|Year : 2013 | Volume
| Issue : 2 | Page : 224-229
Dosimetry of MammoSite® applicator: Comparison between Monte Carlo simulation, measurements, and treatment planning calculation
Mina Oshaghi1, Mahdi Sadeghi2, Seied Rabi Mahdavi3, Alireza Shirazi3
1 Department of Biomedical Engineering, Science and Research Branch, Islamic Azad University, Karaj, Iran
2 Agricultural, Medical & Industrial Research School, Nuclear Science and Technology Research Institute, Karaj, Iran
3 Department of Medical Physics, Faculty of Medicine, Tehran University of Medical Sciences, P.N: 021-88058647, Tehran, Iran
|Date of Web Publication||13-Jun-2013|
Seied Rabi Mahdavi
Department of Medical Physics, Faculty of Medicine, Tehran University of Medical Sciences, P.N: 021-88058647, Tehran
Source of Support: None, Conflict of Interest: None
Purpose: To investigate the dosimetric characteristics of accelerated partial breast irradiation technique by MammoSite® applicator using thermoluminescent dosimeter (TLD) and Monte Carlo simulation to comparing them with treatment planning system calculation for planning target volume (PTV) and organs at risk such as skin, lung and chest wall.
Materials and Methods: The Monte Carlo MCNP-5 code was used to simulate dose rate in the PTV that is a MammoSite® balloon with 1 cm margin around it. Experimental dosimetry was carried out within a female-equivalent chest phantom with TLD dosimeter after insertion of 192 Ir source into the MammoSite® applicator. Three dimensional planning (TP) was done for dose delivery to the specific points within the phantom by means of FlexiPlan software.
Results: Statistical comparisons were done between TP calculation, Monte Carlo simulation and TLD. Our results showed good agreement for surface doses between simulation and measurement. The mean skin dose for the simulation and TLD result was 61.7% and 56.8% of prescription dose, respectively. The maximum dose to the chest wall for Monte Carlo and TLD were 114.4% and 111.8% of prescription dose, respectively. The maximum dose to the lung for Monte Carlo and TLD results were 28.4% and 27.3% of prescription dose, respectively. Using Monte Carlo simulation and an average female chest phantom, it was possible to demonstrate the accuracy on the calculated dose rate in the PTV of a MammoSite® dose delivery system with 192 Ir HDR sources.
Conclusions: The results showed acceptable agreement between simulation, treatment planning, and experimental dosimetry results.
Keywords: Brachytherapy, MammoSite® , 192 Ir, Mont Carlo simulation, thermoluminesce dosimeter, treatment planning
|How to cite this article:|
Oshaghi M, Sadeghi M, Mahdavi SR, Shirazi A. Dosimetry of MammoSite® applicator: Comparison between Monte Carlo simulation, measurements, and treatment planning calculation. J Can Res Ther 2013;9:224-9
|How to cite this URL:|
Oshaghi M, Sadeghi M, Mahdavi SR, Shirazi A. Dosimetry of MammoSite® applicator: Comparison between Monte Carlo simulation, measurements, and treatment planning calculation. J Can Res Ther [serial online] 2013 [cited 2019 Sep 17];9:224-9. Available from: http://www.cancerjournal.net/text.asp?2013/9/2/224/113361
| > Introduction|| |
Breast cancer is a worldwide problem, it has been reported that each year over 1.15 million women worldwide are diagnosed with breast cancer and 502,000 die from the disease.  In Iran, breast cancer has the highest incidence among all cancer types in female with 1 in every 8-10 women being affected during her lifetime. ,,
The MammoSite® radiotherapy system is an alternative treatment option for patients with early-stage breast cancer to overcome the longer schedules associated with external beam radiation therapy. The applicator system consists of a semiflexible catheter with an inflatable balloon attached to the distal end. It can be placed easily either intraoperatively or postoperatively into the cavity after lumpectomy of the breast tumor. After inflation with a combination of saline and radiographic contrast, the balloon position is evaluated via computerized tomography (CT) or ultrasound to assure proper geometry, symmetry, and conformance. The catheter shaft has a single lumen that permits the transport of 192 Ir HDR brachytherapy source into the center of the balloon. 
The treatment schedule for the MammoSite® monotherapy is 34 Gy delivered in 10 fractions at 1.0 cm from the balloon surface with a minimum of 6 hours between fractions on the same day. ,, The MammoSite® brachytherapy not only decrease the treatment time but also can limit the dose to the normal tissues such as the lung and heart in appropriate condition. Accuracy of treatment planning and source correct placement in the balloon is very important to achieve optimal dose coverage in the target volume.
