Absorbed dose standards for radiation therapy : IAEA Consultants' meeting, Vienna, 25-28 November 1996

The dose delivered to a patient from a radiation therapy beam is usually measured with an ionisation chamber. Before the introduction of linear accelerators, the measurements were all taken in air, then converted to dose to water (representing the patient) using a suitable conversion factor. Accordingly, the ionisation chambers were calibrated in air against primary standards of 'exposure', a quantity based on the amount of ionisation generated in a known mass of air. With the introduction of higher energy beams, it became necessary to take the measurement directly in a water tank. More than holf of all the primary standards laboratories are developing or already offering instrument calibrations in terms of absorbed dose to water. There is currently no internationally accepted code of practice for dosimetry based on absorbed dose. Accordingly, a group of eight consultants met at the IAEA , Vienna in November 1996 to discuss the development of a new code of practice. The consultants' group recommended that a new IAEA code of practice be produced for radiation therapy dosimetry based on standards of absorbed dose to water. Ther present TRS 277 would still stand as the code of practice for measurement based on air kerma. The new code would cover all therapy beams (photons, electrons, protons, heavy ions) except neutrons. (author)