Secondary Dose Research Articles

Comprehensive Beam Modeling, Secondary Dose Computations and Treatment Planning for a 1.5 T MRgRT System

Magnetic resonance imaging guided radiotherapy (MRgRT) combines soft tissue visualization with online adaptive radiotherapy. The MRgRT workflow is characterized by many steps which needs to be performed swiftly and therefore greatly benefits from automation. The purpose of this study was to model a MRgRT system for dose computation and intensity modulated radiotherapy (IMRT) planning within a 1.5 T magnetic field in an independent treatment planning system (iTPS) with automation functionalities. A beam model for a 1.5 T MRgRT system was commissioned in an iTPS using beam profiles and output factor measurements. The cryostat of the MRgRT system was modeled using a cylindrical multi-layer combination of epoxy and aluminum and matched to source-to-surface dependent cryostat scatter measurement in air, and corrected for angular dependent cryostat transmission readings. The beam model was used with a fast Monte Carlo dose engine to account for the electron return and electron streaming effects within an external magnetic field. The multi-leaf collimator (MLC) was characterized with a novel synchronous and asynchronous sweeping gap (aSG) test. The aSGs have a shift (between 0 - 40 mm) between adjacent leaves which remains constant over the sweep. Measured doses map the shadowing due to the tongue-and-groove (TG) exposure and TG and leaf tip (LT) parameters are derived to match this within the iTPS. Clinical IMRT plans of 10 prostate, 5 lymph node and 5 rectal plans created in the default clinical TPS were recomputed in the iTPS and evaluated with the corresponding plan-specific quality assurance measurements. In addition, three IMRT plans were optimized within the iTPS, delivered with the MRgRT system and evaluated using PSQA using a 3%/3mm gamma criterium with a 10% threshold. Monte Carlo dose computation in the iTPS closely resembled the measured profiles in the presence of the 1.5 T magnetic field. The cryostat scatter simulations were <0.7% compared to the measurements and literature. The aSG/SGs test demonstrated that the TG width gradually increased up to 1 mm for a distance of 15 mm away from the leaf tip end. These effects were closely replicated in the iTPS with a difference of <1.5% for 95% of the aSGs. The difference between the clinical dose and the dose calculation in the iTPS was, on average, <0.5% within the volume receiving 50% of the maximum dose. The gamma pass rate was 100% for 98% of the PSQA results. The plans optimized within the iTPS were successfully delivered at the MRgRT system. We successfully modeled and commissioned a 1.5 T MRgRT system in an independent TPS. The beam model can be used for a second opinion dose computation in a magnetic field and for direct treatment planning. Next, the beam model will be incorporated in a semi-automated and adaptive workflow using the iTPS.

Development of the Independent Dose Verification Method for the Ring Gantry PET/CT Linac

The RefleXion Xi is a new external beam radiotherapy delivery modality combining kilovoltage fan-beam CT and on-board PET for Biology-guided Radiotherapy. Although the machine shares similar components with the Tomotherapy machine, it has unique hardware features such as stationary beam delivery system with couch increments. The goal of this study is to develop an in-house independent secondary dose calculation method for this ring gantry PET/CT Linac. The method aggregates the beam intensities at each discrete firing gantry angle and couch moving position. The non-uniform intensity map is decomposed into a series of segments of uniform beam intensities, and then coordinates of beam segments were modified based on the relative distance from the dose calculation point to keep the calculation point in the same spot along the gantry rotation. The dose to the calculation point of each segment is determined by using measured tissue-maximum-ratio, output factors, and off-axis ratio by independent binary collimators. Two-dimensional convolution method is applied to integrate all dose contribution components in segment areas for efficient calculation. The final dose is obtained by summing all dose to the calculation point of segments for all firing gantry angle and couch position. Twenty patients with different treatment sites including head and neck, prostate, and lung regions were analyzed. Comparison of the point dose calculated by the independent program to that calculated by the planning system has shown reasonable agreement within ±5%. The independent dose verification program has been developed as an initial patient specific QA for improving patient safety. Further study will be performed to enhance the accuracy and reliability by including additional leakage and scatter models.

