Brachytherapy Dose Calculations Research Articles

Brachytherapy Dose Prediction Based on Monte Carlo Simulations Using Artificial Neural Networks

The current brachytherapy dose calculation is still based on the oversimplified water kernel superposition algorithm recommended by AAPM TG43. Monte Carlo (MC) method provides high accuracy in dose calculation. However, the MC simulation is computationally intensive and too slow to use in the time-sensitive clinical workflow. This study aims to develop a fast neural network (SK-UNet) to predict 3D brachytherapy dose distribution calculated by the MC simulations. We hypothesize that the tracks of Ir-192 inside applicators are essential in dose map prediction and the Selective Kernel Networks (SK) can be employed to emphasize the contribution of the applicator in training. We then included SK in UNet to select appropriate receptive field sizes as needed. The network inputs were CT images of patient anatomy, binary masks of HRCTV, bladder, rectum, and source tracks, and the output was the predicted dose map. Totally, 120 cases were used for training and 30 for testing. All clinically common applicators (e.g., vaginal, tandem and ovoid, multi-channel applicator, free needles, etc.) were involved in the study. Model performance was evaluated by the mean absolute error (MAE) of DVH dosimetric metrics and dose distribution metrics between ground truth (GT) and prediction. A smaller MAE indicated a more accurate dose prediction. Dose distribution metrics include the overdose volume index (ODI), target conformity (TC), dose homogeneity index (DHI), and conformal index (COIN). GT was dose calculated by MC simulations. 3D Gamma analysis was also involved. As shown in Table 1, compared with GT, SK-Net showed comparable DVH dosimetric metrics with 0.28±0.19 deviation for HRCTV D90%, 0.21±0.16 deviation for bladder D2cc, and 0.25±0.22 deviation for rectum D2cc. In dose distribution metrics, the deviations between GT and prediction were TC = 0.04±0.02, ODI = 0.03±0.03, DHI = 0.02±0.02, and COIN = 0.02±0.03. The gamma passing rate was 93%±6%. The 3D dose map prediction for each patient takes about 6s on average in an NVIDIA GeForce RTX 3060 GPU compared to hours with MC simulations. SK-Net demonstrated comparable performance to the MC simulation but with a significantly shorter execution time, taking only 6 seconds on average. It offered a solution to the trade-off between accuracy and speed and has the potential to serve as an alternative to the time-consuming MC simulation in brachytherapy. The developed technique is believed to have the potential for future application with other types of radiation sources and cancers.

185 Development of Test Cases for Comparison of Model-Based Dose Calculations in Low-Energy Brachytherapy

185 Development of Test Cases for Comparison of Model-Based Dose Calculations in Low-Energy Brachytherapy

A MC-based anthropomorphic test case for commissioning model-based dose calculation in interstitial breast 192-Ir HDR brachytherapy.

To provide the first clinical test case for commissioning of 192 Ir brachytherapy model-based dose calculation algorithms (MBDCAs) according to the AAPM TG-186 report workflow. A computational patient phantom model was generated from a clinical multi-catheter 192 Ir HDR breast brachytherapy case. Regions of interest (ROIs) were contoured and digitized on the patient CT images and the model was written to a series of DICOM CT images using MATLAB. The model was imported into two commercial treatment planning systems (TPSs) currently incorporating an MBDCA. Identical treatment plans were prepared using a generic 192 Ir HDR source and the TG-43-based algorithm of each TPS. This was followed by dose to medium in medium calculations using the MBDCA option of each TPS. Monte Carlo (MC) simulation was performed in the model using three different codes and information parsed from the treatment plan exported in DICOM radiation therapy (RT) format. Results were found to agree within statistical uncertainty and the dataset with the lowest uncertainty was assigned as the reference MC dose distribution. The dataset is available online at http://irochouston.mdanderson.org/rpc/BrachySeeds/BrachySeeds/index.html,https://doi.org/10.52519/00005. Files include the treatment plan for each TPS in DICOM RT format, reference MC dose data in RT Dose format, as well as a guide for database users and all files necessary to repeat the MC simulations. The dataset facilitates the commissioning of brachytherapy MBDCAs using TPS embedded tools and establishes a methodology for the development of future clinical test cases. It is also useful to non-MBDCA adopters for intercomparing MBDCAs and exploring their benefits and limitations, as well as to brachytherapy researchers in need of a dosimetric and/or a DICOM RT information parsing benchmark. Limitations include specificity in terms of radionuclide, source model, clinical scenario, and MBDCA version used for its preparation.

