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ORIGINAL ARTICLE
Year : 2012  |  Volume : 8  |  Issue : 3  |  Page : 394-398

The effect of rib and lung heterogeneities on the computed dose to lung in Ir-192 High-Dose-Rate breast brachytherapy: Monte Carlo versus a treatment planning system


1 Department of Nuclear Engineering and Physics, Amir Kabir University of Technology, Tehran, Iran
2 Department of Radiotherapy- Oncology, Imam Khomeini Hospital, Cancer Institute, Tehran, Iran
3 Department of Medical Physics and Biomedical Engineering, Tehran University of Medical Sciences, Tehran, Iran

Date of Web Publication17-Nov-2012

Correspondence Address:
Seyed Ali Vaezzadeh
Department of Medical Physics and Biomedical Engineering, Tehran University of Medical Sciences, Tehran
Iran
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/0973-1482.103519

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 > Abstract 

Aims: This study investigates to what extent the dose received by lungs from a commercially available treatment planning system, Ir-192 high-dose-rate (HDR), in breast brachytherapy, is accurate, with the emphasis on tissue heterogeneities, and taking into account the presence of ribs, in dose delivery to the lung.
Materials and Methods: A computed tomography (CT) scan of a breast was acquired and transferred to the 3-D treatment planning system and was also used to construct a patient-equivalent phantom. An implant involving 13 plastic catheters and 383 programmed source dwell positions were simulated, using the Monte Carlo N-Particle eXtended (MCNPX) code. The Monte Carlo calculations were compared with the corresponding commercial treatment planning system (TPS) in the form of percentage isodose and cumulative dose-volume histogram (DVH) in the breast, lungs, and ribs.
Results: The comparison of the Monte Carlo results and the TPS calculations showed that a percentage of isodose greater than 75% in the breast, which was located rather close to the implant or away from the breast curvature surface and lung boundary, were in good agreement. TPS calculations overestimated the dose to the lung for lower isodose contours that were lying near the breast surface and the boundary of breast and lung and were relatively away from the implant.
Conclusions: Taking into account the ribs and entering the actual data for breasts, ribs, and lungs, revealed an average overestimation of the dose by a factor of 8% in the lung for TPS calculations. Therefore, the accuracy of the TPS results may be limited to regions near the implants where the treatment is planned, and is a more conservative approach for regions at boundaries with curvatures or tissues with a different material than that in the breast.

Keywords: Breast cancer, brachytherapy, heterogeneity, MCNPX, treatment planning system


How to cite this article:
Yazdi HS, Shamsaei M, Jaberi R, Shabani HR, Allahverdi M, Vaezzadeh SA. The effect of rib and lung heterogeneities on the computed dose to lung in Ir-192 High-Dose-Rate breast brachytherapy: Monte Carlo versus a treatment planning system. J Can Res Ther 2012;8:394-8

How to cite this URL:
Yazdi HS, Shamsaei M, Jaberi R, Shabani HR, Allahverdi M, Vaezzadeh SA. The effect of rib and lung heterogeneities on the computed dose to lung in Ir-192 High-Dose-Rate breast brachytherapy: Monte Carlo versus a treatment planning system. J Can Res Ther [serial online] 2012 [cited 2019 Dec 15];8:394-8. Available from: http://www.cancerjournal.net/text.asp?2012/8/3/394/103519


 > Introduction Top


The late decade has been a truly remarkable period of growth and technical development for brachytherapy. The ten-year survival data [1],[2] has proved that brachytherapy is a reliable and safe treatment for low-risk patients, for which several methods and programs have been presented up to now. Treatment planning systems (TPS) are the most commercially applicable programs that perform dose-rate calculations in the tissue, using pre- calculated data of a given or unbounded dimension in homogenous water mathematical phantoms.