The purpose of this study is to investigate the dosimetric characteristics of high dose rate source in MammoSite® brachytherapy. Monte Carlo simulation was used to estimate the dose distribution around balloon surface, its 10 mm margin and critical organs. In addition, treatment planning was performed by using Flexiplan software.
For experimental dosimetry by using thermoluminescent dosimeter (TLD), half of a female chest phantom was built and after placement of TLD in embedded place, percentage of received dose was investigated. The results of all three methods were compared by International Commission on Radiation Units and Measurements (ICRU) report and finally, by buffering the results, percentage of dose received by organs at risk (such as skin and lung) was determined.
| > Materials and Methods|| |
Today, the application of 192 Ir sources in HDR_afterloader equipment is a common practice in clinical brachytherapy. According to recommendations with regard to the TG43U1 methodology, the tolerances in the manufacturing process of the source are also included. The active core of the source is pure iridium cylinder with density of 22.4 g/cm 3 , with an active length of 3.5 mm and a diameter of 0.6 mm. The active core is covered by a stainless-steel-304 capsule and leading to outer dimensions of the source of 0.8 mm in diameter and 4.6 mm of total length. The 304 stainless steel cable has been modeled in the Monte Carlo calculations as a cylinder of 5 mm length and 0.5 mm in diameter. 
The mathematical phantom used in this study represents half of a female thorax and is composed of organs based on the dimensions of an average female human being. The dimensions of the organs were obtained from Scutt an anthropomorphic chest phantom. 
The breast was made from a half sphere of Plexiglas material with 8 cm radius and lung was also a cylinder from cork with 9 cm radius along the x axis and 28 cm of length parallel to the y axis. The chest wall thickness was 2 cm and the hemi-thorax dimensions were 28×30×30 cm in the x, y, and z directions, respectively. Whole phantom was made from 85 layers, 44 layers in breast part and 41 layers in thorax. Layers thicknesses vary according to the anatomic position of that layer in the thorax phantom. A MammoSite® applicator was inserted into the cavity inside the breast part of the phantom and inflated to diameter of 5 cm with 62 ml of sterile normal saline. It's essential to note that, due to the clear margin between balloon surface and lumpectomy cavity in phantom, we did not use contrast material in MammoSite® balloon.
In addition, some slabs were introduced into the phantom in order to obtain absorbed doses in skin, chest wall, and lung surface for planning purpose. Thickness was 2-3 mm and 5 mm for the other slabs in breast and lung, respectively.
To protect phantom layer during transport, a belt was tailored to keep all layers in position as well as pins at four corners of the chest box.
Dose measurement is acknowledged to be a vital part of quality assurance (QA) process in radiology and thermoluminescent dosimetry is the recommended method of entrance dose measurement for 'simple' examinations. 
Experimental dosimetry was performed by TLD. LiF: Mg, Ti (TLD-100) crystals has long been the most commonly used TLD. This form of Lithium fluoride is doped with Mg and Ti and its nearly tissue-equivalent. In this study, we were used 31 square shape TLD-100 chips (Harshaw Chemical Co. Cleveland/Ohio) with size of 3.2×3.2×0.9 mm for dosimetry. TLD calibration is considered necessary in brachytherapy dosimetry.  So at first, element correction coefficient (ECC) was determined for each TLD and then all of the TLDs were calibrated with 60 Co beam in source surface distance SSD=100 cm and field size=12×12.
TLD was performed within the Plexiglas layers with 1 mm thickness. Square shaped location of TLD was prepared to match with the dimensions of TLD size for each specific layer, which were placed in the balloon surface and its 10 mm margin to investigate dose coverage in planning target volume (PTV). They also were placed in skin, chest wall, and lung surface to determine the percentage of received dose by critical organs. Ten TLDs were placed in balloon surface and PTV, 3 TLDs in the first layer of the breast part of the phantom that we call it skin surface, and 10 TLDs in chest wall and lung surface. Also six TLDs were placed in the distance of 1, 10, and 20 mm of balloon surface and for back ground measurement two TLDs were kept in the same condition.
Ideal dosimetric goals of MammoSite® brachytherapy include 90% coverage of PTV by at least 90% prescribed dose (PD). The maximum skin dose should be reduced to as low as achievable and it has not been exceeded from 145% of the PD at any point. The volume of breast tissue receiving 150% (V 150 ) of the PD had to be reduced to as low as achievable and designed not to exceed 50 ml. The volume of breast tissue receiving 200% (V 200 ) of the PD has to be reduced to as low as possible and not larger than 10 ml. 
