What is Ion Chamber Radiotherapy and How Does it Work?

Ion chamber radiotherapy is a type of radiation detector used to detect and measure charge from the number of ion pairs created within a gas caused by incident radiation. It consists of a gas-filled chamber with two electrodes; an anode and a cathode, in which a voltage is applied to maintain an electric field. This electric field accelerates the ions produced by the radiation to be collected by the electrodes, which is then read by an electrometer and converted into absorbed dose. The ionization chamber is used in medical physics and radiation therapy to ensure that the dose delivered from a therapy unit or radiopharmaceutical is as intended.

The gas amplification curve describes the behavior of an ionization chamber as a function of the applied voltage. Open-air ionization chambers are the defining instrument of the Roentgen unit and, as such, are fundamentally linked to the absorbed dose. Multi-cavity ionization chambers can measure the intensity of the radiation beam in several different regions, providing information on the symmetry and flatness of the beam. When atoms or gas molecules between the electrodes are ionized by incident ionizing radiation, ion pairs are created and the resulting positive ions are created and the dissociated electrons move to the electrodes of opposite polarity under the influence of the electric field.

The response of an ionization chamber depends to a large extent on the voltage applied between the outer electrode and the center electrode. The studies focused on the statistical and physical performance of the ionization chamber dosimeter used in radiotherapy departments in hospitals around the world. Since its sensitivity is proportional to its volume, leakage has a relatively greater effect on small cameras, making open-air ionization chambers more suitable for clinical applications. The CC13 ionization chamber is designed for absolute and relative dosimetry of photon, electron and proton beams in radiation therapy.

It is important to note that due to its relatively low electric field strength, an ion chamber has no multiplication effect in its operating region. There are two basic configurations; integral units with cameras and electronics in one housing, and two-piece instruments with separate ion chamber probes attached to electronics modules by flexible cables.