Feasibility of EPID Based In Vivo Dosimetry for On-Couch Adaptive Radiotherapy

CBCT-based online adaptive radiotherapy allows treatment plans to be tailored to the anatomy of the day. For this purpose, dose optimization and computation are performed on a synthetic CT (sCT), i.e., a density map of the planning CT deformably registered onto the acquired CBCT. Plan-specific quality assurance of adaptive treatment sessions is currently limited to the vendors own secondary dose calculation on the sCT. The use of EPID recordings promises to detect deviations in beam delivery or patient position, incorrect sCT data and anatomical changes. The purpose of this research was to evaluate the feasibility of using a new commercial technology available for in-vivo dose reconstruction based on EPID exit beam measurements. EPID images were recorded from on-couch adapted, hypofractionated treatment plans created for a prostate patient. Secondary dose calculation of the adapted plan and 3D reconstructions from EPID images of the absolute dose delivered to the patient were performed with the in-vivo dosimetry system. Comparison of reconstructed and planned delivery was conducted by means of gamma analysis and DVH metrics. Comparisons of the intended plan to a second volumetric check (collapsed cone - CC algorithm), and to the EPID in-vivo calculation, produced gamma passing rates averaged over the 7 fractions of 98.3% and 88.8% at TG-218/219 recommended criteria (Table 1). DVH metrics showed average deviations of -2.9% / -3.3% for PTV D95% and -1.2% / -0.2% for PTV D50% between intended dose and secondary / EPID in-vivo dose. The pronounced discrepancies for fraction 3 in terms of both reduced gamma passing rates and increased deviations for the EPID reconstructed dose coincide with a large air cavity in the rectum showing up in the patient's CBCT prior to treatment. Secondary dose calculation based on the density information of the CBCT as an additional option for QA confirmed origin and magnitude of this effect. In-vivo EPID dosimetry of adaptive clinical plans is feasible. It was shown exemplary that it reveals fractions with noticeable anatomical changes between the sCT and the patient anatomy during treatment. As new recommendations were published on in-vivo dosimetry through TG 307, the development of new clinical tools and evaluation of existing commercial solutions is essential, also in the context of the growing use of on-couch adaptive technology.

Sensitivity and specificity of secondary dose calculation for head and neck treatment plans.

Secondary dose calculation (SDC) with an independent algorithm is one option to perform plan-specific quality assurance (QA). While measurement-based QA can potentially detect errors in plan delivery, the dose values are typically only compared to calculations on homogeneous phantom geometries instead of patient CT data. We analyzed the sensitivity and specificity of an SDC software by purposely introducing different errors and determined thresholds for optimal decisions. Thirty head and neck VMAT plans and 30 modifications of those plans, including errors related to density and beam modelling, were recalculated using RadCalc with a Monte Carlo algorithm. Decision thresholds were obtained by receiver operating characteristics (ROC) analysis. For comparison, measurement-based QA using the ArcCHECK phantom was carried out and evaluated in the same way. Despite optimized decision thresholds, none of the systems was able to reliably detect all errors. ArcCHECK analysis using a 2%/2mm criterion with a threshold of 91.1% had an area under the curve (AUC) of 0.80. Evaluating differences in recalculated and planned DVH parameter of the target structures in RadCalc with a 2% threshold an AUC of 0.86 was achieved. Out-of-field deviations could be attributed to weaknesses in the beam model. Secondary dose calculation with RadCalc is an alternative to established measurement-based phantom QA. Different tools catch different errors; therefore, a combination of approaches should be preferred.

Generation, validation, and benchmarking of a commercial independent Monte Carlo calculation beam model for multi-target SRS