Development and validation of a scatter-corrected CBCT image-guided method for cervical cancer brachytherapy.

Multiple patient transfers have a nonnegligible impact on the accuracy of dose delivery for cervical cancer brachytherapy. We consider using on-site cone-beam CT (CBCT) to resolve this problem. However, CBCT clinical applications are limited due to inadequate image quality. This paper implements a scatter correction method using planning CT (pCT) prior to obtaining high-quality CBCT images and evaluates the dose calculation accuracy of CBCT-guided brachytherapy for cervical cancer. The CBCT of a self-developed female pelvis phantom and five patients was first corrected using empirical uniform scatter correction in the projection domain and further corrected in the image domain. In both phantom and patient studies, the CBCT image quality before and after scatter correction was evaluated with registered pCT (rCT). Model-based dose calculation was performed using the commercial package Acuros®BV. The dose distributions of rCT-based plans and corrected CBCT-based plans in the phantom and patients were compared using 3D local gamma analysis. A statistical analysis of the differences in dosimetric parameters of five patients was also performed. In both phantom and patient studies, the HU error of selected ROIs was reduced to less than 15 HU. Using the dose distribution of the rCT-based plan as the baseline, the γ pass rate (2%, 2mm) of the corrected CBCT-based plan in phantom and patients all exceeded 98% and 93%, respectively, with the threshold dose set to 3, 6, 9, and 12 Gy. The average percentage deviation (APD) of D90 of HRCTV and D2cc of OARs was less than 1% between rCT-based and corrected CBCT-based plans. Scatter correction using a pCT prior can effectively improve the CBCT image quality and CBCT-based cervical brachytherapy dose calculation accuracy, indicating promising prospects in both simplified brachytherapy processes and accurate brachytherapy dose delivery.

Technical note: Analysis of brachytherapy source movement by high-speed camera.

To assess the accuracy of the movement of a brachytherapy source using a high-speed camera for evaluating source position, dwell time, and transit dose. A high-speed camera was used to record the source position of an Ir-192 source relative to a ruler within a custom positioning jig in a remote afterloading system. The analyzed frames can be used to assess dwell positions and times. Treatment plans had multiple dwell times equal to 0.1, 0.5, 1.0, and 2.0 s in 2.5- and 5-mm step sizes. Images were acquired at a rate of 146 frames/s. Acquired images were processed to automatically track the actual source using the correlation between a template image and each frame. The brachytherapy dose calculation formalism (AAPM TG43-U1) was applied to each frame to evaluate the transit dose contribution to the total dose. The differences in measured source positions from the nominal for dwell times equal to 0.1, 0.5, 1.0, and 2.0 s in treatment plans were approximately ≤1mm. The corresponding differences in measured dwell times from the nominal values at 5 mm steps were -15, -9, -5, and 5 ms, respectively. The source velocities at 5mm steps were approximately 393mm/s. The dose differences at 5mm from the source movement with and without the transit dose for these dwell times were 38%, 7%, 3%, and 2%, respectively. Recording a brachytherapy source using a high-speed camera allowed the evaluation of positional and dwell time accuracies as well as dosimetry assessments, such as the transit dose, based on the application of AAPM TG-43U1.

