The main quantity of interest in radiotherapy dosimetry is absorbed dose to water, i.e. the energy that is deposited by the radiotherapy beam in water per unit mass. The most common method to measure dose in radiotherapy is by using air-filled ionization chambers via the charge released in their active volume by ionizations. These ionization chambers are typically absolute calibrated in 60Co beams in terms of absorbed dose to water. If a measurement is carried out in another radiation quality (e.g. proton beams), the different response of the chamber in that radiation quality compared to 60Co photons due to a different water-to-air stopping power ratio, a different amount of electron-ion pairs produced in air per deposited energy and chamber-specific geometry effects is taken into account by applying a beam quality correction factor kQ. Because kQ might be sensitive to several factors, it is recommended that absolute absorbed dose to water measurements should be performed within so-called reference conditions (e.g. defined field size and water depth), and therefore such measurements are referred to as reference dosimetry
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. In addition to kQ, also several other corrections may be necessary (e.g., recombination or air density correction).