Carlo Based Treatment Planning System Research Articles

169Yb-based high dose rate intensity modulated brachytherapy for focal treatment of prostate cancer

PurposeThis study compares conventional 192Ir-based high dose rate brachytherapy (HDR-BT) with 169Yb-based HDR intensity modulated brachytherapy (IMBT) for focal prostate cancer treatment. Additionally, the study explores the potential to generate less invasive treatment plans with IMBT by reducing the number of catheters needed to achieve acceptable outcomes. Methods and MaterialsA retrospective dosimetric study of ten prostate cancer patients initially treated with conventional 192Ir-based HDR-BT and 5-14 catheters was employed. RapidBrachyMCTPS, a Monte Carlo-based treatment planning system was used to calculate and optimize dose distributions. For 169Yb-based HDR IMBT, a custom 169Yb source combined with 0.8 mm thick platinum shields placed inside 6F catheters was used. Furthermore, dose distributions were investigated when iteratively removing catheters for less invasive treatments. ResultsWith IMBT, the urethra D10 and D0.1cc decreased on average by 15.89 and 15.65 percentage points (pp) and the rectum V75 and D2cc by 1.53 and 11.54 pp, respectively, compared to the conventional clinical plans. Similar trends were observed when the number of catheters decreased. On average, there was an observed increase in PTV V150 from 2.84 pp with IMBT when utilizing all catheters to 8.83 pp when four catheters were removed. PTV V200 increased from 0.42 to 2.96 pp on average. Hotspots in the body were however lower with IMBT compared to conventional clinical plans. Conclusions169Yb-based HDR IMBT for focal treatment of prostate cancer has the potential to successfully deliver clinically acceptable, less invasive treatment with reduced dose to organs at risk.

RapidBrachyTG43: A Geant4-based TG-43 parameter and dose calculation module for brachytherapy dosimetry.

The AAPM TG-43U1 formalism remains the clinical standard for dosimetry of low- and high-energy -emitting brachytherapy sources. TG-43U1 and related reports provide consensus datasets of TG-43 parameters derived from various published measured data and Monte Carlo simulations. These data are used to perform standardized and fast dose calculations for brachytherapy treatmentplanning. Monte Carlo TG-43 dosimetry parameters are commonly derived to characterize novel brachytherapy sources. RapidBrachyTG43 is a module of RapidBrachyMCTPS, a Monte Carlo-based treatment planning system, designed to automate this process, requiring minimal user input to prepare Geant4-based Monte Carlo simulations for a source. RapidBrachyTG43 may also perform a TG-43 dose to water-in-water calculation for a plan, substantially accelerating the same calculation performed using RapidBrachyMCTPS's Monte Carlo dose calculationengine. TG-43 parameters , , , and were calculated using three commercial source models, one each of I, Ir, and Co, and were benchmarked to published data. TG-43 dose to water was calculated for a clinical breast brachytherapy plan and was compared to a Monte Carlo dose calculation with all patient tissues, air, and catheters set towater. TG-43 parameters for the three simulated sources agreed with benchmark datasets within tolerances specified by the High Energy Brachytherapy Dosimetry working group. A gamma index comparison between the TG-43 and Monte Carlo dose-to-water calculations with a dose difference and difference to agreement criterion of 1%/1 mm yielded a 98.9% pass rate, with all relevant dose volume histogram metrics for the plan agreeing within 1%. Performing a TG-43-based dose calculation provided an acceleration of dose-to-water calculation by a factor of165. Determination of TG-43 parameter data for novel brachytherapy sources may now be facilitated by RapidBrachyMCTPS. These parameter datasets and existing consensus or published datasets may also be used to determine the TG-43 dose for a plan inRapidBrachyMCTPS.

Transmission probability filter optimization for Agility MLC in Monaco treatment planning system.

