Principle
The use of microwaves for medical applications is based on the same principle as using microwaves to heat up food. Microwave ablation (MWA) was first applied to the treatment of liver cancer 30 years ago in Japan. MWA is well-suited to endoscopic or surgical hemostasis because of its rapid heating and high-temperature capability, which can lead to local hemostasis in a short time. Nonetheless, once the region surrounding the probe has been treated, the energy remains within the treatment area, which is usually under one to two centimeters in diameter. This method was later improved through the introduction of water-cooling and multiple antennas. The area affected by MWA is larger than that of RFA and it can reach temperatures exceeding 100℃. Thus, even low-temperature regions of the body, such as adjacent blood vessels, have little effect on MWA. This method makes it possible to more effectively destroy tumors. In China, MWA is nearly as popular as RFA for the treatment of tumors.

MWA is similar to RFA in that it involves the placement of the applicator tip directly within the tumor tissue; however, the MWA applicator tip is generally thicker. This method uses high-frequency electromagnetic waves (910MHz – 10GHz), usually of either 915MHz or 2.45GHz. Polar molecules predominantly comprised of water are induced to vibrate within the tissue at 20 to 50 million times per second. The vibration of the molecules increases the temperature inside the cells, thereby causing partial coagulation necrosis. As with RFA, the new generation of MWA systems enables the real-time monitoring of temperature at the applicator tip, which reduces the risk of overheating and makes it possible to adjust the output energy according to the size of the ablation zone.
 

Benefits of MWA
MWA provides the same advantages as RFA as well as higher energy levels to deal with tumors of large volume. Many experts believe that MWA is able to destroy tumors more effectively.

Tumors in the lung, kidney, and low-temperature regions are a prime candidate for MWA. In this process, the microwave antenna itself can complete the current loop, such that MWA does not require an electrode patch. This helps to further reduce costs and the potential risk of scalding.
 

Limits of MWA
The high temperature of MWA represents a double-edged sword: MWA is highly efficient in killing tumor tissue but risks causing damage to normal tissue. Without precise monitoring, MWA can produce excessively large ablation areas and lead to complications, such as peripheral vascular injury, to which RFA treatment is largely immune. Furthermore, MWA equipment costs at least double that of RFA, and health insurance in Taiwan does not cover this type of treatment.

MWA is generally not recommended for patients who are pregnant or for those with implanted electronic devices. Massive ascites and cholangio-intestinal anastomosis have also been shown to increase the risk of complications, thereby precluding the use of MWA.
 

When to use

1. MWA is commonly used to treat tumors in the liver, kidney, and lungs. In limited cases, experts have also used RFA to treat tumors in the thyroid and breast.
2. MWA methods and the tumors to which this treatment is applied are similar to those of RFA. This treatment can be performed percutaneously or intraoperatively, using ultrasound or computed tomography for guidance. Microwave applicator tips have been developed for surgical applications (non-percutaneously); however, they tend to result in a larger ablation zone.
3. MWA has proven highly effective in the treatment of liver tumors exceeding four centimeters in diameter.
4. MWA treatment is dangerous for patients with poor coagulation and compromised liver function; therefore, this treatment should only be applied after careful consideration.  
    

Preoperative considerations of MWA

1. Regular testing of blood, liver and kidney function, and coagulation.
2. Patients who receive the CT-guided procedure may need to undergo transcatheter arterial embolization to facilitate positioning. It is not required for patients who receive the ultrasound-guided procedure.
3. Consent forms.
4. Patients receiving local anesthesia do not have to fast, but they must not eat excessively. Patients undergoing systemic anesthesia must observe fasting for no less than six hours.
    

Intraoperative considerations of MWA

1. Use of local anesthesia, intravenous anesthesia, or general anesthesia as required.
2. Use of ultrasound-guided or CT-guided procedures as needed for treatment.
3. Selection of percutaneous treatment, endoscopic therapy, or conventional surgical treatment.
4. Intraoperative conscious patients in any discomfort should contact healthcare providers.
    

Postoperative considerations of MWA

1. Patients that undergo percutaneous treatment should lie down or lie on one side for at least six hours to facilitate hemostasis. This is not necessary for patients that undergo endoscopic or conventional surgery.
2. Patients that undergo percutaneous treatment may begin drinking or eating six hours after treatment. They must continue fasting if vomiting ensues and/or they feel discomfort. Patients that undergo endoscopic or conventional surgery should follow the advice of their physician with regard to the intake of food and liquids.
3. It is normal for a portion of the wound to be painful and/or exhibit signs of bleeding. If the pain becomes too severe, a physician or nurse should be contacted for advice. If the skin around the pinhole were subjected to thermal injury, appropriate treatment should be applied accordance with the severity of the burns.
4. Routine tests of blood and inflammation should be reviewed the following day to check for complications.
5. The gauze covering the percutaneous wound may be removed after 24 hours.
6. Ultrasound is unable to distinguish changes in the tumor after treatment. Thus, computerized tomography or magnetic resonance imaging must be used to track tumor progress. This is generally conducted at the first examination one month after treatment to ensure that any viable tumors are dealt with in a timely manner. Further regular examinations are then conducted every three to four months.
    

Risks and Complications
MWA can induce complications such as intra-bleeding as well as burning in the tissue surrounding the ablation zone, the burning of skin around the antenna insertion site or wire contact site, damage to blood cells, and blood clots.