

ORIGINAL ARTICLE 



Year : 2016  Volume
: 41
 Issue : 2  Page : 115122 

Dosimetry of indigenously developed ^{192}Ir highdose rate brachytherapy source: An EGSnrc Monte Carlo study
Sridhar Sahoo^{1}, T Palani Selvam^{1}, SD Sharma^{1}, Trupti Das^{2}, AC Dey^{2}, BN Patil^{2}, K.V.S. Sastri^{2}
^{1} Radiological Physics and Advisory Division, Bhabha Atomic Research Centre, Navi Mumbai, Maharashtra, India ^{2} Board of Radiation Isotope and Technology, Vashi, Navi Mumbai, Maharashtra, India
Date of Submission  01Dec2015 
Date of Decision  12Jan2016 
Date of Acceptance  12Jan2016 
Date of Web Publication  3May2016 
Correspondence Address: Sridhar Sahoo Radiological Physics and Advisory Division, Bhabha Atomic Research Centre, Room No. 204, CT and CRS Building, Anushaktinagar, Mumbai  400 094. Maharashtra India
Source of Support: None, Conflict of Interest: None  Check 
DOI: 10.4103/09716203.181639
Abstract   
Clinical application using highdose rate (HDR) ^{192}Ir sources in remote afterloading technique is a wellestablished treatment method. In this direction, Board of Radiation and Isotope Technology (BRIT) and Bhabha Atomic Research Centre, India, jointly indigenously developed a remote afterloading machine and ^{192}Ir HDR source. The twodimensional (2D) dose distribution and dosimetric parameters of the BRIT ^{192}Ir HDR source are generated using EGSnrc Monte Carlo code system in a 40 cm dia × 40 cm height cylindrical water phantom. The values of airkerma strength and dose rate constant for BRIT ^{192}Ir HDR source are 9.894 × 10^{−8} ± 0.06% UBq^{−1} and 1.112 ± 0.11% cGyh^{−1}U^{−1}, respectively. The values of radial dose function (g_{L}(r)) of this source compare well with the corresponding values of BEBIG, Flexisource, and GammaMed 12i source models. This is because of identical active lengths of the sources (3.5 mm) and the comparable phantom dimensions. A comparison of g_{L}(r) values of BRIT source with microSelectronv1 show differences about 2% at r = 6 cm and up to 13% at r = 12 cm, which is due to differences in phantom dimensions involved in the calculations. The anisotropy function of BRIT ^{192}Ir HDR source is comparable with the corresponding values of microSelectronv1 (classic) HDR source.
Keywords: Brachytherapy; ^{192}Ir highdose rate source; TG43; EGSnrc Monte Carlo
How to cite this article: Sahoo S, Selvam T P, Sharma S D, Das T, Dey A C, Patil B N, Sastri K. Dosimetry of indigenously developed ^{192}Ir highdose rate brachytherapy source: An EGSnrc Monte Carlo study. J Med Phys 2016;41:11522 
How to cite this URL: Sahoo S, Selvam T P, Sharma S D, Das T, Dey A C, Patil B N, Sastri K. Dosimetry of indigenously developed ^{192}Ir highdose rate brachytherapy source: An EGSnrc Monte Carlo study. J Med Phys [serial online] 2016 [cited 2019 Apr 22];41:11522. Available from: http://www.jmp.org.in/text.asp?2016/41/2/115/181639 
Introduction   
In brachytherapy, sealed radioactive sources are placed near or inside a tumor, to deliver prescribed radiation dose to tumor by intracavitary, interstitial, or surface mold technique. In this treatment modality, a high radiation dose can be delivered locally to the tumor with rapid dose falloff in the surrounding normal tissue.