In the Monte Carlo-based treatment planning system (TPS) Monaco, transmission probability filters (TPF) are utilized to describe the transmission through the multi leaf collimator (MLC). By having knowledge of the TPF parameters for various photon beam energies, adjusting the MLC transmission parameters becomes easier, enhancing the accuracy of the Monte Carlo algorithm in achieving a dose distribution that closely aligns with the irradiated dose at the Versa HD linear accelerator (linac). The objective of this study was to determine the TPF parameters for 6MV, 10MV, 6MV flattening filter free (FFF) and 10MV FFF for a Versa HD linac equipped with Agility MLC. The TPF parameters were adjusted using point dose measurements and vendor-provided fields specifically designed to fine-tune the MLC. After adjusting the TPF parameters, a gamma passing rate (GPR) analysis was conducted on 25 treatment plans to ensure that the Monte Carlo model, with the updated TPF parameters, accurately matched the actual linac delivery. The TPF values ranged from 0.0018 to 0.0032 for leaf transmission and 1.15 to 1.25 for Leaf Tip leakage across the different energies. The average GPR ranged from 97.8% for 10MV FFF to 98.5% for 6MV photon energies. Additionally, the TPF parameters for 6MV obtained in this study were consistent with previously published TPF values for 6MV photon energy. Hence, it was concluded that optimizing the TPF does not need to be performed for every individual Versa HD linac with Agility MLC. Instead, the published parameters can be applied to other Versa HD linacs to enhance clinical accuracy. In conclusion, this study determined the TPF parameters for 6MV and previously unpublished photon energies 10MV, 6MV FFF and 10MV FFF. These parameters can be easily transferred to other facilities, resulting in improved agreement between the dose distribution from the TPS and the linac.

Correction factors for commissioning and patient specific quality assurance of stereotactic fields in a Monte Carlo based treatment planning system

Validation of small field dosimetry is crucial for stereotactic radiosurgery (SRS) and stereotactic body radiotherapy (SBRT). Accurate and considered measurement of linear accelerator dose must be compared to precise and accurate calculation by the treatment planning system (TPS). Monte Carlo calculated distributions contain statistical noise, reducing the reliance that should be given to single voxel doses. The average dose to a small volume of interest (VOI) can minimise the influence of noise, but for small fields introduces significant volume averaging. Similar challenges present during measurement of composite dose from clinical plans when a small volume ionisation chamber is used. This study derived correction factors for VOI averaged TPS doses calculated for small fields, allowing correction to an isocentre dose following account for statistical noise. These factors were used to determine an optimal VOI to represent small volume ionisation chambers during patient specific quality assurance (PSQA). A retrospective comparison of 82 SRS and 28 SBRT PSQA measurements to TPS calculated doses from varying VOI was completed to evaluate the determined volumes. Small field commissioning correction factors of under 5% were obtained for field sizes of 8 mm and larger. Optimal spherical VOI with radius between 1.5 and 1.8 mm and 2.5 to 2.9 mm were determined for IBA CC01 and CC04 ionisation chambers respectively. Review of PSQA confirmed an optimal agreement between CC01 measured doses and a volume of 1.5 to 1.8 mm while CC04 measured doses showed no variation with VOI.

Development of Monte Carlo based treatment planning system for BNCT

A treatment planning system (TPS) plays a major role for a successful radiotherapy, directly influencing the calculated dose accuracy, and thus the treatment quality. Multifunctional Arithmetic for Neutron Transportation Analysis (NeuMANTA), a TPS especially designed for complete workflow for BNCT, which aims to provide high precision dose calculation with maximizing the efficacy. The dedicated MC engine COMPASS seamlessly embedded in NeuMANTA can be called for dose distribution calculation in background. Optimized cross-section library, parallel computing, and various variance reduction techniques are applied to speed up the particle transportations. A benchmark was performed between NeuMANTA and MCNP6, and results agreed well with no significant statistical difference, however, NeuMANTA needs only half of the calculation time. The accuracy of estimated doses, and complete treatment workflow are the main features in NeuMANTA.

GPU-accelerated Monte Carlo-based online adaptive proton therapy: A feasibility study.