The Investigation of Tissue Composition Effects on Dose Distributions Using Monte Carlo Method in Permanent Prostate Brachytherapy

Objective: Radiation dose calculations in the prostate brachytherapy practices have very high importance in terms of the success of treatment. The purpose of the present study is to determine whether there is a significant dose difference between the radiation dose calculations performed in water medium and prostate cancer-diagnosed patients by using the Monte Carlo method.
 Methods: The radiation dose calculations were performed on 20 prostate patients by using the BrachyDose Monte Carlo code. Phantom geometry derived from real patients computed tomography (CT) data was created to use in dose calculations. Water material was assigned to all voxels within the prostate volume for dose comparison with CT derived phantom. 125I (Amersham, OncoSeed, 6711), 103Pd (Theragenics Co., TheraSeed, 200) and 131Cs (IsoRay Medical) commercial brachytherapy seed models were used in dose calculations. 
 Results: It was observed that there are significant dose differences between the water medium and the prostate tissue. The differences between D90 dose values in prostate tissue and water medium were calculated as 7.2-10.5%, 9.1-13.4% and 5.4-8.3% for 125I, 103Pd and 131Cs brachytherapy seed sources, respectively. 
 Conclusions: It was concluded that material compositions of different organs and tissues in the human body should be considered for more accurate brachytherapy dose calculations.

A Monte Carlo dose calculation system for ophthalmic brachytherapy based on a realistic eye model.

There is a growing trend towards the adoption of model-based calculation algorithms (MBDCAs) for brachytherapy dose calculations which can properly handle media and source/applicator heterogeneities. However, most of dose calculations in ocular plaque therapy are based on homogeneous water media and standard in-silico ocular phantoms, ignoring non-water equivalency of the anatomic tissues and heterogeneities in applicators and patient anatomy. In this work, we introduce EyeMC, a Monte Carlo (MC) model-based calculation algorithm for ophthalmic plaque brachytherapy using realistic and adaptable patient-specific eye geometries and materials. We used the MC code PENELOPE in EyeMC to model Bebig IsoSeed I25.S16 seeds in COMS plaques and 106 Ru/106 Rh applicators that are coupled onto a customizable eye model with realistic geometry and composition. To significantly reduce calculation times, we integrated EyeMC with CloudMC, a cloud computing platform for radiation therapy calculations. EyeMC is equipped with an evaluation module that allows the generation of isodose distributions, dose-volume histograms, and comparisons with Plaque Simulator three-dimensional dose distribution. We selected a sample of patients treated with 125 I and 106 Ru isotopes in our institution, covering a variety of different type of plaques, tumor sizes, and locations. Results from EyeMC were compared to the original plan calculated by the TPS Plaque Simulation, studying the influence of heterogeneous media composition as well. EyeMC calculations for Ru plaques agreed well with manufacturer's reference data and data of MC simulations from Hermida et al. (2013). Significant deviations, up to 20%, were only found in lateral profiles for notched plaques. As expected, media composition significantly affected estimated doses to different eye structures, especially in the 125 I cases evaluated. Dose to sclera and lens were found to be about 12% lower when considering real media, while average dose to tumor was 9% higher. 106 Ru cases presented a 1%-3% dose reduction in all structures using real media for calculation, except for the lens, which showed an average dose 7.6% lower than water-based calculations. Comparisons with Plaque Simulator calculations showed large differences in dose to critical structures for 106 Ru notched plaques. 125 I cases presented significant and systematic dose deviations when using the default calculation parameters from Plaque Simulator version 5.3.8., which were corrected when using calculation parameters from a custom physics model for carrier-attenuation and air-interface correction functions. EyeMC is a MC calculation system for ophthalmic brachytherapy based on a realistic and customizable eye-tumor model which includes the main eye structures with their real composition. Integrating this tool into a cloud computing environment allows to perform high-precision MC calculations of ocular plaque treatments in short times. The observed variability in eye anatomy among the selected cases justifies the use of patient-specific models.