^{192} Ir highdose rate (HDR) sources are widely used in brachytherapy treatment because of remote afterloading technology that reduces exposure to hospital personnel, high source activity, higher dose rate, short treatment time, and more important is comfort to patient. Many HDR ^{192} Ir source models such as microSelectronv1 (classic), microSelectronv2, BEBIG GmBH, VariSource (classic), VariSource (VS2000), Flexisource, and GammaMed 12i are available worldwide for clinical applications.^{[1],[2],[3],[4],[5],[6],[7]}
Board of Radiation and Isotope Technology (BRIT) and Bhabha Atomic Research Centre (BARC) jointly developed a remote afterloading HDR machine (Karknidon) for brachytherapy treatments. Seven machines have been already been fabricated, which underwent several tests involving a dummy source. Performance of these machines is found to be satisfactory. This machine will utilize indigenously developed ^{192} Ir HDR source. Two such prototype active sources have been indigenously made recently by BRIT and BARC, to standardize methodology of active core fabrication inside a hot cell using a remote controlled laser welding equipment. Active sources are yet to be loaded in the HDR treatment units.
[Table 1] compares the source geometries, which includes encapsulation material/thickness and distal, and proximal end thicknesses of different ^{192} Ir HDR sources including BRIT ^{192} Ir HDR source, including the details of cable length modeled in the Monte Carlo calculations. The proximal and distal end thickness of BRIT ^{192} Ir HDR source is different from the other HDR source models.  Table 1: Comparison of source designs, encapsulation material/thickness and cable length modeled in Monte Carlo calculations of different ^{192}Ir highdose rate sources
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A study published by Li et al.^{[8]} provides recommendations on dosimetric prerequisites for routine clinical use of photon emitting brachytherapy sources with average energy higher than 50 keV. As per the recommendations, for commercially distributed sources, single sourcebased dose distribution used for clinical treatment planning should be based on two doserate determinations, one of which is theoretical method such as Monte Carlo method and the other an experimental measurement. However, for conventionally encapsulated ^{192} Ir sources similar in design to existing ones, a single dosimetric study either Monte Carlo simulation techniques, or other transport equation solutions, or experimental dosimetry methods is sufficient.^{[8],[9]}
The objective of this work is to calculate American Association of Physicists in Medicine (AAPM) TG43 dosimetry parameters and dose distribution in liquid water for the BRIT ^{192} Ir HDR source and utilize the same for the indigenous development of treatment planning software.^{[10],[11]} DOSRZnrc and FLURZnrc usercodes ^{[12]} of the EGSnrc Monte Carlo code system ^{[13]} are used for this purpose. The calculated dosimetry parameters are compared with published results of other ^{192} Ir HDR sources.^{[1],[2],[3],[4],[5],[6],[7]}
Materials and Methods   
In this ^{192} Ir HDR source, the radioactive material is in the form of ^{192} Ir slugs of 0.6mmdia × 3.5mmlength. The active source is encapsulated in stainless steel316L capsule of thickness 0.2 mm, which is welded remotely by argon gas laser welding process. The distal end of the capsule is spherical in shape with radius of 0.55 mm and length of the proximal end is 2 mm. The total length of the capsule is 6 mm and diameter is 1.1 mm. A schematic diagram of the BRIT ^{192} Ir HDR source is shown in [Figure 1]a.  Figure 1: Schematic diagram of the Board of Radiation and Isotope Technology ^{192}Ir highdose rate source simulated in the EGSnrc Monte Carlo code. Dimensions shown are in millimeters (not to scale). (b) The cartesian coordinate system, P(R,Z) used in the EGSnrc simulations. The coordinate of P will be (r,θ) in polar coordinate system. The origin of the coordinate system is chosen at the center of the active source
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Monte Carlo calculations
DOSRZnrc usercode ^{[12]} of the EGSnrc Monte Carlo code system ^{[13]} is used for modeling of the BRIT ^{192} Ir HDR source in liquid water. The DOSRZnrc usercode ^{[12]} calculates absorbed dose and kerma in cylindrical regions in an RZ cylindrical geometry. The material, mass density, and geometric details of the source needed for simulations are taken from source supplier. [Figure 1]b shows the coordinate system used in the DOSRZnrc simulations. In the Monte Carlo calculations, the source is immersed in a 40 cm dia × 40 cm height cylindrical water phantom to get full scatter conditions up to a distance of 20 cm from the source.^{[14]} The density of water was taken as 0.998 gcm ^{−3} at 22°C, consistent with AAPM TG43U1 formalism.^{[11]} A grid system was simulated with thin cylindrical shells of different thickness (δR, δZ) around source axis where δR represents radial distance and δZ represents axial distance. The thickness of shells, δR = δZ is 0.5 mm (up to R=Z=5 cm), 1 mm (up to R=Z=10 cm), and 2 mm (up to R=Z=14 cm). The photon energy spectrum of the ^{192} Ir source is taken from literature.^{[14]}
The origin of the coordinate system is chosen at the geometric center of the active core. In the simulation, we modeled 1 mm long stainless steel316L cable at proximal end of the source. The distal end of the source is a rounded tip of 0.5mmthickness and 0.55mmradius, which cannot be simulated in DOSRZnrc code. The thickness part (0.5 mm) is divided into 10 slabs each of thickness 0.05 mm and the radius (0.55 mm) is divided into 11 cylinders each of radius 0.055 mm. This geometrical modeling makes a steplike shape, which tries to mimic the real rounded tip.
