Dosimetry of Co-60 Teletherapy Units and Calibration of Some Ionization Chamber

Abstract

Curative treatment of cancer fully depends on the early detection and optimum dose delivery to the tumor. There is a need for an accuracy of ±5% in the delivery of absorbed dose in some treatment situations. In addition, International Atomic Energy Agency (IAEA) has set an acceptance limit of ±1.5% variation for the comparison of N D,W results of ionization chambers used in radiotherapy. In this perspective, absorbed dose to water D W was determined in two 60Co teletherapy units at reference condition applying TRS-398 protocol with secondary standard dosimetry system available at Secondary Standard Dosimetry Laboratory (SSDL) at the Institute of Nuclear Science and Technology (INST), Atomic Energy Research Establishment (AERE), Savar, Dhaka. This absorbed dose to water DW was determined with a view to evaluate the chamber accuracy to be useful for the accurate treatment of cancer patients. The relative percentage deviation between the secondary standard chambers was found to be -0.594%. In this study, the calibration coefficients N D,W for nine ionization chambers were determined in terms of the aforementioned protocol. The percentage of deviation of some calibration coefficients of the ionization chambers taken under calibration were found within the IAEA limit of ±1.5% and some were found beyond the IAEA limit. In addition, some additional characteristic features, namely percent depth dose PDD and output factors were determined for the 60 Co teletherapy units used in this study. The PDD variation with different depths showed a good trend for the two 60 Co teletherapy units. The determined output factors for the respective 60 Co teletherapy units between the two machines indicated the desired upwards trends with field sizes are almost negligible. The accuracy cross check of the secondary standard dosimetry system in terms of TLD intercomparision program was performed. The result of deviation between the delivered dose by the secondary standard dosimetry laboratory and the IAEA measured dose sustained within the limit ±3.5%.

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