What Is Microwave Ablation?

Microwave ablation is a minimally invasive thermal therapy that destroys a targeted volume of tissue, usually a tumor, by driving an electromagnetic field from a needle-like antenna inserted into the target. The field oscillates polar water molecules in the surrounding tissue, and the resulting dielectric loss deposits heat directly in a volume around the antenna, raising temperature past the threshold for coagulative necrosis. Clinical systems operate in the industrial, scientific, and medical bands at 915 megahertz or 2.45 gigahertz, with some devices at 5.8 gigahertz. The technique belongs to the family of image-guided thermal ablation methods alongside radiofrequency ablation, cryoablation, and high intensity focused ultrasound.

The physical distinction from radiofrequency ablation matters clinically. Radiofrequency ablation relies on resistive current flow through tissue, so it depends on electrical conductivity and stalls once tissue desiccates and impedance rises. Microwave energy propagates as a radiated field and does not require a closed conductive circuit, which lets it continue heating through charred or desiccated tissue, reach higher peak temperatures, and treat low-conductivity targets such as lung. A detailed treatment of the underlying physics and hardware appears in a review of microwave tissue ablation biophysics, technology, and applications.

Antenna and System Design

The applicator is a thin coaxial antenna, typically between 1.5 and 2.5 millimeters in diameter for percutaneous use, since larger diameters raise the risk of bleeding and pneumothorax. Common radiating structures include the monopole, the dipole, and slot and choke designs that confine the radiating section and suppress currents traveling back along the cable, which would otherwise cause comet-tail heating along the insertion track. Impedance matching to tissue is difficult because permittivity and conductivity change sharply as tissue heats and water evaporates, so antennas are designed to keep reflected power tolerable across that swing. Many applicators are actively cooled with circulating saline or gas so that the shaft stays cool while high power, often between 60 and 150 watts, is delivered at the tip. Applicator behavior is characterized by measuring the resulting zone in tissue, as in the experimental evaluation of a directional 2.45 gigahertz applicator in ex vivo and in vivo liver. Multiple antennas can also be driven together, either in phase to create constructive field overlap or sequentially, to cover targets larger than a single applicator can treat.

Treatment Planning and Dosimetry

Predicting the ablation zone requires coupling electromagnetic field solutions with a bioheat model, most often the Pennes equation, which adds a perfusion term representing heat carried away by blood flow. Perfusion and large adjacent vessels produce a heat sink effect that can leave viable tumor at the margin, a principal cause of local recurrence. Planning software uses tissue-specific dielectric and thermal properties, many of them temperature dependent, to estimate the volume reaching a cytotoxic thermal dose. Because at 915 megahertz and 2.45 gigahertz field penetration in soft tissue is only a few centimeters, ablation zone size is set largely by conduction outward from the directly heated core and by treatment time. Procedures are guided by ultrasound, computed tomography, or magnetic resonance imaging, and reviews of mechanism of action, clinical results, and devices compare reported zone geometry across commercial systems.

Clinical Use

Hepatic tumors are the most established indication, covering both hepatocellular carcinoma and colorectal metastases, where ablation offers an option for patients who cannot tolerate resection. Outcome data for microwave ablation of hepatocellular carcinoma describe local control rates and complication profiles for percutaneous, laparoscopic, and open approaches. Reported complications include thermal injury to adjacent bowel or bile ducts, pneumothorax in thoracic procedures, and post-ablation syndrome.

Applications

Microwave ablation has applications in a range of clinical areas, including:

• Liver tumor treatment, including primary and metastatic disease
• Lung nodule ablation, where low tissue conductivity favors microwave over radiofrequency energy
• Renal and adrenal tumor therapy
• Benign thyroid nodule and breast lesion treatment
• Bone metastasis palliation and osteoid osteoma treatment
• Cardiac arrhythmia and varicose vein procedures using related microwave applicators