lengths and gauge sizes.

MWA vs. RFA: Current Status

MWA has largely replaced RFA in most academic centers for liver ablation. MWA advantages: faster, larger zones, less heat-sink. RFA advantages: longer track record, widely available, less expensive. For tumors <2 cm, both modalities are equivalent. For tumors 2-5 cm, MWA has a practical advantage (larger single-probe ablation zone).

Indications

Hepatocellular Carcinoma

BCLC Stage 0 (very early) and Stage A (early): ablation is a curative-intent treatment. SURF trial (2023): RFA/MWA vs. surgical resection for single HCC <3 cm -- ablation was non-inferior to resection for recurrence-free survival; paradigm-changing. Milan criteria HCC: ablation as bridge to transplant. Tumors <3 cm: ablation alone is potentially curative (complete necrosis rates >90%). Tumors 3-5 cm: ablation feasible but higher local recurrence rates; consider combination with TACE.

Colorectal Liver Metastases (CRLM)

Unresectable CRLM with limited number and size. EORTC/CLOCC trial: RFA + systemic chemotherapy improved overall survival vs. chemotherapy alone for unresectable CRLM. Used as an adjunct to surgery (ablate + resect strategy for bilobar disease). Best outcomes for CRLM <3 cm with adequate margins.

Other Liver Tumors

Cholangiocarcinoma (limited evidence). Liver metastases from breast, neuroendocrine, and other primaries (case-by-case).

Planning and Technique

Preprocedural Imaging

Contrast-enhanced MRI (preferred) or CT to characterize tumor size, location, relationship to vessels and bile ducts. Plan the needle trajectory to avoid critical structures. Identify potential complications (subcapsular tumor near bowel, diaphragm, gallbladder).

Guidance Modality

CT: most commonly used; excellent spatial resolution. Ultrasound: real-time guidance; used for visible tumors; limited by deep or small lesions. MRI: limited availability; useful for lesions visible only on MRI. Fusion imaging: combines pre-procedure MRI/CT with real-time ultrasound for targeting MRI-only-visible lesions.

Procedure

Position patient and plan trajectory. Place ablation probe(s) into the tumor under image guidance. Confirm probe position (CT or US). Deliver ablation energy per protocol (time and power settings vary by device). Ablation margin assessment: goal is 5-10 mm margin beyond the tumor in all directions. Track ablation (cauterize the needle track upon withdrawal to prevent tumor seeding). Post-procedure imaging: immediate CT to assess for complications (hemorrhage, pneumothorax).

Ablation Margin

The most critical factor predicting local recurrence. A0 margin (no margin): high recurrence risk. A1 margin (>5 mm): adequate. Emerging role of ablation confirmation software (e.g., Innoblative, CAS-One) to assess margins using CT fusion.

Complications

Hemorrhage: subcapsular hematoma, intraperitoneal hemorrhage (1-3%). Abscess/infection: higher risk with bilioenteric anastomosis (<1% overall). Thermal injury to adjacent structures: bile duct injury (stricture), bowel injury, diaphragmatic injury. Tumor seeding: along the needle track (rare, <1%); prevented by track ablation. Post-ablation syndrome: pain, low-grade fever (self-limited). Pneumothorax: for dome lesions with transpulmonary approach. Liver failure: rare; risk with large ablation volumes in cirrhotic liver.

Protective Techniques

Hydrodissection: injection of D5W (non-conductive) or normal saline between the ablation zone and adjacent structures (bowel, diaphragm, gallbladder) to create a thermal buffer. Pneumodissection: CO2 injection for thermal insulation (especially near the diaphragm). Balloon interposition: for colonic protection. Artificial ascites/pleural effusion: fluid instillation to separate the liver from adjacent organs.

<image>Comparison of radiofrequency ablation and microwave ablation physics. Two panels: Left panel shows RFA with an electrode in a liver tumor, alternating current flowing between the electrode and grounding pads on the thighs, with ions oscillating in the tissue generating frictional heat; a temperature gradient from the electrode outward, with charring zone shown at >100 degrees C near the electrode. Right panel shows MWA with an antenna generating an electromagnetic field, water molecules rotating in response, generating heat through dielectric hysteresis; higher and more uniform temperatures throughout the ablation zone. A comparison chart below shows maximum temperature, ablation time, heat-sink susceptibility, and typical ablation zone size for each modality.</image>

<image>Illustration of percutaneous liver tumor ablation with margin assessment. Three panels: (1) Pre-ablation contrast-enhanced CT showing a 2.5 cm enhancing HCC tumor in the right lobe with the planned ablation zone (dashed circle) extending 10 mm beyond the tumor margin in all directions; (2) Ablation probe positioned in the center of the tumor under CT guidance, with the probe tip and active zone labeled; (3) Post-ablation CT showing a non-enhancing ablation zone larger than the original tumor with adequate circumferential margin. An overlay of pre- and post-ablation images demonstrates the achieved margin using fusion software.</image>

<image>Illustration of hydrodissection technique for thermal protection during liver ablation. A coronal view showing an ablation probe in a subcapsular hepatic tumor adjacent to the hepatic flexure of the colon. A spinal needle is positioned between the liver capsule and the colon, with D5W fluid being injected to create a fluid buffer displacing the colon away from the ablation zone. The thermal ablation zone is shown NOT extending into the fluid-filled space. An inset cross-section shows the protective fluid layer separating the liver from the bowel.</image>

Key Clinical Pearls

MWA has largely replaced RFA as the preferred thermal ablation modality due to faster, larger, and more predictable ablation zones. The ablation margin is the single most important factor predicting local recurrence -- aim for at least 5-10 mm circumferential margin. The SURF trial showed ablation is non-inferior to resection for single HCC <3 cm -- this supports ablation as a curative-intent treatment. Heat-sink effect from large vessels (>3 mm) remains a challenge, especially for RFA; MWA partially overcomes this but does not eliminate it. Hydrodissection is an essential technique for protecting adjacent structures -- learn it early. Tumor seeding is rare but preventable with track ablation on probe withdrawal. Patients with bilioenteric anastomosis are at high risk for post-ablation abscess -- give prophylactic antibiotics.

References

• Vietti Violi N et al. Radiofrequency ablation versus surgical resection for small HCC: randomized controlled trial (SURF). Gastroenterology 2023
• Ruers T et al. Local treatment of unresectable colorectal liver metastases: results of a randomized phase II trial (EORTC/CLOCC). J Clin Oncol 2017
• Defined by the SIR Quality Improvement Guidelines for Percutaneous Ablation. J Vasc Interv Radiol 2019
• Defined by Defined by the EASL Clinical Practice Guidelines on HCC: Role of Ablation. J Hepatol 2018

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