The geometry factor, G (r,θ) accounts for the distribution of radioactivity in the active source volume, which is calculated either by point or line source approximation. TG43U1 report recommends the use of line source based geometry factors for evaluation of 2D dose distributions. This approximation is suitable for dose estimation at larger distances from the source. However, this approximation introduces errors up to 3% at radial distances close to the sources.^{[15]} Therefore, we have calculated the exact geometry factor, up to 1 cm distances around the source. The difference of 0.8% at 0.2 cm and 0.3% at 0.5 cm along transverse axis is observed between exact and line source based geometry factor.
The dose value for a particular location in polar coordinate (r,θ) is estimated by converting the polar coordinate into corresponding cartesian coordinate (R, Z). When this coordinate (R, Z) is not falling with in the centre of voxel, dose is estimated from dose values of four nearest points, (R_{1}, Z_{1}), (R_{2}, Z_{2}), (R_{3}, Z_{3}), and (R_{4}, Z_{4}) using bilinear interpolation. Accuracy of the interpolation is improved by dividing the dose values with the exact geometry function, up to a distance of 1 cm from the active center of the source and line sourcebased geometry function thereafter. These dose values in polar coordinate are used to estimate anisotropy function for this source. The abovedescribed interpolation approach has been also applied elsewhere.^{[16],[17]}
The airkerma strength, S_{k} of the source is defined as the product of airkerma rate at distance, in free space, measured along the transverse bisector of the source and the square of the distance.^{[11]} In the study, airkerma strength per Bq (U/Bq) is calculated for the BRIT ^{192} Ir source using the FLURZnrc code.^{[12]} In this calculation, the source was in vacuum. This is consistent with the updated TG43U1 formalism. As detailed secondary electrons transport is not important, ECUT = 1 MeV (kinetic energy) is set in the FLURZnrc simulations. The photon fluence energy spectrum in 10 keV interval, along the transverse axis, at 100 cm is scored and subsequently converted into airkerma per initial photon, k_{air} (Gy/initial photon) using the massenergyabsorption coefficient of air.^{[18]} The composition of air considered is as recommended by the TG43U1 protocol (40% humidity).^{[11]} The k_{air} values were then converted to S_{k} per unit activity (cGycm ^{2} h ^{−1} Bq ^{−1} or UBq ^{−1}).
The PEGS4 dataset needed for the above calculations is based on XCOM compilations.^{[19]} We set AE = 521 keV and AP = 10 keV while creating PEGS4 dataset, where the parameters AP and AE are the lowenergy threshold for the production of secondary bremsstrahlung photons and knockon electrons. All DOSRZnrc simulations utilize the PRESTAII electron step length and EXACT boundarycrossing algorithms. The electron step size parameter is ESTEP set to 0.25. To increase the speed of the calculations, for all simulations, electron range rejection technique is used by setting ESAVE = 2 MeV. The value of photon transport cutoff parameter PCUT used in all simulations is 10 keV. The value of ECUT used in absorbed dose calculations is 521 keV.
