Establishment of air kerma reference standard for low dose rate Cs‐137 brachytherapy sources

Sunil Dutt Sharma,G Chourasiya,P Srinivasan,Sudhir Kumar

doi:10.1120/jacmp.v12i4.3553

Sunil Dutt Sharma, G Chourasiya + Show 2 more

Open Access

https://doi.org/10.1120/jacmp.v12i4.3553

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Abstract

A guarded cylindrical graphite ionization chamber of nominal volume 1000 cm3 was designed and fabricated for use as a reference standard for low‐dose rate 137Cs brachytherapy sources. The air kerma calibration coefficient (NK) of this ionization chamber was estimated analytically using Burlin's general cavity theory, as well as by the Monte Carlo simulation and validated experimentally using Amersham CDCS‐J‐type 137Cs reference source. In the analytical method, the NK was calculated for 662 keV gamma rays of 137Cs brachytherapy source. In the Monte Carlo method, the geometry of the measurement setup and physics‐related input data of the 137Cs source and the surrounding material were simulated using the Monte Carlo N‐Particle code. The photon energy fluence was used to arrive at the reference air kerma rate (RAKR) using mass energy absorption coefficient. The energy deposition rates were used to simulate the value of charge rate in the ionization chamber, and the NK was determined. The analytical and Monte Carlo values of NK of the cylindrical graphite ionization chamber for 137Cs brachytherapy source are in agreement within 1.07%. The deviation of analytical and Monte Carlo values from experimental values of NK is 0.36% and 0.72%, respectively. This agreement validates the analytical value, and establishes this chamber as a reference standard for RAKR or AKS measurement of 137Cs brachytherapy sources.PACS numbers: 87.53.Bn, 87.53.Jw, 87.56.bg, 87.55.Qr

Highlights

Sealed gamma rays emitting radioactive sources with average energy higher than 50 keV are extensively used for treatment of various types of cancers employing the technique of brachytherapy.[1]
Measuring the source strength in terms of reference air kerma rate (RAKR) or air kerma strength (AKS) is recommended for gamma-emitting brachytherapy sources, including low-activity 137Cs tube sources used for intracavitary brachytherapy applications.[1,7,14,18,19,20,21,22,23,24,25,26,27,28]
RAKR is the kerma rate to air, in air, at a reference distance of one meter corrected for attenuation and scattering, and refers to the quantity determined along the transverse bisector of the source

Summary

Introduction

Sealed gamma rays emitting radioactive sources with average energy higher than 50 keV are extensively used for treatment of various types of cancers employing the technique of brachytherapy.[1]. Measuring the source strength in terms of reference air kerma rate (RAKR) or air kerma strength (AKS) is recommended for gamma-emitting brachytherapy sources, including low-activity 137Cs tube sources used for intracavitary brachytherapy applications.[1,7,14,18,19,20,21,22,23,24,25,26,27,28]. A guarded cylindrical graphite ionization chamber of nominal sensitive volume 1000 cm was designed and fabricated locally for strength measurement of low-activity 137Cs brachytherapy sources. This newly designed ionization chamber will be maintained as the national RAKR reference standard for LDR Cs-137 brachytherapy sources and will be used for calibrating the hospital’s dosimeters. The air kerma calibration coefficient (NK) of this ionization chamber was estimated by three independent methods: (i) analytically using Burlin’s general cavity theory,

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