Three dimensional planning was done on computed tomography (CT) images of the phantom with FlexiPlan software (Nucleotron Co., Veenedaal/ Netherlands) to deliver PD to the reference points. CT images were obtained to determine the volume of the balloon, balloon-to-skin distance, maximal point skin dose per fraction, percent of the volume that received 100% of the prescription dose, V 100 , percent of the volume that received 150% of the prescription dose, V 150 , and percent of the volume that received 200% of the prescription dose, V 200 .
After treatment planning, irradiation in to the TLDs was done for three times. Due to the source activity at irradiation time, time of exposure in each stage was determined by the Flexiplan software. After exposure, all of the TLD chips were read by using LTM reader (PTW Co., Freiburg/Germany). Considering that the treatment schedule for the MammoSite® monotherapy is 3.4 Gy delivered at PTV in each fraction, percentage of the dose received by each TLD was determined.
The radiation transport simulations performed in the present study were carried out with the Monte Carlo n-particle transport computer code version 5 (MCNP5) and the F6 energy deposition tally. , The simulations were operated in the photon and electron transport mode, through materials such as water, lung tissue, and air. Monte Carlo simulation was used to estimate the dose distribution around balloon surface as well as its 1 cm margin and critical organs.
In the Monte Carlo simulation, breast and chest wall were considered to be water equivalent and the lung composition was obtained from International Commission on Radiological Protection Publication 23 with a density of 0.297 g/cm 3 . 
In this study for phantom simulation, the center of the balloon (or the center of the coordinated system) was placed 4 cm from the base of the breast that is a half sphere with 8 cm radius. The scoring voxels were defined in the x-y plane of the PTV at 1.0 cm from balloon surface also for investigating the percentage of the dose received by each TLD, scoring voxels were simulated exactly in the TLD places in the phantom. The axis of "z0" was assumed longitudinally along the 192 Ir brachytherapy source and x and y axes were situated vertically and laterally, respectively, relative to the source position.
The source was simulated as two cylinders with a common central axis for source material and filtration around.
For benchmarking, the relative dose calculated by Monte Carlo MCNP5 were compared with those data at TG43 results.  The dose rate constant λ, the radial dose function g(r), the anisotropy function F(r,θ), were calculated in water for gamma photons.  The detectors were simulated at polar angles of 0-180° in 30° increase and at radial distances of r=0.5-10 cm relative to the source center.
| > Results|| |
The hemithorax phantom was prepared from Plexiglas layers, a full picture of 30×30×30 phantom is shown in [Figure 1].
|Figure 1: Picture of the female chest phantom that was built in this study; (a) 1. Breast, 2. Belt to keeping the breast layers together. 3. Chest wall and lung. 4. Woodseler for keeping the thorax layers together and (b) 5. MammoSite® balloon, 6. The stem inserted MammoSite® applicator|
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The phantom includes 85 layers (44 layers in breast part and 41 layers in thorax) and can be used with single channel MammoSite® , Multi-Channel Contura and with little changes for Multi-Channel Strut-Adjusted Volume Implant (SAVI) applicator. In this phantom, dosimetry performed with thermoluminescent detector, and with replace Plexiglas layers in the reference points with radiochromic films, dosimetry also can be performed by radiochromic dosimeter.
Treatment planning was done by Flexiplan software. Isodose curve were set at balloon surface, PTV (Balloon and its 1 cm margin), skin, chest wall, and lung surface to show the received dose at those points [Figure 2].
|Figure 2: Treatment planning report in one of the phantom CT-scan slices. Slice of breast, isodose curve (in PTV, skin, chest wall, and lung) and peculiarities of MammoSite® applicator are shown in this picture|
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The 2-D anisotropy function describes the variation of dose in the longitudinal plane of a brachytherapy source. For calculation of the anisotropy function, the *F8 tally was used and the results are presented in [Table 1]. The anisotropy function, F(r,θ) table, showed homogeneous doses scores near the surface of source.
The radial dose function, g(r), was determined in order to characterize the effects of absorption and scatter in the medium along the reference radial (θ0 =90°) axis of the source. [Figure 3] shows the radial dose function curve with respect to the distance from surface of source. We compared our results with the Sadeghi et al. results.  In this study the dose rate constant λ, was determined to be 1.108±0.032 cGy/U/h. In the Sadeghi et al. study, λ was determined to be 1.113±0.033 cGy/U/h.
|Figure 3: Comparison of the radial dose function of 192Ir line source and Sadeghi et al. work|
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In Monte Carlo simulation, in order to achieve uncertainty less than 1.5% for each tally volume at different materials, the number of photon histories was set at 2E8 per photon.
