Calibration and Standardization of Ionization Chamber and Absorbed Dose to Water for Photon Beam by Two IAEA Dosimetry Protocols

Abstract

A high degree of accuracy, reliability and reproducibility become an important and necessary issue for safe and effective radiation treatment of cancer patients to ensure confidence in both the dose delivered to the tumour, as well as to the nearby healthy. According to ICRU-24, the accuracy of dose deliver to the tumour should meet ±5% of the prescribed value. For the accuracy of dose measurement, several national and international protocols such as TRS-277, TRS-398, TG-51 and DIN-6800-2 are currently being used for the dosimetry of ⁶⁰Co from teletherapy unit, high energy photon and electron beam from medical linear accelerator. Each protocol is introduced with various correction coefficients for the accurate dose determination. Among the protocols TRS-277 (air kerma, N_k) and TRS-398 (absorbed dose to water: N_D,W) are widely used in Bangladesh as well as all over the world that are published by the International Atomic Energy Agency (IAEA). The TRS-277 involved various correction factors as the chamber calibration factor involved in terms air kerma and hence uncertainty increases than TRS-398. The purpose of this research was to studies on (i) the calibration of ionization chamber against reference standard with ⁶⁰Co unit of  National Institute of Cancer Research and Hospital (NIRCH), Dhaka,  (ii) accuracy involve in the two different dosimetry protocol TRS-277 and TRS-398 and comparison for high energy photon beam 4-, 6-, 15-MV dosimetry from medical linac at Khwaja Yunus Ali Medical College & Hospital, Sirajgonj, and (iii)  traceability of present measurement with international standard by participating IAEA/WHO TLD intercomaprison program. An empirical relation of output of ⁶⁰Co teletherapy unit of NIRCH has been established. A total number of 13 ionization chamber (12 thimble and 1 parallel plate) are calibrated with ⁶⁰Co quality in-terms of absorbed dose to water (N_D,W) against reference standard chamber of SSDL, Bangladesh Atomic Energy Commission. The deviation of calibration factor by manufacturer’s stated value and present measurement lies in between 0.13-3.38% with maximum uncertainties of ±2.8% (coverage factor k=2). Hence, an empirical relation (exponential) of chamber volume and calibration factor has also been obtained. The average deviation of absorbed dose measurement between two different dosimetry protocol TRS-277 and TRS-398 were found within ±1.59% for 4 MV-15 MV linac photon beams which shows an excellent in agreement with other measurement in the literature. The traceability of the present measured values of absorbed dose to water for 4 MV, 6 MV (2 beams), 15 MV (2 beams) photons beams is  also studied by the participation of IAEA/WHO TLD intercomparison program and found excellent in agreement.

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