To develop an online graphic processing unit (GPU)-accelerated Monte Carlo-based adaptive radiation therapy (ART) workflow for pencil beam scanning (PBS) proton therapy to address interfraction anatomical changes in patients treated with PBS. A four-step workflow was developed using our in-house developed GPU-accelerated Monte Carlo-based treatment planning system to implement online Monte Carlo-based ART for PBS. The first step conducts diffeomorphic demon-based deformable image registration (DIR) to propagate contours on the initial planning CT (pCT) to the verification CT (vCT) to form a new structure set. The second step performs forward dose calculation of the initial plan on the vCT with the propagated contours after manual approval (possible modifications involved). The third step triggers a reoptimization of the plan depending on whether the verification dose meets the clinical requirements or not. A robust evaluation will be done for both the verification plan in the second step and the reopotimized plan in the third step. The fourth step involves a two-stage (before and after delivery) patient-specific quality assurance (PSQA) of the reoptimized plan. The before-delivery PSQA is to compare the plan dose to the dose calculated using an independent fast open-source Monte Carlo code, MCsquare. The after-delivery PSQA is to compare the plan dose to the dose recalculated using the log file (spot MU, spot position, and spot energy) collected during the delivery. Jaccard index (JI), dice similarity coefficients (DSCs), and Hausdorff distance (HD) were used to assess the quality of the propagated contours in the first step. A commercial plan evaluation software, ClearCheck™, was integrated into the workflow to carry out efficient plan evaluation. 3D Gamma analysis was used during the fourth step to ensure the accuracy of the plan dose from reoptimization. Three patients with three different disease sites were chosen to evaluate the feasibility of the online ART workflow for PBS. For all three patients, the propagated contours were found to have good volume conformance [JI (lowest-highest: 0.833-0.983) and DSC (0.909-0.992)] but suboptimal boundary coincidence [HD (2.37-20.76mm)] for organs-at-risk. The verification dose evaluated by ClearCheck™ showed significant degradation of the target coverage due to the interfractional anatomical changes. Reoptimization on the vCT resulted in great improvement of the plan quality to a clinically acceptable level. 3D Gamma analyses of PSQA confirmed the accuracy of the plan dose before delivery (mean Gamma index=98.74% with a threshold of 2%/2mm/10%), and after delivery based on the log files (mean Gamma index=99.05% with a threshold of 2%/2mm/10%). The average time cost for the complete execution of the workflow was around 858s, excluding the time for manual intervention. The proposed online ART workflow for PBS was demonstrated to be efficient and effective by generating a reoptimized plan that significantly improved the plan quality.

Performance evaluation of a Monte Carlo-based treatment planning system in out-of-field dose estimation during dynamic IMRT with different dose rates

AimThe present study aimed to investigate the effect of dose rate variations in the out-of-field regions on the dose calculation accuracy of an algorithm based on the Monte Carlo (MC) method implemented in Monaco (V.5.11.03) Treatment Planning System (TPS). Materials and methodsNine fields of the IMRT plan for the CShape structure were created in the CIRS IMRT Thorax phantom, and the related doses were calculated using the MC-based algorithm of the Monaco TPS. Afterward, the CIRS IMRT Thorax phantom was irradiated by an Elekta Synergy linear accelerator and the 160-leaf MLC Agility 6 MV photon beams using the Dynamic Intensity Modulated Radiation Therapy (D-IMRT) technique. To this end, 3 ion chambers were inserted in 3 cavities, which were equivalent to water, bone, and lung, and were located in the out-of-field regions within the phantom. Finally, the dose measurements were performed, and the values were compared with the doses calculated by the Monaco TPS. ResultsFindings revealed that the Monaco TPS underestimated the dose by an average of 40% for most of the out-of-field measurement distances. Moreover, at distant positions of the out-of-field edge, out-of-field doses were significantly underestimated for all media and dose rates. Finally, the percentage of the TPS error and out-of-field dose values were different for each dose rate. ConclusionsResults in the present research showed remarkable differences between the measured and calculated doses. Thus, it was concluded that the dose calculation accuracy of the Monaco TPS was not reliable for out-of-field regions at different dose rates, and the application of the current commercial treatment planning systems in these regions could lead to underestimating the risk of secondary cancer development.