Optic Disc Dose Comparison Between 125I and 103Pd Collaborative Ocular Melanoma Study (COMS) Plaques Based on Current Clinical Practice.

PurposeThe purpose of this study is to compare optic disc dose (ODD) between 125I and 103Pd Collaborative Ocular Melanoma Study (COMS) plaques in ocular brachytherapy.MethodsA previously validated in-house brachytherapy dose calculation program was used for ODD calculations. ODD was calculated as a function of tumor margin-to-optic disc distance (DT) up to 5 mm for various tumor basal dimensions (BDs), for a prescription depth of 5 mm, and for standard and notched COMS plaques loaded with 125I (model: IAI-125A) and 103Pd (model: IAPd-103A) seeds. ODD calculations were repeated for prescription depths from 2 mm to 10 mm in 1 mm intervals. A prescribed dose of 85 Gy (irradiation time: 120 hours) was normalized to each prescription depth. Dose conversion factors (DCFs) for each prescription depth were calculated by taking a ratio of [total reference air kerma (TRAK) per seed]prescription depth to [TRAK per seed]5 mm. ODD reduction by notched COMS plaques was calculated for each prescription depth by subtracting ODD for notched COMS plaques from ODD for standard COMS plaques.ResultsTrends of ODD as a function of DT for various BDs are similar between the two seed types in both standard and notched COMS plaques. However, due to the energy difference, there exists a transition distance (dt) for each BD in each plaque at which ODD for 125I COMS plaques equals that for 103Pd COMS plaques. For small BDs, at DT <dt, ODD for 103Pd COMS plaques is higher than that for 125I COMS plaques while at DT >dt, the opposite is observed. For the largest 1-3 BD(s), contrarily, dt occurs within the tumor, and thus, ODD for 125I COMS plaquesis always higher. Trends of ODD reduction by notched COMS plaques as a function of DT for various BDs are the same for the two seed types except that maximum ODD reduction by 103Pd COMS notched plaques is larger. DCF increases with increasing prescription depth for both seed types.ConclusionsThere exist ODD differences between 125I and 103Pd COMS plaques and the differences depend on DT, BD, plaque size, and prescription depth.

PO09: Model-Based Dose Calculations for Post-Mastectomy Chestwall HDR Plesiotherapy: Should Bolus Be Used and How Much?

Purpose: The novel application of HDR plesiotherapy for post-mastectomy chestwall boost irradiation offers dosimetric advantages over conventional therapy via electrons due to the skin mould's ability to conform to highly curved and large surface areas while delivering a localized dose to a superficial target. Most brachytherapy dose calculation algorithms compute dose to homogenous water medium following the AAPM Task Group-43 (TG-43) formalism. For HDR plesiotherapy, TG-43-based dose calculation overestimates the coverage to a superficial target by ignoring the loss of backscatter at the target-air interface.

THUBrachy: fast Monte Carlo dose calculation tool accelerated by heterogeneous hardware for high-dose-rate brachytherapy

The Monte Carlo (MC) simulation is regarded as the gold standard for dose calculation in brachytherapy, but it consumes a large amount of computing resources. The development of heterogeneous computing makes it possible to substantially accelerate calculations with hardware accelerators. Accordingly, this study develops a fast MC tool, called THUBrachy, which can be accelerated by several types of hardware accelerators. THUBrachy can simulate photons with energy less than 3 MeV and considers all photon interactions in the energy range. It was benchmarked against the American Association of Physicists in Medicine Task Group No. 43 Report using a water phantom and validated with Geant4 using a clinical case. A performance test was conducted using the clinical case, showing that a multicore central processing unit, Intel Xeon Phi, and graphics processing unit (GPU) can efficiently accelerate the simulation. GPU-accelerated THUBrachy is the fastest version, which is 200 times faster than the serial version and approximately 500 times faster than Geant4. The proposed tool shows great potential for fast and accurate dose calculations in clinical applications.