The doses received by balloon surface, PTV, skin, chest wall, and lung were normalized to PD at 1 cm from balloon surface in all three methods and all of the differences was normalized to MCNP results. Treatment planning, experimental dosimetry, and Monte Carlo simulation results and their differences are shown in [Table 2].
|Table 2: Treatment planning, Experimental dosimetry and Monte Carlo simulations results and their differences, Balloon center was determined as the center of coordinate system|
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Treatment planning result shows that, V 150 and V 200 were obtained 39.2 ml and 6.4 ml in PTV, respectively. Additionally, 94.7% of target volume received 100% of PD and 50% of PD was received by 34.3% of the total breast volume.
After averaging the results from all three methods, it was found that 100.7% ±0.9 of the PD has been delivered to the PTV and the percentage dose delivered to the balloon surface was 194.9%±1.8 of PD. Percentage of dose delivered to the skin surface at 20 mm, chest wall at 9 mm, and lung at 29 mm from the balloon surface were 59.9% ±1.9, 102.9% ±3.3, and 29.5%±1.1 of PD, respectively [Table 2].
| > Discussion|| |
The MammoSite® brachytherapy, as with any other radiotherapy techniques, aims at delivering the PD to the treated volume while simultaneously avoiding complications to the adjacent normal tissues. Accurate source locations at the balloon center as well as minimal balloon deformation are important for desired dose delivery with the MammoSite® applicator. Uncertainties in source position and a small or large deformation of the balloon shape (elliptical shape) may perturb the PD to the treated volume. For this reason, symmetry of the MammoSite® balloon was checked with CT images.
Successful treatment planning and dose delivery depends on the appropriate delineation of a treatment target and a thorough QA program that assures accurate treatment with homogeneous dosimetric coverage of the target. Several studies have shown that CT-based three-dimensional planning is paramount in order to achieve reliable target coverage with maximal normal tissue sparing for MammoSite® approaches.
The most important goals of QA program are to achieve a desired level of accuracy and precision in dose delivery. For intracavitary and plaque brachytherapy, an uncertainty of ±15% in the delivery of PD is a more realistic level; and larger uncertainties may be present in multiplane interstitial implants. 
The difference between the calculation results of Monte Carlo simulation and values expressed by the treatment planning system, and dosimetry by TLD showed acceptable agreement (less than 8%) between results. This indicates that the experimental dosimetry results and treatment planning are verifying each other.
Our treatment planning system was verified by acceptable difference between treatment planning and Monte Carlo simulation results.
To assure that acceptable dose homogeneity is not exceeded while striving to achieve target coverage, the volume of tissue receiving higher doses will be limited.
To compare the results of this study with the maximum tolerable dose of each organ, Radiation Therapy Oncology Group RTOG 0413 protocol revealed that actual volume of tissue receiving 150% and 200% of the PD should be limited to ≤50 and ≤10 ml, respectively. Ideally, less than 60% of the whole breast volume should receive ≥50% of the PD.
It's generally accepted that the minimal balloon surface-skin distance should be ≥7 mm. However, if the balloon-skin thickness is 5-7 mm, skin dose at any point should be ≤145% of prescription dose to assure that the skin dose does not cross the acceptable limits and t<15% ipsilateral lung volume may receive 30% of the PD. 
The curve of dose volume histogram (DVH) in treatment planning shows that, the actual volume V 150 and V 200 were 39.2 and 6.4 ml, respectively. Altered skin and lung received 28.4% and 59.6% of the PD, respectively [Table 3].
|Table 3: Comparing the results of present study with the RTOG-0413 protocol|
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The MammoSite® technique treats only a portion of the breast for relatively small number of fractions with large fraction sizes. The spherical dose distribution within the surrounding tissue of the MammoSite® technique is to ensure that the PTV receives the PD while protecting the normal healthy organs from excessive or unnecessary dose. To note that in our country, Iran, MammoSite method has not been used until now, in the present study, we used Monte Carlo simulation and experimental dosimetry to verify our treatment planning and dose delivery system. Three different methods of dose quantification were compared within a hemithorax phantom that was designed.
Using Monte Carlo simulation and an average female chest phantom, it was possible to demonstrate the accuracy of treatment planning calculation on dose in the PTV of a MammoSite® dose delivery system with 192 Ir HDR source. Our initial results showed acceptable agreement between simulation, treatment planning and experimental dosimetry.
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[Figure 1], [Figure 2], [Figure 3]
[Table 1], [Table 2], [Table 3]