A Monte Carlo investigation of the dose distribution for 60Co high dose rate brachytherapy source in water and in different media

https://doi.org/10.1016/j.apradiso.2018.02.016Get rights and content

Highlights

  • Dosimetric study for BEBIG 60Co HDR brachytherapy source in liquid water phantom and different media.

  • Modeling and simulation Monte Carlo using EGS5 code.

  • The results showed that the dose distribution differed in bone, adipose tissue and water.

Abstract

In this study, the dosimetric characterization for The BEBIG 60Co High Dose Rate (HDR) brachytherapy source model Co0.A86 was investigated and the validity of the EGS5 Monte Carlo code to reproduce the dosimetric parameters in water phantom was checked. In addition, the dose distribution for different tissue phantoms was calculated. The BEBIG 60Co HDR brachytherapy source was modeled using EGS5 Monte Carlo simulation code. A description of the source design, geometry and materials used in this work were provided. According to the update TG43-U1 formalism of AAPM, the air kerma strength, the dose rate constant, 2D rectangular dose distribution in water were calculated, moreover, the results of the radial dose function were obtained in water and different tissue phantoms; bone, lung, adipose tissue, breast and muscle. The obtained results were tabulated and presented in graphical formats for the comparison with available data. The calculated value of the air kerma strength of this study, 3.0419 U Bq−1, agree well with that of the other Monte Carlo calculation. The 2D look-up along-away rectangular dose were obtained in water, the results were similar to the published data for all distances larger than 1 cm, for the distances near to the source region on the transversal source axis small differences are apparent. The radial dose function were presented in graphical format for the comparison between the dose distribution in water and different tissue phantoms. The EGS5 results obtained in this study shows good consistency with the published data for the dosimetric parameters of the of the BEBIG 60Co HDR brachytherapy source. It seems that the radial dose function calculated in water differed in tissue phantoms due to the atomic composition and densities for media that are not taken account by the TG43-U1 formalism.

Introduction

Brachytherapy is one of the most common treatment modalities for cancer treatment; it has played an important role for management of different cancer types. Brachytherapy is a technique of radiation therapy by placing radioactive sources either within or immediately at short distances from the tumor. The radiation from a radioactive source is emitted isotropically. Depending on the length of time radioactive sources remain in place, brachytherapy can be provided using permanent low dose rate (LDR) or temporary high dose rate (HDR). Moreover, in term of dosimetry, the delivered dose to the tumors can be controlled by manipulating the dwell time and position of the radioactive source (Nag and Gupta, 2000).

In high dose rate, brachytherapy, the dose received is obtained by following the American Association of Physicists in Medicine (AAPM) TG-43 formalism (Rivard et al., 2004). The source design, geometry and composition is provided by the manufacturer for the purpose of Monte Carlo input data calculation. To validate this input data for simulation, a dosimetric study is required according to the TG-43 formalism.

60Co is used in high dose rate (HDR) afterloader equipment as an alternative to the 192Ir HDR source (Baltas et al., 2008). Many clinical examples shows similar dose distributions for 60Co and 192Ir despite the fact that the two sources have different physical characteristics. The main advantage of considering 60Co for HDR brachytherapy is assigned to cost and logistics improvements due to the longer half-life of 60Co (t1/2 = 5.27 years vs. 74 days) (Richter et al., 2008; Stefan Strohmaier and Grzegorz Zwierzchowski, 2011). The most common treatment sites for brachytherapy are prostate, breast, lung, head and neck.

The dose distribution around the source should be obtained experimentally or by simulation in order to be used as input parameters in the treatment planning system (TPS) according to the TG-43 report (Rivard et al., 2004; Berkay Camgoz and Mehmet Nurullah Kumru, 2011; Ghassoun, 2013). In this formalism, the dose parameter calculations were based on water phantom. TG-43 does not consider both patient heterogeneities (atomic composition and density) and finite patient dimensions effects, as full scattering conditions (large phantom) are assumed. Monte Carlo calculation for HDR dosimetry have been carried out in the literature in water phantom, and the 60Co dose distribution in tissues are still unknown. The main purpose of this study is to reproduce the TG-43 parameters such as the air kerma strength and the 2D dose rate table in Cartesian coordinates, and to calculate the dose distribution around 60Co HDR brachytherapy source in bone, lung, adipose tissue, breast, muscle and water by the means of EGS5 Monte Carlo code (Hiramaya et al., 2005).

Section snippets

Geometry design and source description

The BEBIG 60Co HDR brachytherapy source, model Co0.A86 used in this work was composed of pure and homogenous cylindrical active core of 60Co, with a length of 3.5 mm and a diameter of 0.5 mm (density: 8.09 g/cm3). The active core is covered by an air shell of 0.1 mm of thickness then encapsulated in a cylindrical stainless steel capsule of 0.2 mm for thickness and 1 mm for the external diameter. For this study we considered 4 mm and 0.9 mm for the length and diameter, respectively, of the

Results and discussion

The calculated value of the air kerma strength per unit source activity for the BEBIG 60Co HDR source at a distance of 1 m from the source center in a cubic air phantom was found to be 3.0419 10−7 ± 0.007 U Bq−1. Palani et al. (2010) obtained in their work a value of 3.043 10−7 ± 0.006 U Bq−1 using EGSnrc Monte Code, also the calculated value of Sk obtained by Sahoo et al. (2010) was 3.04 10−7 ± 0.005 U Bq−1 using Monte Carlo based MCNP code. Anwarul et al. (2012) obtained a value of 3039.10−7

Conclusion

The EGS5 results obtained in this study showed good consistency with the published data; the air kerma strength, the dose rate constant and the 2D look-up along-away dose rate table, of the BEBIG 60Co HDR brachytherapy source, were calculated according to the TG43-U1 recommendations. In addition, the results obtained were presented in tabled and graphical formats. The air kerma strength was consistent with the results of Palani and Sahoo within 0.11% and 0.19% respectively. 2D look-up

Acknowledgments

This study was supported by the High Energy Accelerator Research Organization, KEK of Japan. The authors are grateful to all members of Applied Research Laboratory of KEK.

References (17)

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