Up to 2 × 10^{9} primary photon histories are simulated. All Monte Carlo simulations were run on a 32bit Intel (R) Core i3, 3.2 GHz computer. The statistical uncertainties on the calculated estimates have a coverage factor k = 1. The uncertainties on the dose values varies between 0.1% and 1% for the regions up to Z = 2 cm, R = 0.2–14 cm. For regions Z = 2–5 cm and R = 0.2–0.5 cm, the uncertainties varied between 1% and 2%. For regions Z = 5–15 cm and R = 0.2–0.5 cm, the uncertainties varied between 2% and 3%. The uncertainty on airkerma calculation is <0.10%.
Results and Discussion   
Twodimensional dose rate distribution
Absorbed dose per unit airkerma strength (in cGyh ^{−1} U ^{−1}) is presented along both axial and radial distances up to 14 cm from the centre of the active source in [Table 2].  Table 2: Dose rate per unit airkerma strength (cGyh^{1}U^{1}) around the Board of Radiation and Isotope Technology 192Ir highdose rate source in a 40 cm diameter × 40 cm height cylindrical liquid water phantom of density 0.998 g/cm^{3}
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Dose rate constant, Λ
The value of airkerma strength is 9.894 × 10^{−8} ± 0.06% UBq ^{−1}. The calculated value of Λ for BRIT ^{192} Ir HDR source is 1.112 ± 0.11% cGyh ^{−1} U ^{−1} and is in excellent agreement with the published values of Λ for other ^{192} Ir HDR source models [Table 1] other than VariSource (classic). This is due to same active length (3.5 mm) of the ^{192} Ir HDR sources.
The value of Λ for BRIT ^{192} Ir HDR source is higher by 6.5% when compared to VariSource (classic).^{[4]} This is because the active length of VariSource (classic) is 1 cm. For a given radionuclide, the main influencing factor which affects Λ is the geometry factor. The values of Λ when corrected for geometry factor are comparable [Table 1].
Radial dose function
The values of g_{L} (r) for BRIT ^{192} Ir HDR source calculated for distances r = 0.25–20 cm are presented in [Table 3]. [Figure 2] compares the plot of g_{L} (r) with distance for different HDR ^{192} Ir sources. The g_{L} (r) values of BRIT ^{192} Ir HDR source are almost same with that of the BEBIG, Flexisource, and GammaMed 12i source models. This is due to similar active lengths and comparable phantom dimensions used in the calculations. The 40 cm diameter × and 40 cm height cylindrical water phantom is simulated for BRIT ^{192} Ir HDR and GammaMed 12i sources whereas BEBIG and Flexisource models utilized 40 cm radius spherical water phantom. Granero et al. observed 1% difference in g_{L}(r) values, for ^{192} Ir point source, at r = 10 cm, between an unbounded spherical phantom of 40 cm in radius and cylindrical phantom of 40 cm in diameter and 40 cm in height.^{[20]}  Table 3: Radial dose function for the Board of Radiation and Isotope Technology 192Ir highdose rate source
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 Figure 2: Comparison of the radial dose functions of various ^{192}Ir highdose rate sources
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The g_{L}(r) values for the source models such as VariSource (classic, and VS2000), microSelectron (vlclassic and v2) models are based on 30 cm dia spherical water phantom. Hence, the values of g_{L}(r) fall rapidly for these models as compared to BRIT source model, due to lack of back scattering of photons. A comparison of g_{L}(r) values of BRIT and source models of VariSource and microSelectron shows differences by about 2% at r = 6 cm and up to 13% at r = 12 cm.