A novel and fast methodology to calculate doses in LDR brachytherapy

We present the concept of a new methodology for faster simulation of the doses in brachytherapy with permanent implants, based on the knowledge of the seeds arrangement, adding previously simulated doses in an equivalent medium in terms of the atomic composition of the organ in question.To perform the doses calculations we use Monte Carlo simulations. We simulated a cylindrical I-125 seed and compared our results against published data. Our proposal is to have the doses simulated previously in different arrangement of seed-absorbents, and then, considering the spacial positions of the seeds after the implants, these doses can be directly added, obtaining a very fast computation of the total dose. Two phantoms of prostates with permanent implant seeds in 2D and 3D arrangements were simulated.The results of the proposed methodology were compared with two complete Monte Carlo simulations in 2D and 3D designs. Differences in doses were analysed, obtaining statistical discrepancies of less than 1% and reducing the simulation time by more than 4 orders of magnitude. With the proposed methodology, it is possible to perform rapid dose calculations in brachytherapy, using laptop or desktop computers.

Optic disc dose reduction in ocular brachytherapy using 125I notched COMS plaques: A simulation study based on current clinical practice

PurposeAlthough notched Collaborative Ocular Melanoma Study (COMS) plaques have been widely used, optic disc dose reduction by notched COMS plaques has not been discussed in the literature. Therefore, this study investigated optic disc dose reduction in ocular brachytherapy using 125I notched COMS plaques in comparison with optic disc dose for 125I standard COMS plaques.MethodsFor this simulation study, an in‐house brachytherapy dose calculation program was developed using MATLAB software by incorporating the American Association of Physicists in Medicine Task Group‐43 Update (AAPM TG‐43U1) dosimetry formalism with a line source approximation in a homogeneous water medium and COMS seed coordinates in the AAPM TG 129. Using this program, optic disc doses for standard COMS plaques (from 12 to 22 mm in diameter in 2 mm increments) and notched COMS plaques with one seed removed (Case #1, from 12 to 22 mm) and with two seeds removed (Case #2, from 14 to 22 mm) were calculated as a function of tumor margin‐to‐optic disc distance (DT) for various tumor basal dimensions (BDs) for prescription depths from 1 to 10 mm in 1 mm intervals. A dose of 85 Gy for an irradiation time of 168 h was prescribed to each prescription depth. Then absolute and relative optic disc dose reduction by notched COMS plaques (Cases #1 and #2) was calculated for all prescription depths.ResultsOptic disc dose reduction by notched COMS plaques (Cases #1 and #2) had five unique trends related to maximum optic disc dose reduction and corresponding optimal DT for each BD in each plaque. It increased with increasing prescription depth.ConclusionsThe results presented in this study would enable the clinician to choose an adequate plaque type among standard and notched 125I COMS plaques and a prescription depth to minimize optic disc dose.

Development of fast Monte Carlo code for high dose rate brachytherapy

Brachytherapy plays an important role in the treatment of various cancers. The TG-43 formalism is widely used in dose calculation for brachytherapy, but it is not accurate as it assumes the patient as homogeneously constituted of water. Monte Carlo simulation can obtain accurate result, but computing time using traditional code is too long to be accepted in clinical application. This work developed a fast Monte Carlo code THUBrachy for high dose rate brachytherapy dose calculation. THUBrachy can simulate photon with energy up to 3MeV for heterogeneous materials and obtain dose, fluence and dose distribution. It applies several accelerators such as Graphics Processing Unit (GPU), Intel Xeon Phi to perform dose calculation with good accuracy and high speed. THUBrachy adopts several parallel programming models to transplant the code onto different hardware accelerators. Voxel geometry based on CT images is used to describe the tissues of patient. A linear track-length estimator is used for dose calculation. In the preliminary accuracy test, a cuboid water model and a heterogeneous-material model were selected as the phantoms. Dose and fluence distributions were calculated by both THUBrachy and FLUKA which is a well-benchmarked Monte Carlo code. A total of 5×108 photons were simulated to make the uncertainty in all area less than 5%. The maxium difference of the results in low dose area is less than 3% and in high dose area is less than 1%, indicating a good agreement. In the performance test, a phantom consisted by 24 types of human tissue materials was chosen to estimate the performance in practical application. Dose distributions with an uncertainty of 2% or less for the target volume were obtained in less than 1 min by simulating 5×107 photons with a GTX 1080Ti, while FLUKA taking several hours to simulate the same number of particles. There is great potential for clinical application in brachytherapy dose planning using this fast Monte Carlo code.

