Monte 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.

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.

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.

Verification for dose estimation performance of a Monte-Carlo based treatment planning system in University of Tsukuba

The University of Tsukuba is developing not only a linac-based neutron source for BNCT (iBNCT) but also a multi-modal treatment planning system (Tsukuba-Plan) for BNCT. We are currently performing several verifications. Phantom experiments performed in iBNCT were simulated by the Tsukuba-Plan, and the calculation results were compared with the measurements from the experiments. The calculations were in good agreement with the measurements. The results demonstrated that the Tsukuba-Plan can perform to estimate doses properly for BNCT treatment at iBNCT.

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.

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.

Abstract ID: 122 Verification of dose estimation for Monte-Carlo based treatment planning system for boron neutron capture therapy

University of Tsukuba is developing a treatment device for accelerator-based for boron neutron capture therapy (BNCT). In the project, not only the treatment device (neutron source) but also several peripheral devices requiring in BNCT treatment [1] . As part of the development, a Monte-Carlo based treatment planning system (Developing code: Tsukuba-Plan) applicable to BNCT is also being developed. Regarding Monte-Carlo dose calculation engine, the Tsukuba-Plan has employed PHITS as the multi-purpose Monte Carlo Particle and Heavy Ion Transport code System. PHITS allows to calculate behaviors for several radiations such as neutrons, photons, protons and heavy-ions [2] . Therefore the Tsukuba-Plan with PHITS enables to perform dose estimation for not only BNCT but also particle radiotherapy and X-ray therapy. A prototype of the Tsukuba plan has been completed. At present, we are carrying out several verifications for the Tsukuba-Plan. To verify dose estimation accuracy of the system, some experiments with a water phantom were simulated by using the Tsukuba-Plan, and the calculation results were compared with experimental data. In the experiments, gold foils and TLDs were set in the phantom, and then the phantom was set to in front of the beam aperture of the neutron source for BNCT device. Neutron beam was irradiated to the phantom, finally two-dimensional distributions for thermal neutron flux and gamma-ray dose in the phantom were measured. On the other hand, regarding simulation of the experiments with the Tsukuba-Plan, CT images of the water phantom were loaded to Tsukuba-Plan, and the irradiation conditions were represented. And calculations of two-dimensional distributions for both of thermal neutron flux and gamma-ray dose in the phantom were determined by using PHITS. The calculation results obtained from the Tsukuba-Plan were compared with the experimental values. The calculation values were in good agreement with the experimental values within statistical errors and experimental errors. The verification results demonstrated that Tsukuba-Plan enables to perform dose estimation for BNCT. Clinical group of University of Tsukuba plans to perform clinical trial for BNCT by using the neutron source for BNCT treatment device in the near future. Tsukuba-Plan will be applied to the clinical trials in practical use.

Abstract ID: 124 Application expansion of the Monte-Carlo based treatment planning system for BNCT to particle radiotherapy and X-ray therapy

University of Tsukuba is developing an accelerator-based treatment device for boron neutron capture therapy (BNCT). As part of the development, a Monte-Carlo based treatment planning system (developing code: Tsukuba-Plan) is being developed [1] . The Tsukuba-plan has employed PHITS as the Monte-Carlo transport calculation code [2] . PHITS can calculate neutron, photon as well as proton and heavy-ions including carbon-ions. Therefore, Tsukuba-Plan with PHITS enables to perform dose estimations for not only BNCT but also particle radiotherapy and X-ray therapy. In BNCT protocols for malignant brain tumor, X-ray therapy is added after BNCT to enhance the treatment effect. And in the future, proton therapy will be also applied in stead of the X-ray therapy in order to improve therapeutic dose distribution. Therefore, the treatment planning system is required dose estimation for the combined multi-modality therapy. Based on this background, the aim of this study is to expand application field of the Tsukuba-plan to conventional external radiotherapies. University of Tsukuba Hospital has X-ray therapy, proton therapy and BNCT. Thus, first, we have expanded Tsukuba-plan to proton therapy and X-ray therapy. Regarding proton therapy, we have constructed 155 MeV and 200 MeV proton beam source and a geometry struction data for the proton therapy device installed in our Hospital. Verification for calculation accuracy for proton irradiation were performed. Phantom experiments performed using the proton therapy device in advance were represented by the Tsukuba-Plan. The proton dose distributions in the phantom were determined, and the calculations were compared with the experimental values. Calculations for both of proton therapy and X-ray therapy were in good agreement with experimental values, respectively. The results demonstrated that the Tsukuba-Plan enables to estimate accurately doses for proton irradiation and X-ray irradiations. To put Tsukuba-Plan to practical use for proton therapy and X-ray therapy, Tsukuba-Plan is improved further and several verifications are performed. This work was supported by JSPS KAKENHI Grant Nos. JP16K15343.