The Ir-192 HDR breast brachytherapy implantation method, as a modern and advanced brachytherapy program, not only presents a boost in irradiation following an external beam therapy, but is also prevalently applied as a powerful method for early-stage and deep-seated tumors. [3],[4],[5] In this internal beam therapy, some surrounding organs such as the ribs and lungs may receive an undesirable dose, which will affect the outcome results in clinical practice. In other words the effect of the actual delivered dose on the targetorgans, deviate from those for the homogenous water phantom and make a serious error in actual dose calculation. [6],[7],[8] Therefore, it seems to be important to evaluate the effect of such parameters in dose calculation.

Anagnostopoulos et al. [9] considered the patient's body as a homogeneous water medium. The study on esophageal brachytherapy showed that if the patient's heterogeneities were not taken into account, the spinal cord dose is overestimated at the order of 13% and the sternum dose was underestimated at the order of 15%. Pantelis et al. [10] showed that if the source position near or far from the surface of patient was not taken into consideration, the central breast dose was overestimated at the order of 5%, and all other points of the breast were overestimated at 10% in the HDR brachytherapy of the breast. Similarly, the effect of heterogeneities like cortical bone and air, and source position on dose distribution is done by Chandola et al. [11]

In this study, the result of tissue homogeneity of the TPS program and delivered dose to non-target organs (rib and lung) have been evaluated in the course of Ir-192 HDR breast brachytherapy, and also, in comparison with other studies, [7],[8] the effect of heterogeneity based on an anthropomorphic phantom with real density and ingredients of organs using the Monte Carlo N-Particle eXtended (MCNPX) code have been assessed.


 > Materials and Methods Top


Based on the clinical TPS approach, anIr-192 HDR breast brachytherapy involves irradiation of the left breast of a patient with an implant of 13 plastic catheters including a 383 programmed dwell position, using a Felxitron HDR machine (Nucletron, Elekta Ltd., Crawley, UK). This arrangement is based on the Primary Access Regional Information System (PARIS), as it is a remarkable method for one- and two-surface implants. The Flexiplan software version 2.2.4 (Nucletron, Elekta Ltd., Crawley, UK) was used for a TPS dose study that employs TG- 43 formalism. This formalism introduces a method to obtain uniform 2-D dose distribution around the asymmetrical cylindrical sources, with a linear source approximation on microselectron HDR Ir-192. [12] The sources are implanted in the tumor with a volume of 51.95 cm3, with pitches of 0.1 cm and kerma strengths of dwell positions of 19/868/551 (U/day) in order to deliver a prescribed dose of 6Gy on the surface of the planning treatment volume (PTV). A series of axial patient CT slices that are used in the clinical treatment planning procedure are also used for the construction of the MCNPX phantom that resembles the patient's anatomic characteristics. [Figure 1]a shows the central axial slice of the MCNPX phantom, relative to the corresponding CT patient axial slice [Figure 1]b.
Figure 1: Central axial slice of (a) the MCNP phantom constructed to resemble the anatomic characteristics of the patient: 1-tumor, 2-breast, 3-applicator, 4-rib, and 5-lung, with respect to (b) the corresponding computed tomography axial slices of the patient

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The mathematical phantom of the left part of the patient's chest, presented with dimensions of 30 cm and 20 cm in the x-axis and y-axis directions, respectively. The breast was described by an ellipsoidal segment, defined by the intersection of a 516.34 cm 3 volume ellipsoid, with an oblique plane [see inset of [Figure 1]a]. The lung was simulated by an ellipsoid of 18 cm length and 1147.05 cm 3 volume. For the purpose of validation in the MCNP phantom, the whole phantom was first considered as liquid water, as well as treated in the TPS program, and next, the actual data of the related organs were entered in the MCNPX program, as indicated in [Table 1] and [Table 2].
Table 1: Effective linear attenuation coefficients and medium to air mass energy absorption coefficient ratios
of Ir-192 for the tissue materials used in this study (ICRU-44 1989)


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Table 2: Elements and density of each organ

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Monte Carlo simulation using MCNPX code