Beam modeling and commissioning for Monte Carlo photon beam on an Elekta Versa HD LINAC

PurposeThis study aims at analyzing beam data commissioning along with verifying Monte Carlo-based treatment planning system on the basis of the manufacturer guidelines for Elekta Versa HD Linear Accelerator. Moreover, evaluating the beam match process in terms of quality index, photon profile and multi leaf collimator (MLC) offset is aimed as well. Materials and methodsThe process of collecting beam data for Monaco 5.51 Treatment Planning System commissioning was done based on the instructions provided by the manufacturer as well as AAPM TG-106. Monte Carlo analysis was done for output factors in water, percent depth dose and beam profiles. A set of eight static and intensity modulated radiation therapy fields were used to verify the MLC parameters. ResultsThe difference between the measured and modeled penetrative quality (D10) was achieved to be 0.54%. The output factors for 6 MV photon energy were measured and the difference between the measured and Monte Carlo output results was smaller than 1% for all the fields. The average percentage of passing the gamma criteria for commissioning test fields was (95+-4)%, however, the minimum agreement was 87.5% belonging to “7SEGA”. Additionally, the agreement between both LINAC is 96%, however, the second LINAC reveals a positive offset in the point of approximately −4 cm on the x-axis at the crossplane. ConclusionTest commissioning was successfully verified using a homogeneous phantom for point dose measurements, post modelling MLC parameters and patient QA plans. All plan parameters pass the gamma criteria. 6 MV photon beam was successfully commissioned for Elekta VersaHD LINAC and is ready for clinical implementation.

OL67 - Dosimetric validation of a Monte Carlo based treatment planning system for pencil beam scanning proton therapy treatments in low density tissues

OL67 - Dosimetric validation of a Monte Carlo based treatment planning system for pencil beam scanning proton therapy treatments in low density tissues

Comparison of Skin Dose for Brain Cancer Patients Between a 1.5 T MR-Linac and a Conventional Linac

The MR-Linac (MRL) is a new technology, and due to the presence of the electron return effect (ERE) elicited by the strong magnetic field, there is concern about there being elevated surface dose due to the ERE. The hypothesis is that the skin doses for the MRL is greater than for the conventional Linac. Six brain cancer patients being treated on our MRL were included in the present study and were planned with an IMRT technique using the MRL’s Monte Carlo based treatment planning system (TPS-MRL). Each patient had a back-up IMRT 6 MV plan for a conventional (no magnetic field) Linac generated on our standard TPS, which used a collapsed cone-based dose engine (TPS-conventional). These back-up plans were clinically delivered during an MRL preventative maintenance day, which provided an opportunity to compare skin dose measurements between MRL and conventional Linac. For one fraction on each Linac, an optically stimulated luminescence dosimeter (OSLD) was placed at the same location on the skin proximal to the target. Within TPS-MRL and TPS-conventional, a dose point was created at the OSLD location and at a skin depth equivalent to the water equivalent thickness of the OSLD (0.6 mm). Five of six patients had higher OSLD readings on the MRL than for the conventional Linac, ranging from 11.5%-24.8% higher (the one exception had the MRL 7.5% less than the conventional Linac). The OSLD measurement for the MRL ranged from 11.8-45.7 Gy; the OSLD doses for the conventional Linac ranged from 10.0-39.4 Gy. The root mean square error between measurement and TPS was 7.3% and 17.4% for MRL and conventional, respectively. This may be explained because the conventional Linac TPS uses a superposition-convolution algorithm, and the MRL TPS uses a theoretically more accurate Monte Carlo algorithm. Skin dose, as determined in the present study with in vivo OSLD measurements, were higher on the MRL than a conventional Linac for brain patients. Furthermore, the skin dose as estimated by the TPS-MRL is more accurately modeled compared to the TPS-conventional. Although the exact cause(s) of the apparent skin dose enhancement can only be speculated, the potential implications of the measurements in the present study include enhanced adverse skin reactions on the MRL compared to conventional Linac. However, the fact that the MRL-TPS models the surface dose with a Monte Carlo dose engine can enable clinicians to incorporate strategies for reducing surface dose at the planning stage.