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.

Local Hypothermia as a Radioprotector of the Rectal Wall During Prostate Stereotactic Body Radiation Therapy

Tocompare the single-fraction dose-related incidence of rectal obstruction and/or bleeding in normothermic and hypothermic rectums of a rat model. A 1.9-cm length of rectum was irradiated with a single fraction in 57 Sprague-Dawley rats using a dedicated image-guided small animal irradiator and Monte Carlo-based treatment planning system. All rats had a rectal temperature control apparatus placed during irradiation and were stratified to achieve either a normothermic (37°C) or hypothermic (15°C) rectal wall temperature. Radiation was delivered to a 1-cm-diameter cylindrical volume about the cooling device and rectal wall. The radiation dose was escalated from 16Gy up to 37Gy to assess the dose response in each arm. The primary endpoint of this study was rectal obstruction and/or bleeding during a follow-up of 180 to 186days. Histologic scoring was performed on all study rats. Probit analysis showed a dose associated with a 50% incidence of rectal obstruction of 24.6Gy and 40.8Gy for normothermic and hypothermic arms, respectively. The occurrence of obstruction and/or bleeding correlated with the posttreatment histologic score for normothermic rats; however, there was no difference in histologic score between normothermic and hypothermic rats at the highest dose levels evaluated. A significant radioprotective effect was observed using local hypothermia during a single large dose of radiation for the functional endpoint of rectal obstruction and/or bleeding. A confirmatory study in a large animal model with anatomic and physiologic similarities to humans is suggested.

Clinical experience with planning, quality assurance, and delivery of burst-mode modulated arc therapy.

“Burst‐mode” modulated arc therapy (hereafter referred to as “mARC”) is a form of volumetric‐modulated arc therapy characterized by variable gantry rotation speed, static MLCs while the radiation beam is on, and MLC repositioning while the beam is off. We present our clinical experience with the planning techniques and plan quality assurance measurements of mARC delivery. Clinical mARC plans for five representative cases (prostate, low‐dose‐rate brain, brain with partial‐arc vertex fields, pancreas, and liver SBRT) were generated using a Monte Carlo–based treatment planning system. A conventional‐dose‐rate flat 6 MV and a high‐dose‐rate non‐flat 7 MV beam are available for planning and delivery. mARC plans for intact‐prostate cases can typically be created using one 360° arc, and treatment times per fraction seldom exceed 6 min using the flat beam; using the nonflat beam results in slightly higher MU per fraction, but also in delivery times less than 4 min and with reduced mean dose to distal organs at risk. mARC also has utility in low‐dose‐rate brain irradiation; mARC fields can be designed which deliver a uniform 20 cGy dose to the PTV in approximately 3‐minute intervals, making it a viable alternative to conventional 3D CRT. For brain cases using noncoplanar arcs, delivery time is approximately six min using the nonflat beam. For pancreas cases using the nonflat beam, two overlapping 360° arcs are required, and delivery times are approximately 10 min. For liver SBRT, the time to deliver 800 cGy per fraction is at least 12 min. Plan QA measurements indicate that the mARC delivery is consistent with the plan calculation for all cases. mARC has been incorporated into routine practice within our clinic; currently, on average approximately 15 patients per day are treated using mARC; and with the exception of LDR brain cases, all are treated using the nonflat beam.PACS number(s): 87.55.D‐, 87.55.K‐, 87.53.Ay. 87.56.N‐

Influence of monte carlo variance with fluence smoothing in VMAT treatment planning with Monaco TPS.

