Absorbed dose-to-water measurement of an HDR 192Ir source with Farmer ionization chambers in a sandwich setup

This study extends a sandwich method for ion chamber measurements of absorbed dose to water of a Nucletron microSelectron-v2 high dose rate (HDR) Ir-192 brachytherapy source presented in the previous paper (Araki et al 2013 Med. Phys. 40 092101) to Farmer-type ion chambers. The goal in this study is to verify the calculation dose based on theAAPMTG-43 protocol by direct absorbed dose-towater measurement with the Farmer chamber. The measurement device uses two Farmer chambers in a so-called sandwich assembly. A microSelectron-v2HDR Ir-192 source and a PTW30013 Farmer chamber were modeled with the EGSnrc/egs_ chamber code and its beam quality conversion factor, kIr, for 60 Co to Ir-192 was calculated as a function of a distance from the Ir-192 source. From calculation results, the optimal distance for the Farmer chamber measurements was determined to be 8 cmfrom the Ir-192 source center, considering the depth dependency of kIr and the chamber positioning, and the required measurement time. The developed device consists of 1 mm-thick poly-methyl methacrylate (PMMA) waterproof sleeves located at both sides of the Ir-192 source with a separation of 16 cm. The chamber readings measured with the Farmer chambers inserted into thePMMAsleeves were converted to the absorbed dose to water using a 60 Co calibration factor and kIr. The average dose of two Farmer chamber measurements was compared with values based on TG-43 for eight different Ir-192 sources at four institutes. kIr for the PTW30013 Farmer chamber ranged from 1.174 to 0.985 at distances of 1.5-10 cm from the source and it was 0.992 at a distance of 8 cm. The measured dose to water was 0.73% lower than TG-43 values and its reproducibility, the chamber positioning accuracy, and the combined standard uncertainty were 0.28%, 0.25%, and 0.68%, respectively. The sandwich measurement with the more popular Farmer chamber is useful and convenient for daily dose management of 192 Ir sources in clinics.