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Measurement of Absorbed Dose-To-Water for An HDR Ir-192 Source with a Farmer Ionization Chamber in a Sandwich Setup

F Araki

F Araki1*, T Kouno1, T Ohno1, S Kawamura2, K Kakei3, (1) Kumamoto University, Kumamoto,(2) Miyazaki University Hospital, Miyazaki,(3) Kumamoto University Hospital, Kumamoto,

SU-E-T-102 Sunday 3:00PM - 6:00PM Room: Exhibit Hall

Purpose: This study developed a dedicated device for ion chamber measurements of absorbed dose-to-water for an HDR Ir-192 brachytherapy source. The device uses two Farmer chambers in a so-called sandwich assembly. The accuracy of the sundwich method was examined by comparing chamber measurements with absorbed dose-to-water determination based on the AAPM TG-43 protocol.

Methods:A microSelectron-v2 HDR Ir-192 source was modeled with the EGSnrc/egs_chamber code. The accuracy of modeling was confirmed by comparing calculated results for gL(r) and F(r, θ) with those of TG-43. The PTW30013 Framer chamber was also modeled with the egs_chamber code and its beam quality conversion factors, kQ, of Ir-192 to Co-60 were calculated as a function of distance from the Ir-192 source. From calculation results, we developed a dedicated device for the sandwich setup with 1 mm PMMA waterproof sleeves located at both side of 8 cm from the Ir-192 source. The chamber readings measured with the Farmer chambers inserted into the 1 mm PMMA sleeves were converted to the absorbed dose to water using kQ. The average dose of two Farmer chamber measurements with the sandwich setup were compared with the TPS values based on TG-43 for eight Ir-192 sources at four institutes.

Results:Calculated gL(r) and F(r, θ) values agreed well with those of TG-43. The calculated kQ values for the PTW30013 Framer chamber ranged from 1.103 to 0.986 at distance of 2~10 cm from the source and it was 0.992 at distance of 8 cm. The doses to water measured with the sandwich setup were -0.73 +/- 0.28% (n=10) compared to TG-43 values.

Conclusion:The dose to water measurement by the Farmer chamber with the sandwich setup is useful for daily dose management for Ir-192 sources.

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