The study aimed to investigate the interplay between Monte Carlo Variance (MCV) and fluence smoothing factor (FSF) in volumetric modulated arc therapy treatment planning by using a sample set of complex treatment planning cases and a X-ray Voxel Monte Carlo-based treatment planning system equipped with tools to tune fluence smoothness as well as MCV. The dosimetric (dose to tumor volume, and organ at risk) and physical characteristic (treatment time, number of segments, and so on) of a set 45 treatment plans for all combinations of 1%, 3%, 5% MCV and 1, 3, 5 FSF were evaluated for five carcinoma esophagus cases under the study. Increase in FSF reduce the treatment time. Variation of MCV and FSF gives a highest planning target volume (PTV), heart and lung dose variation of 3.6%, 12.8% and 4.3%, respectively. The heart dose variation was highest among all organs at risk. Highest variation of spinal cord dose was 0.6 Gy. Variation of MCV and FSF influences the organ at risk (OAR) doses significantly but not PTV coverage and dose homogeneity. Variation in FSF causes difference in dosimetric and physical parameters for the treatment plans but variation of MCV does not. MCV 3% or less do not improve the plan quality significantly (physical and clinical) compared with MCV greater than 3%. The use of MCV between 3% and 5% gives similar results as 1% with lesser calculation time. Minimally detected differences in plan quality suggest that the optimum FSF can be set between 3 and 5.

Online Magnetic Resonance Image Guided Adaptive Radiation Therapy: First Clinical Applications

To demonstrate the feasibility of online adaptive magnetic resonance (MR) image guided radiation therapy (MR-IGRT) through reporting of our initial clinical experience and workflow considerations. The first clinically deployed online adaptive MR-IGRT system consisted of a split 0.35T MR scanner straddling a ring gantry with 3 multileaf collimator-equipped (60)Co heads. The unit is supported by a Monte Carlo-based treatment planning system that allows real-time adaptive planning with the patient on the table. All patients undergo computed tomography and MR imaging (MRI) simulation for initial treatment planning. A volumetric MRI scan is acquired for each patient at the daily treatment setup. Deformable registration is performed using the planning computed tomography data set, which allows for the transfer of the initial contours and the electron density map to the daily MRI scan. The deformed electron density map is then used to recalculate the original plan on the daily MRI scan for physician evaluation. Recontouring and plan reoptimization are performed when required, and patient-specific quality assurance (QA) is performed using an independent in-house software system. The first online adaptive MR-IGRT treatments consisted of 5 patients with abdominopelvic malignancies. The clinical setting included neoadjuvant colorectal (n=3), unresectable gastric (n=1), and unresectable pheochromocytoma (n=1). Recontouring and reoptimization were deemed necessary for 3 of 5 patients, and the initial plan was deemed sufficient for 2 of the 5 patients. The reasons for plan adaptation included tumor progression or regression and a change in small bowel anatomy. In a subsequently expanded cohort of 170 fractions (20 patients), 52 fractions (30.6%) were reoptimized online, and 92 fractions (54.1%) were treated with an online-adapted or previously adapted plan. The median time for recontouring, reoptimization, and QA was 26 minutes. Online adaptive MR-IGRT has been successfully implemented with planning and QA workflow suitable for routine clinical application. Clinical trials are in development to formally evaluate adaptive treatments for a variety of disease sites.

Investigation of the clinical potential of scattering foil free electron beams

Electron beam therapy has been an important radiation therapy modality for many decades. Studies have been conducted recently for more efficient and advanced delivery of electron beam radiation therapy. X-ray contamination is a common problem that exists with all of the advanced electron beam therapy techniques such as Bolus Electron conformal therapy, segmented electron conformal therapy, and modulated electron arc therapy. X-ray contamination could add some limitations to the advancement and clinical utility of those electron modalities. It was previously shown in the literature that the scattering foil is one of the major accelerator parts contributing to the generation of bremsstrahlung photons. Thus, in this work we investigate the dosimetric characteristics of scattering foil free (SFF) electron beams and the feasibility of using those beams for breast cancer boosts. The SFF electron beams were modeled and simulated using the Monte Carlo method. CT scans of six previously treated breast patients were used for the treatment plan generation utilizing our in-house Monte Carlo-based treatment planning system. Electron boost plans with conventional beams and the SFF beams were generated, respectively, for all patients. A significant reduction of the photon component was observed with the removal of the primary scattering foil for beam energies higher than 12 MeV. Flatness was greatly affected but the difference in flatness between conventional and SFF beams was much reduced for small cone sizes, which were often used clinically for breast boosts. It was found that the SFF electron beams could deliver high-quality dose distributions as conventional electron beams for boost treatments of the breast with an added advantage of a further reduced dose to the lung and the heart.