A calibration system of therapy-level dosimeter in Japan organized by ANTMA calibration system of therapy-level dosimeter in Japan organized by ANTM

(Purpose) In Japan, the therapy-level dosimeter calibration had been carried out in terms of exposure until 2012. Therefore, the conversion factor kD,X was necessary to convert the calibration factor Nx obtained in air measurement to the water absorbed dose calibration factor ND,w. In 2011, Japanese National Standard Laboratory (National Metrology Institute of Japan, NMIJ) established the national measurement standard of therapy level absorbed dose to water of Co60 and started to deliver this standard. According to the NMIJ activity, the ANTM (Association for Nuclear Technology in Medicine, the SSDL in Japan) has launched a new dosimeter calibration system in terms of absorbed dose to water from October 2012, too. After that, ND,w determined directly in water is delivered to radiotherapy institutions in substitution Nx.(Method) The calibration procedure by ANTM follows Standard Dosimetry 12 published by Japan Society of Medical Physics (JSMP). The concept and formalism of Standard Dosimetry are almost same to IAEA TRS 398. Ionization chambers to be calibrated are fixed at 5cm depth in water phantom. For cylindrical type chambers, appropriate waterproof sleeves are used not only to protect water but also to fix their position in the phantom. The sleeves are made of PMMA with a wall of 1mm in thickness at ionization effective volume of chamber. For flat type chambers that are water proof or have water proof cap are put into water directly.ANTM is a calibration laboratory certified with the Japan Calibration Service System (JCSS) and can issues certificate with JCSS symbol mark. The JCSS is a standard dissemination system based on Metrological Law. The principles of JCSS are the calibration traceable to the national standard and the certification of calibration laboratory. Calibration laboratories certified are proven to have sufficient technical and management competence to carry out calibration. Calibration certificate issued by JCSS certified calibration laboratory states both calibration value traceable to the national standard and its uncertainty. Users who use dosimeters calibrated can estimate the uncertainty of their own dose measurements.(Results) During 27 months from October 2012 to December 2014, 5,193 ionization chambers were calibrated in terms of absorbed dose to water. A small differences were seen between calibration factors ND,w and Nx multiplied by kD,X for cylindrical chambers. ND,w are slightly lower than Nc・kD,x for cylindrical type. The main reason is air gap between chamber wall and water-proofing sleeve. However, there is no difference between calibration factors for flat type chambers. The reason is The standard deviation of calibration factors in consecutive two calibration using absorbed dose to water improved clearly. For Chambers calibrated with water absorbed dose 2 times or more the changes of calibration factors were small compared to the former calibration system using exposure in air. This shows that the new calibration system of ANTM is well organized and calibration factors are delivered to radiotherapy institutions with less uncertainty.(Conclusion) A new system of dosimeter calibration system using absorbed dose to water has started in Japan. The differences of calibration factor are so small that the system is operated stably.World Congress on Medical Physics and Biomedical Engineerin