Multimodal Monte Carlo treatment system capable of microdosimetry with PHITS

Boron neutron capture therapy (BNCT) is a radiation therapy that combines neutrons and boron drugs. An industry-academia-government collaboration team is currently developing the linac-based treatment device and several peripheral devices to establish this therapy. In the project, a demonstration device, “iBNCT001”, for a linac-based neutron source applicable to BNCT treatment is being developed. We are developing Tsukuba Plan, a multimodal Monte Carlo-based treatment planning system, in addition to the linac-based neutron source device. Tsukuba-Plan has adopted a Particle and Heavy Ion Transport code System (PHITS) as the Monte Carlo-based dose calculation engine. PHITS has a microdosimetric function that can compute a lineal energy distribution and determine relative biological effectiveness through combination with the stochastic microdosimetric kinetic model parameters. By utilising these functions, Tsukuba-Plan is expected to estimate the weighted doses administered to the tumour region and normal tissues in the irradiation field more accurately.

Experimental validation of Monte Carlo based treatment planning system in bone density equivalent media.

IntroductionAdvanced, Monte Carlo (MC) based dose calculation algorithms, determine absorbed dose as dose to medium-in-medium (Dm,m) or dose to water-in-medium (Dw,m). Some earlier studies identified the differences in the absorbed doses related to the calculation mode, especially in the bone density equivalent (BDE) media. Since the calculation algorithms built in the treatment planning systems (TPS) should be dosimetrically verified before their use, we analyzed dose differences between two calculation modes for the Elekta Monaco TPS. We compared them with experimentally determined values, aiming to define a supplement to the existing TPS verification methodology.Materials and methodsIn our study, we used a 6 MV photon beam from a linear accelerator. To evaluate the accuracy of the TPS calculation approaches, measurements with a Farmer type chamber in a semi-anthropomorphic phantom were compared to those obtained by two calculation options. The comparison was made for three parts of the phantom having different densities, with a focus on the BDE part.ResultsMeasured and calculated doses were in agreement for water and lung equivalent density materials, regardless of the calculation mode. However, in the BDE part of the phantom, mean dose differences between the calculation options ranged from 5.7 to 8.3%, depending on the method used. In the BDE part of the phantom, neither of the two calculation options were consistent with experimentally determined absorbed doses.ConclusionsBased on our findings, we proposed a supplement to the current methodology for the verification of commercial MC based TPS by performing additional measurements in BDE material.

A Multimodality Image Guided Precision Radiation Research Platform: Integrating X-ray, Bioluminescence, and Fluorescence Tomography With Radiation Therapy

Small animal irradiation is crucial to the investigation of radiobiological mechanisms. The paradigm of clinical radiation therapy is trending toward high-precision, stereotactic treatment. However, translating this scheme to small animal irradiation is challenging owing to the lack of high-quality image guidance. To overcome this obstacle, we developed a multimodality image guided precision radiation platform. The platform consists of 4 modules: x-ray computed tomography (CT), bioluminescence tomography (BLT), fluorescence molecular tomography (FMT), and radiation therapy. CT provides animal anatomy and material density for radiation dose calculation, as well as body contour for BLT and FMT reconstruction. BLT and FMT provide tumor localization to guide radiation beams and molecular activity to evaluate treatment outcome. Furthermore, we developed a Monte Carlo-based treatment planning system (TPS) for 3-dimensional dose calculation, calibrated it using radiochromic films sandwiched in a water-equivalent phantom, and validated it using invivo dosimeters surgically implanted into euthanized mice (n = 4). Finally, we performed image guided irradiation on mice bearing orthotopic breast and prostate tumors and confirmed radiation delivery using γH2AX histology. The Monte Carlo-based TPS was successfully calibrated by benchmarking simulation dose against film measurement. For invivo dosimetry measured in the euthanized mice, the average difference between the TPS calculated dose and measured dose was 3.86% ± 1.12%. Following the TPS-generated treatment plan, we successfully delivered 20 Gy dose to an animal bearing an orthotopic prostate tumor using 4 BLT-guided radiation beams and 5 Gy dose to an animal bearing an orthotopic breast tumor using a single FMT-guided radiation beam. γH2AX histology presented significantly more DNA damage in irradiated tumors and thus validated the dose delivery accuracy. Combined with Monte Carlo TPS, this multimodality CT/BLT/FMT image guided small animal radiation platformcan specifically localize tumors, accurately calculate dose distribution, precisely guide radiation delivery, and molecularly evaluate treatment response. It provides an advanced toolset for radiobiology and translational cancer research.

