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An ionization chamber measures the charge from the number of ion pairs created within a gas caused by the incident radiation. These ion pairs can be created by both incident photons and secondary electrons. This collection volume should be sufficiently small to ensure electronic equilibrium with the surrounding medium but large enough to ensure a stable electric field within the volume. Many ionization chambers have been used for Gamma Knife, and in our clinical environment we have two ionization chambers, the Exradin A16 [12] (Standard Imaging, Middleton, WI, USA) and the PR05 from Capintec Inc. (Florham Park, NJ, USA). To measure the absolute dose or dose rate, the chamber needs to have a valid American Dosimetry Calibration Laboratory (ADCL) calibration coefficient. The ionization chamber is placed in the ABS or solid water phantom [13] which is aligned to the RFP and irradiated with the 16-mm collimator from all the sectors. Using the American Association of Physicists in Medicine (AAPM) Task Group 21 protocol [14], certain characteristic properties of the chamber are used to determine the total correction factor that is applied with the exposure calibration from the ADCL. At installation, the dose rate should be greater than 300 cGy/min.