A practical approach to estimating optic disc dose and macula dose without treatment planning in ocular brachytherapy using 125I COMS plaques

BackgroundIt has been reported that proximity of the tumor to the optic disc and macula, and radiation dose to the critical structures are substantial risk factors for vision loss following plaque brachytherapy. However, there is little dosimetry data published on this. In this study, therefore, the relationship between distance from tumor margin and radiation dose to the optic disc and macula in ocular brachytherapy using 125I Collaborative Ocular Melanoma Study (COMS) plaques was comprehensively investigated. From the information, this study aimed to allow for estimation of optic disc dose and macula dose without treatment planning.MethodsAn in-house brachytherapy dose calculation program utilizing the American Association of Physicists in Medicine Task Group-43 U1 formalism with a line source approximation in a homogenous water phantom was developed and validated against three commercial treatment planning systems (TPS). Then optic disc dose and macula dose were calculated as a function of distance from tumor margin for various tumor basal dimensions for seven COMS plaques (from 10 mm to 22 mm in 2 mm increments) loaded with commercially available 125I seeds models (IAI-125A, 2301 and I25.S16). A prescribed dose of 85 Gy for an irradiation time of 168 h was normalized to a central-axis depth of 5 mm. Dose conversion factors for each seed model were obtained by taking ratios of total reference air kerma per seed at various prescription depths (from 1 mm to 10 mm in 1 mm intervals) to that at 5 mm.ResultsThe in-house program demonstrated relatively similar accuracy to commercial TPS. Optic disc dose and macula dose decreased as distance from tumor margin and tumor basal dimension increased. Dose conversion factors increased with increasing prescription depth. There existed dose variations (<8%) among three 125I seed models. Optic disc dose and macula dose for each COMS plaque and for each seed model are presented in a figure format. Dose conversion factors for each seed model are presented in a tabular format.ConclusionsThe data provided in this study would enable clinicians in any clinic using 125I COMS plaques to estimate optic disc dose and macula dose without dose calculations.

Comparison of Two Commercially Available Model Based Dose Calculation Algorithms and TG-43 for APBI Brachytherapy

Brachytherapy dose calculation algorithms that account for heterogeneous patient geometries have recently become commercially available. However, it is as yet unknown how the treatment plans produced by these new algorithms will compare to the current clinical standard, TG-43 algorithm when applied to patients. Ten clinical APBI brachytherapy treatment plans were randomly chosen from our clinical database. These plans were recomputed with the Oncentra® Advanced Collapsed Cone Engine (ACE) version 4.5 and Acuros™ BV version 13 and compared to the current standard TG-43-based algorithms treated at our institution with a Strut-Adjusted Volume Implant (SAVI).

Physics elements of an algorithm for brachytherapy dose calculation in homogeneous media for 192Ir source

Unlike for external beam treatment, Model Based Dose Calculation Algorithms for Brachytherapy are still under development, and have been recently encouraged by the American Association of Physicists in Medicine. This delay is at least partially caused by the extreme complications associated with the multidimensional considerations underlying Brachytherapy dose calculations. These algorithms require detailed mathematical description, based on extensive radiation physics considerations, of the spatial and angular distribution of the points where the decay photon first interacts in matter, called TERMA points, along with the functions describing the radial and angular dose spread around each TERMA point, called Kernel. This work contributes to this global effort by providing these functions for the individual photon lines in the decay of the exactly modeled Nucletron microSelectron v3192Ir source. The underlying physics of all photon and electron interaction mechanisms has been extensively considered. Furthermore, the effect of the absorbing media has been also investigated. Ultimately, three-dimensional integration of dose Kernel contribution at a specified position, running all over TERMA points in the irradiated volume, has been conducted to generate the dose at that specific location. This methodology is much less time, data storage, and CPU consuming compared to repeating the extensive Monte Carlo simulations. The work is merely intended to provide a launching point and basic key physics elements for anticipated future quantitative clinical use.