Small animal irradiator dose distribution verification using radioluminescence imaging.

The main objective of this work was the development of a novel 2D dosimetry approach for small animal external radiotherapy using radioluminescence imaging (RLI) with a commercial complementary metal oxide semiconductor detector. Measurements of RLI were performed on the small animal image-guided platform SmART, RLI data were corrected for perspective distortion using Matlab. Four irradiation fields were tested and the planar 2D dose distributions and dose profiles were compared against dose calculations performed with a Monte Carlo based treatment planning system and gafchromic film. System linearity and RLI image noise against dose were also measured. The maximum difference between beam size measured with RLI and nominal beam size was less than 8% for all the tested beams. The image correction procedure was able to reduce perspective distortion. A novel RLI approach for quality assurance of a small animal irradiator was presented and tested. Results are in agreement with MC dose calculations and gafchromic film measurements.

Impact of magnetic fields on calculated AAPM TG-43 parameters for 192Ir and 60Co HDR brachytherapy sources: A Monte Carlo study

PurposeThe aim of this work is to investigate the influence of an external magnetic field (MF) on The American Association of Physicists in Medicine (AAPM) No. 43 Report (TG-43) parameters for 192Ir and 60Co high dose rate (HDR) brachytherapy sources using Monte Carlo (MC) simulation methods. Materials and methodsWe used the Geant4 toolkit (version 10.1. p01) to simulate the geometry of 192Ir and 60Co brachytherapy sources. AAPM TG-43 parameters (the radial dose function, g(r), and the anisotropy function, F (r, θ)) of both 192Ir and 60Co sources were calculated in the presence of a magnetic field with strengths of 1.5T, 3T, and 7T in the X, Y, and Z directions in a voxelized water phantom. ResultsFor the 192Ir source, the calculated values g(r) and F (r, θ) remained nearly unaffected by the magnetic field for all investigated strengths.For the 60Co source, the differences for the g(r) and F (r,θ) under the 1.5T, 3T, and 7T magnetic field strengths along the direction parallel with the MF were found to be an increase of up to 5%, 15%, and 33%, respectively. However, for the directions perpendicular with the magnetic field, there was a decrease of up to 3%, 6% and 15% under 1.5T, 3T and 7T strengths, respectively. ConclusionOur results highlight the necessity of a Monte Carlo-based treatment planning system (TPS) if cobalt HDR treatments are performed under a magnetic field, especially for strengths greater than 1.5T.

Exposure of remote organs and associated cancer risks from tangential and multi-field breast cancer radiotherapy.

