Separated calibration of radiotherapy dosimeter

The dosimeter used in the dose measurement for radiation therapy is called 'dosimeter for radiotherapy' and consists of 'detector' and 'measuring device' that reads signals from the detector. As the detector, 'ionization chamber' is used, and as the measuring device, 'electrometer' that reads output current and electric charge is used. In Japan, 'integral calibration' which carries out calibration for both the ionization chamber and electrometer together as a unit has been performed so far, but 'separation calibration' is being started as a measure to respond to user's usage conditions. The problem with integral calibration is the difficulty in changing calibration conditions (dose or dose rate), and it is only sensitivity calibration at a point of dose value under a certain dose rate. In separation calibration, when the ionization chamber is calibrated at one point, the electrometer can be calibrated at over three points including the verification of the linearity of electrometer, which can improve the reliability of dose measurement. Other ranges are also calibrated, making it possible to measure using the optimum range according to the dose of a measurement target. In face of the start of separation calibration, 'Guideline for electrometers used in dosimeter for radiotherapy' was issued by the Japan Society for Medical Physics. This paper explained the following: (1) background and purpose of the guideline, (2) scope of performance requirements, (3) requirements toward users, (4) use/storage of electrometer by users, (5) users' inspection, and (6) results of electrometer calibration and electrometer calibration constant. (A.O.)