**1.3. Ablative devices**

84 Hyperthermia

microwave ablation (MWA) in tumors [1].

reduce pain and bleeding [3].

*1.2.1. Radiofrequency ablation* 

death occurs [5].

*1.2.2. Microwave ablation* 

**1.2. Principles of tissue damage** 

to destroy an entire tumor by using heat to kill the malignant cells in a minimally invasive fashion together with a sufficient margin of healthy tissue, to prevent local recurrence. Many ablation modalities have been used, including cryoablation, ethanol ablation, laser ablation, and radiofrequency ablation (RFA). The most recent development has been the use of

Nevertheless the local application of heat to treat patients with malignant tumors is not a novel concept. The Edwin Smith papyrus describes the topical application of hot oil or heated metallic implements that were used approximately 5000 years ago to treat patients with tumors [2]. The use of an electrical current to produce thermal tissue necrosis in patients with breast carcinoma also is not new: Metallic or clay-insulated electrodes were inserted into locally advanced breast tumors in the late 19th century to shrink the tumor and

This therapy works by converting radiofrequency waves into heat through ionic vibration. Alternating current passing from an electrode into the surrounding tissue causes ions to vibrate in an attempt to follow the change in the direction of the rapidly alternating current. It is the ionic friction that generates the heat within the tissue and not the electrode itself. The higher the current, the more vigorous the motion of the ions and the higher the temperature reached over a certain time, eventually leading to coagulation necrosis and cell death. The ability to efficiently and predictably create an ablation is based on the energy balance between the heat conduction of localized radiofrequency energy and the heat convection from the circulation of blood, lymph, or extra and intracellular fluid [4]. The amount of radiofrequency produced heat is directly related to the current density dropping precipitously away from the electrodes, thus resulting in lower periphery temperatures. It can be approximated that the heat generated in a region at distance d from the electrode drops as 1/d4. The goal of radiofrequency ablation is to achieve local temperatures that are lethal to the targeted tissue. Generally, thermal damage to cells begins at 42°C; and once above 60°C, intracellular proteins are denatured, the lipid bilayer melts, and irreversible cell

Water molecules are polar, that is, the electric charges on the molecules are asymmetric. The alignment and the charges on the atoms are such that the hydrogen side of the molecule has a positive charge, and the oxygen side has a negative charge. When an oscillating electric charge from radiation interacts with a water molecule, it causes the molecule to flip. Microwave radiation is specially tuned to the natural frequency of water molecules to maximize this interaction. Temperature is a measure of how fast molecules move in a substance, and the vigorous movement of water molecules raises the temperature of water.