BackgroundDosimetric validation of single isocenter multi-target radiosurgery plans is difficult due to conditions of electronic disequilibrium and the simultaneous irradiation of multiple off-axis lesions dispersed throughout the volume. Here we report the benchmarking of a customizable Monte Carlo secondary dose calculation algorithm specific for multi-target radiosurgery which future users may use to guide their commissioning and clinical implementation. PurposeTo report the generation, validation, and clinical benchmarking of a volumetric Monte Carlo (MC) dose calculation beam model for single isocenter radiosurgery of intracranial multi-focal disease. MethodsThe beam model was prepared within SciMoCa (ScientificRT, Munich Germany), a commercial independent dose calculation software, with the aim of broad availability via the commercial software for use with single isocenter radiosurgery. The process included (1) definition & acquisition of measurement data required for beam modeling, (2) tuning model parameters to match measurements, (3) validation of the beam model via independent measurements and end-to-end testing, and finally, (4) clinical benchmarking and validation of beam model utility in a patient specific QA setting. We utilized a 6X Flattening-Filter-Free photon beam from a TrueBeam STX linear accelerator (Siemens Healthineers, Munich Germany). ResultsIn addition to the measured data required for standard IMRT/VMAT (depth dose, central axis profiles & output factors, leaf gap), beam modeling and validation for single-isocenter SRS required central axis and off axis (5 cm & 9 cm) small field output factors and comparison between measurement and simulation of backscatter with aperture for jaw much greater than MLCs. Validation end-to-end measurements included SRS MapCHECK in StereoPHAN geometry (2%/1 mm Gamma = 99.2% ± 2.2%), and OSL & scintillator measurements in anthropomorphic STEEV phantom (6 targets, volume = 0.1–4.1cc, distance from isocenter = 1.2–7.9 cm) for which mean difference was −1.9% ± 2.2%. For 10 patient cases, MC for individual PTVs was −0.8% ± 1.5%, −1.3% ± 1.7%, and −0.5% ± 1.8% for mean dose, D95%, and D1%, respectively. This corresponded to custom passing rates action limits per AAPM TG-218 guidelines of ±5.2%, ±6.4%, and ±6.3%, respectively. ConclusionsThe beam modeling, validation, and clinical action criteria outlined here serves as a benchmark for future users of the customized beam model within SciMoCa for single isocenter radiosurgery of multi-focal disease.

THE COMPARISON OF 2D DOSE PATIENT-SPECIFIC QUALITY ASSURANCE BETWEEN MONTE CARLO-CONVOLUTION AND MODIFIED CLARKSON INTEGRATION ALGORITHM

A sophisticated machine of radiotherapy treatment process follows the complexity of the quality assurance (QA) measurement. Non-measurement QA becomes one of the solutions to reduce the medical physicists’ workload. However, this method has not been clinically established. This study compared two non-measurement methods of patient-specific quality assurance (PSQA) to find the feasible algorithm for the adaptive radiotherapy process. Monte Carlo-based (MC) PSQA used a phase space file of the medical linear accelerator (Linac) to obtain the photon energy fluence and forward projected to the isoplane. In contrast, Modified Clarkson Integration-based (MCI) used a non-uniform fluence map in the isoplane. For the modulated intensity, we used a pair of the dynamic log files of the multileaf-collimator (MLC) and then employed them in the algorithms. The dose distributions of MC and MCI methods were compared to the treatment planning system (TPS) using gamma index analysis. We found that the gamma pass rates (GPR) for MC-TPS and MCI-TPS were 99.54% and 99.57%, respectively. Further, the dose distribution in the off-axis region for the MCI method showed lesser accuracy due to the higher secondary dose contribution. The linac log file information can be used and calculated into a 2D dose distribution using both MC and MCI methods, providing high-accuracy results.

Global dose distributions of neutrons and gamma-rays on the Moon

Dose assessment on the lunar surface is important for future long-term crewed activity. In addition to the major radiation of energetic charged particles from galactic cosmic rays (GCRs), neutrons and gamma-rays are generated by nuclear interactions of space radiation with the Moon’s surface materials, as well as natural radioactive nuclides. We obtained neutron and gamma-ray ambient dose distributions on the Moon using Geant4 Monte Carlo simulations combined with the Kaguya gamma-ray spectrometer measurement dataset from February 10 to May 28, 2009. The neutron and gamma-ray dose rates varied in the ranges of 58.7–71.5 mSv/year and 3.33–3.76 mSv/year, respectively, depending on the lunar geological features. The lunar neutron dose was high in the basalt-rich mare, where the iron- and titanium-rich regions are present, due to their large average atomic mass. As expected, the lunar gamma-ray dose map was similar to the distribution of natural radioactive elements (238U, 232Th, and 40K), although the GCR-induced secondary gamma-ray dose was significant at ~ 3.4 mSv/year. The lunar secondary dose contribution resulted in an additional dose of 12–15% to the primary GCR particles. Global dose distributions on the lunar surface will help identify better locations for long-term stays and suggest radiation protection strategies for future crewed missions.