Comparison of the effectiveness of polymer gel dosimeters (Magic and Pagatug) for organ dose calculation in brachytherapy, nuclear medicine and teletherapy

Purpose: To investigate and compare two polymer gel dosimeters, Magic and Pagatug, as organ dosimeters for 3D measurement of dose distribution in brachytherapy, nuclear medicine and teletherapy. Methods: Magic and Pagatug polymer gels were compared with soft tissue based on irradiation with low energy photons during therapeutic applications. Comparison was simulated using Monte-Carlobased MCNPX code. ORNL phantom–Female was used to model some vital organs (kidneys, ovaries and uterus). The right kidney was proposed to be the source of irradiation and the two organs were exposed to this irradiation. Results : The effective atomic numbers of soft tissue, Magic and Pagatug were 6.86134, 7.07 and 7.2884, respectively. The results showed that Magic and Pagatug, were comparable to soft tissue with regard to application in nuclear medicine and teletherapy. Differences between gel dosimeters and soft tissue were defined as the dose responses. This difference was < 8.1, < 4 and < 76.8 % for teletherapy, nuclear medicine and brachytherapy, respectively. Conclusion: Due to slight differences between the effective atomic numbers of these polymer gel dosimeters and soft tissue, the polymer gels are not suitable for brachytherapy since the photoelectric interaction is dominant for low energy photons, and the interaction relates to Z4. The results demonstrate that the gel dosimeters are best suited for nuclear medicine. Keywords : Magic, Pagatug, Brachytherapy, Nuclear medicine, Teletherapy, Organ dosimetry, Soft tissue

Dosimetric effects of saline- versus water-filled balloon applicators for IORT using the model S700 electronic brachytherapy source

PurposeThe Xoft Axxent Electronic Brachytherapy System (Xoft, Inc., San Jose, CA) is a viable option for intraoperative radiation therapy (IORT) treatment of early-stage breast cancer. The low-energy (50-kVp) X-ray source simplifies shielding and increases relative biological effectiveness but increases dose distribution sensitivity to medium composition. Treatment planning systems typically assume homogenous water for brachytherapy dose calculations, including precalculated atlas plans for Xoft IORT. However, Xoft recommends saline for balloon applicator filling. This study investigates dosimetric differences due to increased effective atomic number (Zeff) for saline (Zeff = 7.56) versus water (Zeff = 7.42). MethodsBalloon applicator diameters range from 3 to 6 cm. Monte Carlo N-Particle software is used to calculate dose at the surface (Ds) of and 1 cm away (D1cm) from the water-/saline-filled balloon applicator using a single dwell at the applicator center as a simple estimation of the dosimetry and multiple dwells simulating the clinical dose distributions for the atlas plans. ResultsSingle-dwell plans show a 4.4–6.1% decrease in Ds for the 3- to 6-cm diameter applicators due to the saline. Multidwell plans show similar results: 4.9% and 6.4% Ds decrease, for 4-cm and 6-cm diameter applicators, respectively. For the single-dwell plans, D1cm decreases 3.6–5.2% for the 3- to 6-cm diameter applicators. For the multidwell plans, D1cm decreases 3.3% and 5.3% for the 4-cm and 6-cm applicators, respectively. ConclusionsThe dosimetric effect introduced by saline versus water filling for Xoft balloon applicator–based IORT treatments is ∼5%. Users should be aware of this in the context of both treatment planning and patient outcome studies.