The MCNPX code version 2.4.0 was used for calculating dose results in this study. [13] This was an extension of the general-purpose MCNP code, [14],[15] and presented an increased versatility in handling a large number of scoring voxels for tallying energy deposition, by defining a 3-D mesh grid of cylindrical, spherical, or rectangular shape, as those used herein, over the phantom geometry. The geometrical characteristics and the material composition of the heterogeneous phantom geometry were simulated as described in [Table 1] and [Table 2]. The 383 source dwell positions of the classic microselectron Ir-192 HDR sources [12] were simulated in a single Monte Carlo (MC), run according to the information acquired by the TPS data, and the full geometrical characteristics of the source design were used as published in Williamson and Li. [12] The dose contribution of each simulated source dwell position was weighted by a factor equal to the ratio of its corresponding irradiation time programmed by the TPS to the total irradiation time. The Ir-192 photon spectrum from Glasgow and Dillman [16] was considered in simulation. The MCNPX utilized the MCPLIB02 photon cross-section library that was contained in the RSICC package DLC-200 / MCNPDATA. Although, it has been reported that the use of the DLC-200 library is challenging for low energy photon emitting sources, its use for the dosimetry of Ir-192 sources is fully justifiable, inflicting no detectable impact on the calculation results. [17]

The gamma spectrum of the Ir-192 HDR brachytherapy source used in this study has been obtained from the Glasgow and Dillman [16] database. The gamma rays have been simulated considering that Ir-192 is uniformly distributed in the source core. The beta spectrum of the Ir-192 source has not been considered in simulation, as its contribution to the dose rate distribution for distances greater than 1 mm from the source is negligible, due to the encapsulation of the source and the catheter in which the source is introduced.

As Ir-192 is a gamma emitter, only a photon was used for the source definition of the MCNPX simulation (PAR = 2) for the source particle, while in the simulation process, both electron and photon transport were considered (mode p e). The water collision kerma approximation was used to score the dose using the RMESH: P pedep card, which defined a rectangular mesh grid, independent of the problem geometry and scores the energy deposition per unit volume per initiated photon (MeV / cm 3 / photon), only for photons. The deposited energy was then transformed to deposited energy per unit mass by dividing every single mesh grid voxel by its corresponding mass density. In order to express the results in terms of the TG-43 formalism, [18] which was used by the TPS, a separate MC run was performed, to calculate the air-kerma strength per unit contained activity (SK / A), in units of U.Bq−1 . For the isodose curves calculated in the MCNP and TPS, the dose grid was equal (the voxel size of the 3-D mesh tally grid was 0.2 cm × 0.2 cm × 0.2 cm along the x-, y-, and z-axes); 6 × 10 7 photons were initiated from the 383 simulated source dwell positions, resulting in a CPU running time of approximately 170 hours (on a Pentium IV 2.4 GHz equivalent PC).


 > Results Top


As the TPS dose calculation relies on data published by Williams and Li, [12] which is based on MC simulation in a spherical water phantom of 15 cm radius, therefore, the condition for preserving the electronic equilibrium cannot be held when source-boundary distance becomes small, due to programming of the source dwell position. In other words, the missing backscatter close to the phantom edge will lead to overestimation of the dose in those regions. The results of a computer run for the TPS program and MCNPX modeling of the breast (organ target) and lung (non organ target) under similar condition are shown in [Figure 2],[Figure 3],[Figure 4],[Figure 5] and [Figure 6].
Figure 2: DVH of breast by MCNPX and TPS

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Figure 3: Comparison of 2-D isodose contours by MCNPX and TPS for a homogeneous phantom Monte Carlo– calculated broken line percentage isodose contours (a) x– z view (b) y – z view

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Figure 4: DVH of lung by MCNPX and TPS

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Figure 5: Comparison of 2-D isodose contours by MCNPX and TPS (Monte Carlo– calculated broken line percentage isodose contours). Actual patient geometry and the presence of the rib heterogeneity (a) x – z view (b) y – z view