Anisotropy function
The anisotropy function, F(r,θ) data, at radial distances r = 0.25–10 cm, at polar angles θ = 0°–180° relative to long axis of the source are presented in [Table 4]. [Figure 3] presents the plot of F(r,θ) of the BRIT ^{192} Ir HDR source for radial distance 1 cm. The ratio of F(r,θ) of the other HDR sources to the BRIT ^{192} Ir HDR source is plotted for radial distance 5 cm, in [Figure 4].  Table 4: Anisotropy function, F (r, θ) of the Board of Radiation and Isotope Technology 192Ir highdose rate source calculated in a 40 cm diameter × 40 cm height cylindrical liquid water phantom of density 0.998 g/cm^{3}
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 Figure 3: Anisotropy function of Board of Radiation and Isotope Technology ^{192}Ir highdose rate source for radial distance r = 1 cm
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 Figure 4: Ratio of anisotropy function of ^{192}Ir clinical sources with Board of Radiation and Isotope Technology ^{192}Ir highdose rate source for radial distance r = 5 cm
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It is observed from the [Figure 4] that for polar angles from 20° to 140°, F(r,θ) values are nearly independent of r and similar for all HDR sources. However, at polar angles close to the longitudinal axis of the source, (0°–20° and 140°–180°), i.e. proximal and distal end of the source, greater differences in F(r,θ) values are observed. This is mainly due to the different encapsulation thickness at proximal and distal end of the source.
Due to similar design, F(r,θ) values of BRIT ^{192} Ir HDR source and microSelectronv1 (classic) source are comparable. Small difference in the distal end thickness of these sources did not show observable differences in the anisotropy.
The F(r,θ) values of BRIT source compares well with microSelectronv2 source for θ = 0°–165° and for angles >165° significant differences up to 6% are observed. The microSelectronv2 source shows more anisotropy than BRIT source. This is due to differences in the geometry in the proximal end.
The VariSource (classic) source shows more anisotropy due to its longer active length (10 mm). Anisotropy is 30% higher than BRIT source along source axis for r = 1 cm.
A difference of about 5% at r = 1 cm and 2% at r = 5, 10 cm, in F(r,θ) values along distal end (θ = 0°) are observed between BRIT and BEBIG source. This is due to difference in distal end thickness, which is 0.5 mm for BRIT source and 0.84 mm for BEBIG source. Similarly, significant differences up to 20% along proximal end are observed between two sources, which is also due to the difference in end thickness.
A significant difference of up to 15% along proximal end is observed between BRIT and Flexisource models. This is due to 0.45 mm end thickness and 5 mm steel cable considered in the Flexisource simulations. A difference of up to 20% along proximal end is also observed between BRIT and GammaMed 12i source models as the later source utilized 0.5 mm end thickness and 60 mm stainless steel cable in the Monte Carlo calculations.
Conclusions   
The AAPM TG43 dosimetric parameters of the BRIT ^{192} Ir HDR source are generated using the EGSnrc Monte Carlo code system. The calculated dose rate constant of BRIT ^{192} Ir HDR source is in excellent agreement with the published values of similar other ^{192} Ir HDR sources, which have similar active length of 3.5 mm. The values of radial dose function of BRIT ^{192} Ir HDR source compare well with the corresponding values of BEBIG, Flexisource, and GammaMed12i sources due to similar active lengths and comparable phantom dimensions. The sources such as VariSource (classic, VS2000) and microSelectron (classic and v2) exhibit significant deviations in the values of radial dose function as compared to the BRIT source which is attributed to the size of water phantom employed in the simulations. The anisotropy function of BRIT ^{192} Ir HDR source is comparable with the corresponding values of microSelectronv1 (classic) HDR source. Significant differences along source axis are observed, when compared with other ^{192} Ir HDR source models. It is proposed to utilize the Monte Carlocalculated dose data as inputs for the indigenous development of brachytherapy treatment planning software. For clinical use, independent validation of this Monte Carlo data generated in this work, either through experimental measurements and/or Monte Carlo simulation using a different code would be helpful in ascertaining its reliability.
Acknowledgments
The authors would like to thank Dr. D. Datta, Head, Radiological Physics & Advisory Division, Bhabha Atomic Research Centre (BARC) and Dr. Pradeepkumar K. S., Associate Director, Health, Safety and Environment Group, BARC for their encouragement and support for this work.
Financial support and sponsorship
Nil.
Conflicts of interest
There are no conflicts of interest.
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[Figure 1], [Figure 2], [Figure 3], [Figure 4]
[Table 1], [Table 2], [Table 3], [Table 4]
