Purpose
The source strength of brachytherapy sources should be verified prior to their clinical use. To fulfil the requirement for the standardization of high-dose rate (HDR) 60Co brachytherapy sources, a homogeneous thick-walled graphite cylindrical ionization chamber of nominal sensitive volume 223.5 cm3 was designed, developed and established as a reference standard.
Methods
Experimental measurements were conducted with HDR 60Co brachytherapy sources at different radiotherapy centres across India to determine the basic characteristics of this ionization chamber. The reference air-kerma rate, , of the HDR 60Co sources was determined from the measured signal using the air-kerma calibration coefficient formalism developed earlier analytically based on Burlin general cavity theory and large cavity theory and validated with Monte-Carlo calculations.
Results
The basic characteristics of this chamber were in conformity with requirements specified in IAEA TRS-374 report. The of HDR 60Co sources determined experimentally using this chamber was within 3 % of the value quoted by the supplier, which is traceable to PTB, Germany. The standardized HDR 60Co sources were used for calibrating the eight well-type ionization chambers of different radiotherapy centres and the values of air-kerma calibration coefficient so obtained agreed within 3 % of the vendor-quoted values.
Conclusion
The performance of the newly developed chamber was found to be satisfactory and it qualifies to be designated as a reference standard for the measurement of of HDR 60Co sources and for calibration of the well-type chambers of radiotherapy centres.
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