Microwave ablation (MWA)
Microwave ablation (MWA) is a minimally invasive medical procedure used to treat certain types of tumors. During MWA, a specialized electrode is inserted into the tumor, guided by imaging techniques such as ultrasound or CT scans. MWA offers several benefits, including reduced recovery time, less pain, and a lower risk of complications compared to traditional surgery.
Mechanism of Action:
Thermal Ablation (Hyperthermic) – Microwave heating is produced as a result of dielectric hysteresis (rotating dipoles).
- Cellular Homeostatic Mechanism – upto 40ºC
- Cytotoxic temperature is needed (> 50 ºC, between 50-100)
- Increased susceptibility to chemical and radiation injury – 42-45 ºC
- Irreversible cellular damage – 46 ºC for 60 mins, 50 ºC for 5 min ( temp > 50 – surrogate marker)
- Tissue vaporisation and gas formation – 105 ºC
Coagulation of cytosolic and mitochondrial enzyme with formation of nucleic acid – histone protein complex leads to coagulation necrosis (used to describe this thermal damage even though the manifestations of cell death following high-temperature thermal ablation may not fulfill strict histopathologic criteria of coagulative necrosis).
Tissue Characterstics:
| Tissue Characteristics | Features |
|---|---|
| Tissue Perfusion | 1. Heat-Sink Effect / Perfusion mediated cooling 2. Foremost factor limiting thermal ablation = Blood flow carries thermal energy away from the targeted tissue. 3. Most veins greater than 3 mm remain patent after RF ablation resulting in less endothelial injury and irregular ablation zones 4. Effect reduced after arterial embolization methods like coils, particles, balloons, or lipiodol agents |
| Thermal Conductivity | 1. Oven Effect 2. Poor Thermal Conductivity or Increased heating efficacy for tumors surrounded by cirrhotic liver/ fat. 3. Increased Thermal Conductivity eg. cystic lesion = fast heat transmission and dissipation = more time required to reach cytotoxic temperature = potentially incomplete or heterogeneous tumor heating |
| Electrical Conductivity (Radiofrequency ablation) | 1. Ionic agent like normal saline increase conductivity = NS injection in and around ablation site results in better local heating effect 2. Non-Ionic agent hamper the electrical conductivity = Dextrose 5% used for hydrodissection and protection of adjacent organ 3. Tumor-organ interface with marked difference in electric conductivity (like lung, bone) = limited heating of adjacent organ = difficult to obtain 1 cm ablative margin |
| Dielectric Properties (Microwave Ablation) | 1. Relative Permittivity - charge storing capacity of material 2. Bulk Conductivity - Energy loss inside material 3. Dielectric properties of tissue correlate well with their water contents, therefore tissue conductivity increases with increasing water content. |
Generation of MWA equipment:
- First-Generation systems – lack active antenna cooling, limited to low power and short durations.
- Second-Generation systems have antenna cooling but limited generator power.
- Third-Generation systems incorporate antenna cooling and high-power generators.
Technical Advantage of MWA over RFA:
- Faster Ablation
- Higher temperature without the limitations related to electrical impedance
- Less sensitivity towards tissue type
- More consistent results
- Beneficial in treating some tumors resistant to treatment with RFA (sarcoma and hemangiopericytoma)
- Relative insensitivity to heat sink
Contraindications:
Absolute contraindications include
- Patient refusal
- Increased intracranial pressure, and
- Local infection.
Relative contraindications include
- Deranged coagulation or patient on anticoagulation
Complications:
- Bleeding – Entry site, Access route, target area
- Infection
- Tumor Seeding – Tract ablation is recommended.
- Needle placement-induced nerve damage
- Post-Ablation Syndrome (transient) – Flu-like symptoms and generally low-grade fever.
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