With the ever-increasing cure rates in breast cancer, radiotherapy-induced cancers have become an important issue. This study aimed to estimate secondary cancer risks for different treatment techniques, taking into account organs throughout the body. Organ doses were evaluated for atangential three-dimensional conformal (3D-CRT) and amulti-field intensity-modulated radiotherapy (IMRT) plan using avalidated, Monte Carlo-based treatment planning system. Effects of wedges and of forward versus inverse planning were systematically investigated on the basis of phantom measurements. Organ-specific cancer risks were estimated using risk coefficients derived from radiotherapy patients or from the atomic bomb survivors. In the 3D-CRT plan, mean organ doses could be kept below 1 Gy for more remote organs than the lung, heart, and contralateral breast, and decreased to afew cGy for organs in the lower torso. Multi-field IMRT led to considerably higher mean doses in organs at risk, the difference being higher than 50% for many organs. Likewise, the peripheral radiation burden was increased by external wedges. No difference was observed for forward versus inverse planning. Despite the lower doses, the total estimated secondary cancer risk in more remote organs was comparable to that in the lung or the contralateral breast. For multi-field IMRT it was 75% higher than for 3D-CRT without external wedges. Remote organs are important for assessment of radiation-induced cancer risk. Remote doses can be reduced effectively by application of atangential field configuration and alinear accelerator set-up with low head scatter radiation.

RapidBrachyMCTPS: a Monte Carlo-based treatment planning system for brachytherapy applications

Despite being considered the gold standard for brachytherapy dosimetry, Monte Carlo (MC) has yet to be implemented into a software for brachytherapy treatment planning. The purpose of this work is to present RapidBrachyMCTPS, a novel treatment planning system (TPS) for brachytherapy applications equipped with a graphical user interface (GUI), optimization tools and a Geant4-based MC dose calculation engine, RapidBrachyMC. Brachytherapy sources and applicators were implemented in RapidBrachyMC and made available to the user via a source and applicator library in the GUI. To benchmark RapidBrachyMC, TG-43 parameters were calculated for the microSelectron v2 (192Ir) and SelectSeed (125I) source models and were compared against previously validated MC brachytherapy codes. The performance of RapidBrachyMC was evaluated for a prostate high dose rate case. To assess the accuracy of RapidBrachyMC in a heterogeneous setup, dose distributions with a cylindrical shielded/unshielded applicator were validated against film measurements in a Solid WaterTM phantom. TG-43 parameters calculated using RapidBrachyMC generally agreed within 1%–2% of the results obtained in previously published work. For the prostate case, clinical dosimetric indices showed general agreement with Oncentra TPS within 1%. Simulation times were on the order of minutes on a single core to achieve uncertainties below 2% in voxels within the prostate. The calculation time was decreased further using the multithreading features of Geant4. In the comparison between MC-calculated and film-measured dose distributions, at least 95% of points passed the 3%/3 mm gamma index criteria in all but one case. RapidBrachyMCTPS can be used as a post-implant dosimetry toolkit, as well as for MC-based brachytherapy treatment planning. This software is especially well suited for the development of new source and applicator models.

DEVELOPMENT OF A MULTIMODAL MONTE CARLO BASED TREATMENT PLANNING SYSTEM.

To establish boron neutron capture therapy (BNCT), the University of Tsukuba is developing a treatment device and peripheral devices required in BNCT, such as a treatment planning system. We are developing a new multimodal Monte Carlo based treatment planning system (developing code: Tsukuba Plan). Tsukuba Plan allows for dose estimation in proton therapy, X-ray therapy and heavy ion therapy in addition to BNCT because the system employs PHITS as the Monte Carlo dose calculation engine. Regarding BNCT, several verifications of the system are being carried out for its practical usage. The verification results demonstrate that Tsukuba Plan allows for accurate estimation of thermal neutron flux and gamma-ray dose as fundamental radiations of dosimetry in BNCT. In addition to the practical use of Tsukuba Plan in BNCT, we are investigating its application to other radiation therapies.

