Comparison of Calibration Methods for an Iridium 192 Source (hdr) and Application to the Absorbed Dose to Water Determination

The aim of this study was to compare the high dose rate (HDR) iridium 192 source calibration in terms of reference air kerma rate with two types of ionisation chambers (a thimble chamber and a well-type chamber). Dose measurements in water have been also performed with thimble chamber and thermoluminescent dosimeters (TLD) in order to validate the absorbed dose to water calculation provided by the TPS used in brachytherapy clinical routine. The uncertainties of each measurement method have been evaluated. The results of this study have shown the consistency of the iridium 192 source calibration obtained by the well type chamber and the thimble ionisation chamber as well as the good agreement between the TPS calculation and the measurements. The traceability of our determined reference air kerma rate values with the standard dosimetry laboratories has been pointed out through this study. Introduction The aim of this study has been to compare two different calibration methods for an iridium 192 source (HDR) in air with a thimble chamber and with a well type chamber and then to validate the absorbed dose to water calculation evaluated by the treatment planning system used for clinical purpose in brachytherapy cancer treatment. Materials and methods a) measurements performed in air The strength of the checked source provided by Mallinckrodt Medical BV and used in the HDR Microselectron, (Nucletron) was determined in terms of reference air kerma rate with the well-type chamber and with the thimble chamber. The thimble ionisation chamber was of type NE 2571 associated with a Keithley 35040 electrometer (calibrated in the Nertherlands Meetinstituut (NMi), Secondary Standard Laboratory (SSDL) in terms of reference air kerma rate per unit current with 10 low and medium energy photons beams qualities (RX from 60 keV to 250 keV and from Cs and Co gammas rays)). The well-type chamber (HDR 1000 plus, Standard Imaging) was connected to a Standard Imaging electrometer type Max 4000, calibrated by the Primary Standard Laboratory BNM-LNHB (Laboratoire National Henri Becquerel / Bureau National de Métrologie, France) [1] and by the Accredited Dosimetry Calibration Laboratory (ADCL), (University of Wisconsin, United states) in terms of reference air kerma rate with a reference iridium 192 source (HDR) (table I). Table I: Values of the calibration factor for the well-type chamber in terms of reference air kerma rate with a reference iridium 192 source provided by 2 standard laboratories.