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Figure 6: DVH of the rib by MCNPX and TPS

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Evaluation of MCNPX and the TPS for homogenous phantom

[Figure 2] shows the delivered dose to different percentage volumes of the breast, based on the MCNPX code and TPS program, when the phantom is considered as a homogenous medium. It shows a good agreement between two obtained results. Inspection of the two isodose contour data presented in [Figure 3] show that the MC and TPS calculations for an isodose contour above 80% are in close agreement. This implies that the missing backscatter for points close to the source, due to the curvature shape of the phantom, is not significant, as the primary radiation dominates the dose delivery in those regions. The deviation for an isodose below 70% in both the axial (z - x) and sagittal (z - y) planes becomes more apparent at points lying close to the surface curvature and relatively away from the source implant.

Evaluation of dose delivery to the breast and lung for a heterogeneous phantom

For validation of dose delivery to the breast and lung, a final MC run was performed, to simulate the actual clinical application. The simulation takes into account not only the shape effect of the surface on dose calculation, but accounts for the presence of rib and lung heterogeneities, to assess any different effect on the dosimetry of breast brachytherapy. [Figure 4] shows the energy deposition in the lung, where the TPS results are greater than those in the MCNP calculation. [Figure 5] shows that isodose contours above nearly 75% are not affected by the boundary shape of the breast or the presence of the rib and lung heterogeneities close to the PTV. However, an overestimation of TPS becomes more apparent at points in the proximity of the lung and near the surface boundary of the breast. This overestimation is due to the combined effect of the missing backscatter from both the lung and the tissue-air interface of the problem. It reduces to nearly 16, 13, and 9%, corresponding to the isodose contours of 40, 50, and 60, respectively.

These severe differences in the derived results from MCNPX and TPS occur due to differences in the rib density and elements in the two programs. The real value of rib density considered in the MCNPX code is equal to 1.62 g / cm 3 instead of 1 g / cm 3 , which is commonly used in TPS. Material elements can be important when using the Ir-192 spectrum. The range of energy for Ir-192 extends from 136 keV to 1060 keV and consequently has a higher photoelectric probability, which varies with atomic number as Z 3 , [17] where Z refers to the atomic number. Therefore, the dose absorption in the rib is higher in MCNPX than that obtained by the TPS program.

Rib absorption dose evaluation by MCNPX and TPS

The dose volume histogram (DVH) calculation in the rib is performed based on its real density and elemental composition, as indicated in [Table 1] and [Table 2], and entered in the MCNPX as material data. [Figure 6] shows that energy deposition in the rib for MCNPX calculation is 8% greater than that in TPS. This difference is quite obvious and is due to the higher atomic number of the rib in comparison to the homogeneous manner of the water phantom in the rib by the TPS program.


 > Discussion Top


The MC simulation of an actual clinical application of Ir-192 HDR breast brachytherapy, involving an implant of 13 plastic catheters and 383 programmed source dwell positions, with a prescribed dose of 6 Gy to be delivered to the PTV, was performed in a patient-equivalent phantom. Comparison between the MC and the corresponding TPS results revealed that all isodose contours above 75% were not affected by the boundary shape of the breast and the presence of the rib and lung heterogeneities at points close to the surface of the PTV. Thus DVH and all possible relevant planning quality indices like maximum dose, average dose, homogeneity, and conformity calculated by the TPS for the PTV were credible.

Overestimation of the dose in the lung for lower isodose contours, as a result of breast brachytherapy, indicates that MCNPX can provide more reliable results for dose distribution in the lung in comparison to TPS.


 > Acknowledgment Top


This study was supported by a grant from the Tehran University of Medical Sciences. The authors would like to thank the Radiotherapy personnel in the Imam Khomeini Hospital, at Tehran, Iran, for their assistance.

 
 > References Top

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    Figures

  [Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5], [Figure 6]
 
 
    Tables

  [Table 1], [Table 2]



 

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