Air-kerma strength determination of an HDR 192Ir source including a geometric sensitivity study of the seven-distance method

Purpose To perform an in-air air-kerma strength (Sĸ) calibration of the Bebig model Ir2.A85-2 192Ir high-dose rate (HDR) brachytherapy source manufactured by Mallinckrodt Medical (Wester-duinweg, Germany) with the NIST-traceable seven-distance technique established by the University of Wisconsin [1‒3]. A comparison was made between the reference air-kerma rate (RAKR) reported on a certificate from the Physikalisch-Technische Bundesanstalt (Berlin, Germany) (PTB) primary laboratory and the Sĸ determined at the University of Wisconsin Madison Radiation Research Center (UWMRRC). A theoretical sensitivity study was performed to investigate the impact that variations in the experimental setup have on the computed Sĸ from the seven-distance algorithm in order to determine if the uncertainty budget for the seven-distance method should be expanded. Methods The manufacturer-reported Sĸ for the source was compared to the Sĸ determined from the University of Wisconsin Accredited Dosimetry Calibration Laboratory (UWADCL) transfer standard well chambers and the seven-distance technique. Monte Carlo techniques (MCNP6) were employed to compare the theoretical Sĸ calibration coefficients of a Standard Imaging (Middleton,WI) HDR1000 Plus well chamber using Standard Imaging model 70010 and model 70110 source holders to determine if a holder dependence was present. Radiochromic film (EBT3, Ashland) exposures were performed to assess the dose distribution of the source in phantom. The seven-distance algorithm was coded in MATLAB®(R2013b) and benchmarked with MCNP6 with the capacity to model distance offset behaviors among nominal positions. This offset model was used in a Monte Carlo simulation coded in MATLAB to determine the average uncertainty in the Sĸ calculations from the seven-distance algorithm. Results The measured Sĸ using the seven-distance technique at the UWMRRC agreed with the RAKR reported on the PTB source certificate and the Sĸ on the Mallinckrodt source certificate to within 0.28% and ‒0.79%, respectively. It was found that the difference between the Sĸ measured from the transfer standard well chambers at the UWADCL and the seven-distance method was between 0.13% and 0.30% at the 95% confidence level. Monte Carlo results showed negligible differences between the simulated Sĸ calibration coefficient for an HDR1000 Plus well chamber using the model 70010 or model 70110 source holder. The autoradiographs from the source in Virtual Water™ showed that the dose distribution is symmetric. Additionally, the sensitivity study performed in MATLAB showed that the Sĸ calculated with the seven-distance algorithm could deviate by 0.24% from randomly generated distance offsets within 1mm in magnitude. Conclusion The differences between the Sĸ measurements determined from the seven-distance technique and the accredited UWADCL measurement results were within the k=2 uncertainty reported for an accredited calibration. Excellent agreement was found between the measured Sĸ and RAKR methods used at the UWMRRC and PTB, respectively. Additionally, the sensitivity study has shown that the seven-distance algorithm accurately determines the Sĸ of a source while having a variable chamber offset among nominal positions; the uncertainty budget for the seven-distance method does not need to be expanded at this time. It has been determined that the current standard used by the UWADCL for well chamber calibrations is valid for the Bebig model Ir2.A85-2 192Ir brachytherapy source. This article is protected by copyright. All rights reserved.