Impact of the vaginal applicator and dummy pellets on the dosimetry parameters of Cs‐137 brachytherapy source

Sedigheh Sina,M Amin Mosleh Shirazi,Mehdi Zehtabian,Ali S Meigooni,Reza Faghihi,Simin Mehdizadeh

doi:10.1120/jacmp.v12i3.3480

Abstract

In this study, dose rate distribution around a spherical 137Cs pellet source, from a low‐dose‐rate (LDR) Selectron remote afterloading system used in gynecological brachytherapy, has been determined using experimental and Monte Carlo simulation techniques. Monte Carlo simulations were performed using MCNP4C code, for a single pellet source in water medium and Plexiglas, and measurements were performed in Plexiglas phantom material using LiF TLD chips. Absolute dose rate distribution and the dosimetric parameters, such as dose rate constant, radial dose functions, and anisotropy functions, were obtained for a single pellet source. In order to investigate the effect of the applicator and surrounding pellets on dosimetric parameters of the source, the simulations were repeated for six different arrangements with a single active source and five non‐active pellets inside central metallic tubing of a vaginal cylindrical applicator. In commercial treatment planning systems (TPS), the attenuation effects of the applicator and inactive spacers on total dose are neglected. The results indicate that this effect could lead to overestimation of the calculated F(r,θ), by up to 7% along the longitudinal axis of the applicator, especially beyond the applicator tip. According to the results obtained in this study, in a real situation in treatment of patients using cylindrical vaginal applicator and using several active pellets, there will be a large discrepancy between the result of superposition and Monte Carlo simulations.PACS number: 87.53.Jw

Highlights

184 Sina et al: Applicator and inactive pellets’ effect on Cs-137 dosimetry parameters by the applicator set
Markman et al[2] investigated the interapplicator attenuating effects for a typical intrauterine and double ovoid combination set by Monte Carlo (MC) calculations, and showed that neglecting interapplicator shielding effects may decrease the accuracy of treatment
Dosimetry technique In 1995, Association of Physicists in Medicine (AAPM) Task Group 43 (TG-43) introduced a protocol for dosimetric evaluation of low-energy brachytherapy sources used for interstitial brachytherapy.[7]. This protocol is used around the globe and provides a user-friendly condition for clinical brachytherapy

Summary

Introduction

184 Sina et al: Applicator and inactive pellets’ effect on Cs-137 dosimetry parameters by the applicator set. Siwek et al[3] showed the effect of applicator and other pellets on dose rate distributions on the tip of the applicator for a typical clinical configuration of pellets experimentally. Pérez Calatayud et al[4] calculated the dose distributions around a single active pellet without the applicator using Geant Monte Carlo (MC) code. They performed a complete Monte Carlo simulation for a typical train source configuration (Cs-137 pellet sources, remote afterloading system plastic guide tube and gynecological applicator) and showed that applying the superposition principle to Monte Carlo data of the individual pellet sources would cause significant differences in dose estimations, especially towards the tip of the applicator. The expertise and skill of the operator in optimal applicator insertion[5] and the ability of the system in limiting the movement of the applicators are very important in gynecological brachytherapy, as the applicator displacement would worsen the local control and increase the morbidity.[6]

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Conclusion