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

Abstract

This study extends a sandwich method for ion chamber measurements of absorbed dose to water of a Nucletron microSelectron-v2 high dose rate (HDR) 192Ir 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 the AAPM TG-43 protocol by direct absorbed dose-to-water measurement with the Farmer chamber. The measurement device uses two Farmer chambers in a so-called sandwich assembly. A microSelectron-v2 HDR 192Ir source and a PTW30013 Farmer chamber were modeled with the EGSnrc/egs_chamber code and its beam quality conversion factor, kIr, for 60Co to 192Ir was calculated as a function of a distance from the 192Ir source. From calculation results, the optimal distance for the Farmer chamber measurements was determined to be 8 cm from the 192Ir 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 192Ir source with a separation of 16 cm. The chamber readings measured with the Farmer chambers inserted into the PMMA sleeves were converted to the absorbed dose to water using a 60Co calibration factor and kIr. The average dose of two Farmer chamber measurements was compared with values based on TG-43 for eight different 192Ir 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 192Ir sources in clinics.