Abstract ID: 186 OpenTRAK: An OpenCL-based GPU Monte Carlo simulation code for Brachytherapy dose calculation

Introduction Monte Carlo simulation (MC) is a major type of model-based dose calculation algorithm (MBDCA) in Brachytherapy. However, due to the large number of primary beams (usually hundreds of millions or more) as needed to obtain small statistical errors, MC is usually not practical in clinical applications. Recently, there arise several NVIDIA CUDA-based graphic processing unit (GPU) MC codes that accelerate the simulation such that they suffice clinical requirements in certain situations. In this work, an Open-CL-based MC code that targets Brachytherapy dose calculations, OpenTRAK, was presented. Methods OpenTRAK implements all photon interactions (photoelectric effect, compton scatter, rayleigh scatter and pair-production) and computes the spatial energy-fluence distribution. The linear track estimator was then used to calculate the collisional kerma (Dm,m) distribution in phantom. OpenTRAK was preliminarily benchmarked against ALGEBRA, a GEant4-based BRAchytherapy dose calculation code [1] , by using a MicroSelectronV2 Ir-192 HDR source. The source phasespace files had been prepared before dose calculations and were used as input for both MC codes. As an preliminary test, a single source was placed in the centre of a uniform water phantom with dimension of 200 × 200 × 200 mm3 with its longitudinal axis aligned with the z-axis of the phantom. Dose ratio map ( D OpenTRAK / D ALGEBRA ) was generated and used for comparison analysis. The codes were run in a Mac Pro with two AMD FirePro D700 GPUs (for OpenTRAK) and a 2.7 GHz 12-Core Intel Xeon E5 CPU (ALGEBRA). Results The ratio map shows that the codes are consistent with each other with Conclusion An OpenCL-based MC code for brachytherapy dose calculation was implemented and preliminarily tested. The code itself needs further fine-tuning/corrections in order to maximize its GPU acceleration. Further benchmarks are planned including source TD-43 parameters extraction, real patient test cases, etc.

Abstract ID: 114 The impact of dual-energy CT tissue segmentation for low-dose rate prostate brachytherapy Monte Carlo dose calculations

Purpose To evaluate the impact of a novel tissue segmentation method based on dual-energy CT (DECT) for low-dose rate (LDR) brachytherapy dose calculations, by comparison with a reference single-energy CT (SECT) segmentation method. Methods A virtual patient geometry is created using the DICOM-RT of a real patient pelvis SECT scan, where known elemental compositions and varying densities are overwritten in each voxel to define a reference phantom. Simulated CT images are generated using XCOM attenuation coefficients, with a 100 kVp spectrum for SECT, and 80 and 140Sn kVp for DECT. Tissue segmentations for Monte Carlo (MC) dose calculations are performed with both SECT and DECT and compared with the reference geometry. For SECT, the method of Schneider et al. [1] is used and for DECT, the eigentissue decomposition of Lalonde & Bouchard [2] used in combination with a Bayesian estimator. A LDR prostate brachytherapy treatment is planned with 125I sources and calculated using the MC code Brachydose for all three cases. Dose distributions and dose-volume histograms (DVH) are compared to the reference dose distribution to investigate the accuracy of the tissue segmentation methods. Results For noiseless images, DECT-based tissue segmentation outperforms the SECT procedure with a relative dose distribution root mean square error (RMSE) of 3.08% versus 8.02%, and provides DVH closest to the reference for all tissues. For a medium level of noise (12 HU), Bayesian eigentissue decomposition performs better with a dose calculation RMSE of 6.11% and 8.49% for DECT and SECT, respectively. Both methods yield similar DVHs for the prostate while DECT segmentation remains more accurate for organs at risk. Conclusion Our study shows that DECT-based tissue segmentation has the potential to provide LDR brachytherapy dose distributions with higher accuracy that conventional SECT in a clinical context, even in the presence of noise.