IMBrachy: A Novel Intensity Modulated High Dose Rate Brachytherapy Delivery System with 169Yb

Conventional brachytherapy often results in less than ideal tumor dose conformity due to the non-symmetrical shape of the tumors, resulting in dose spillage to radiation sensitive healthy tissues. Intensity modulated brachytherapy (IMBT) can dynamically direct the radiation towards the tumor and away from healthy tissues by incorporating partially-shielded brachytherapy sources. The hypothesis is that IMBT can reduce the dose spillage to organs at risk in sites treated with conventional brachytherapy and thereby reduce numerous side effects. IMBrachy is a novel delivery system for IMBT which can incorporate the intermediate energy brachytherapy source 169Yb available from SPECMed. IMBrachy is standalone and can be connected to any commercial afterloader as an add-on device. Specifically, the device dynamically controls the rotation of shielded catheters. The device is compatible with both interstitial and intracavitary applicators. As in conventional brachytherapy, the afterloader is responsible for the motion of the source through the source guide and within the catheter. The device is divided in three main systems: a rotating system, a link assembly and a shield assembly. The rotation of the shield is controlled through a series of moving panels with an interlock system. Each panel is connected to a motor which handles the rotation of a subset of needles. The shielded needles are connected to the rotating mechanism through flexible locking luers that will give radiation oncologists the opportunity to implant the needles at an angle. The motors are equipped with controller sensors that control the motor position. A second set of sensors placed directly on the shield, or at the end of the link assembly, reads the actual position of the shield and provides feedback to the controller sensors. As a proof of principle, an IMBT treatment plan for a prostate cancer patient was simulated using a Monte Carlo based treatment planning system to show the potential advantages of the IMBrachy system. The Wilcoxon signed rank test was used to check for significant differences (p < 0.05) between the dose-volume histogram points. Validation of the functionality of the delivery system, the stability of the connection between the source guide and the shielded catheters, and the resistance to wear of the plastic needle during use has been completed. For the same minimum dose to the hottest 90 % of the planning target volume (PTV), the IMBT plan resulted in a statistically significant reduction in the bladder V75, rectum V75, and urethral D10 by 10 %, 8 %, and 15 %, respectively, while improving the homogeneity within the target (PTV V150 was reduced by 25 %). Delivering IMBT using the IMBrachy device has the potential to reduce radiation sensitive normal tissue toxicity and improve brachytherapy treatment outcomes.

Technical Note: System for evaluating local hypothermia as a radioprotector of the rectum in a small animal model.

The protective effects of induced or even accidental hypothermia on the human body are widespread with several medical uses currently under active research. In vitro experiments using human cell lines have shown hypothermia provides a radioprotective effect that becomes more pronounced at large, single-fraction doses common to stereotactic body radiotherapy (SBRT) and stereotactic radiosurgery (SRS) treatments. This work describes the development of a system to evaluate local hypothermia for a radioprotective effect of the rat rectum during a large dose of radiation relevant to prostate SBRT. This includes the evaluation of a 3D-printed small animal rectal cooling device and the integration with a small animal irradiator. A 3-cm long, dual-lumen rectal temperature control apparatus (RTCA) was designed in SOLIDWORKS CAD for 3D printing. The RTCA was capable of recirculating flow in a device small enough for insertion into the rat rectum, with a metal support rod for strength as well as visibility during radiation treatment planning. The outer walls of the RTCA comprised of thin heat shrink plastic, achieving efficient heat transfer into adjacent tissues. Following leak-proof testing, fiber optic temperature probes were used to evaluate the temperature over time when placed adjacent to the cooling device within the rat rectum. MRI thermometry characterized the relative temperature distribution in concentric ROIs surrounding the probe. Integration with an image-guided small animal irradiator and associated treatment planning system included evaluation for imaging artifacts and effect of brass tubing on dose calculation. The rectal temperature adjacent to the cooling device decreased from body temperature to 15°C within 10-20min from device insertion and was maintained at 15±3°C during active cooling for the evaluated time of one hour. MR thermometry revealed a steep temperature gradient with increasing distance from the cooling device with the desired temperature range maintained within the surrounding few millimeters. A 3D-printed rectal cooling device was fabricated for the purpose of inducing local hypothermia in the rat rectum. The RTCA was simply integrated with an image-guided small animal irradiator and Monte Carlo-based treatment planning system to facilitate an invivo investigation of the radioprotective effect of hypothermia for late rectal toxicity following a single large dose of radiation.