Study of Long-Term Stability of Calibration Coefficient of Therapy-Level Dosimeter and Various Factors Affecting its Stability

The Secondary Standard Dosimetry Laboratories (SSDLs) established through Primary Standard Dosimetry Laboratories are the authorized institutions for calibrating dosimeters used in teletherapy¹ all over the country. These calibrated dosimeters play a pivotal role for treatment planning in accelerator and teletherapy facilities for deciding the amount of dose and exposure to be delivered to the cancer patients during treatment. In India, Radiation Standards Section of Bhabha Atomic Research Centre is the authorized SSDL for providing calibration to teletherapy facilities for around 580 hospitals all over the country. The dosimeters are calibrated as per the standard protocol TRS 398, which is followed by standards laboratories all over the world. The accuracy of setting along with proper functioning of the dosimeters will together contribute for determination of calibration coefficient of the ionization chamber with accuracy. Controlling ambient atmospheric conditions like temperature, pressure and humidity factors within the recommended standard range also plays a major role in deriving calibration coefficient with utmost less uncertainty. A study was carried out on the variation of calibration coefficient of various chambers over a period of 15 years. It was found that variation of calibration coefficient of same chamber over this period varied within the limit measurement uncertainty of ± 1.5% only. Hence based on this study, the period for recalibration of dosimeters can be reviewed for a longer time unless it is subjected to damage or improper handling.