Study on fast algorithm of neutron radiation field under complex terrain scenario based on ensemble learning approach

In this study a prediction algorithm has been proposed to rapidly figure out neutron radiation field for nuclear explosion under complex terrain scenario based on ensemble learning approach, which could be an impossibility for traditional radiation transport simulation methodology. By analyzing the influence of complex surface morphology on the radiation field, a series of characteristic parameters which could characterize the topographic features and their influence on the transport of neutrons and secondary gamma in the atmosphere have been extracted with the application of DEM, and the sample sethas been constructedwith the MC simulation results of terrain samples generated by random algorithm, to be used to train the prediction model for the neutron radiation field of nuclear explosion. In order to verify the actual prediction performance of the model, the study has implemented the prediction for the neutron flux, neutron tissue dose and secondary gamma tissue dose under the authentic urban and mountainous terrain scenarios, and analyzed and compared the results from fast prediction and MC simulation in different evaluation dimensions. The comparisons suggest that both of the results are in good agreement with each other, demonstrating that the fast prediction models preliminarily possess the engineering application value. In addition, a feasible approach to improve the generalization performance of the prediction model for various radiation scenarios has been proposed, which could be deemed as a reference for further research.

A structured FMEA approach to optimizing combinations of plan-specific quality assurance techniques for IMRT and VMAT QA.

Many commercial tools are available for plan-specific quality assurance (QA) of radiotherapy plans, with their functionality assessed in isolation. However, multiple QA tools are required to review the full range of potential errors. It is important to assess their effectiveness in combination with each other to look for ways to both streamline the QA process and to make certain that errors of high impact and/or high occurrence are caught before reaching patient treatment. To develop a structured method to assess the effective risk reduction of combinations of QA methods for IMRT/VMAT treatments. First, a structured prospective risk assessment was performed to establish the major failure modes (FMs) of IMRT/VMAT QA, and assign occurrence (O), severity (S), and baseline detectability (BD) rankings to them. The baseline assumed that chart checks and linear accelerator QA was performed, but no plan-specific secondary dose calculation or measurement was done. Second, the detectability of each FM for two secondary dose calculation methods and four plan measurement methods (point-based dose calculation, Monte-Carlo-based dose calculation, 2D fluence-based measurement, 2.5D phantom-based measurement, log file analysis with dose recalculation, and log file analysis combined with MLC QA) was determined. Third, we used a minimum detectability approach in addition to each FM's occurrence and severity to determine the optimal combination of QA methods.We analyzed the cumulative risk priority number of eight combinations of QA methods. The analysis was done on (1) all FMs, (2) FMs with high severity, (3) FMs with high-risk priority numbers (RPN) of O*S*BD, and (4) on FMs with both high severity and high RPN. Our analysis resulted in 54 FMs, including commissioning, planning, data transfer, and linear accelerator failures. 1D secondary dose calculation plus measurement provided a 19%-22% risk reduction from baseline. 1D/3D secondary dose calculation plus log files created a 25%-32% reduction. 3D secondary dose calculation plus measurement resulted in a 27%-34% reduction. 3D secondary dose calculation plus log files with additional machine QA provided the greatest reduction of 31%-42%. This novel structured approach to comparing combinations of QA methods will allow us to optimize our procedures, with the goal of detecting all clinically significant FMs. Our results show that log-file QA with 3D dose recalculation and supplemental machine QA provides better risk reduction than measurement-based QA. This work builds evidence to justify reducing or eliminating measurement-based PSQA with an independent 3D dose verification, log-file measurement, and appropriate supplementation of machine QA. The process also highlights FMs that cannot be caught by pre-treatment QA, prompting us to consider future directions for on-treatment QA.

METU-DBL: a cost effective proton irradiation facility

The Middle East Technical University Defocusing Beamline (METU-DBL) is designed to deliver protons with selectable kinetic energies between 15–30 MeV, and proton flux between 106–1010 protons/cm2/s, on a maximum 21.55 to 15.40 cm target region with a beam uniformity within ±6%, in accordance with the ESA ESCC No. 25100 specification for single event effects (SEEs) tests in the low energy range. The achieved high proton fluences, allow users to test space-grade materials; electronic circuits, ASICs, FPGAs, optical lenses, structural elements, and coating layers for LEO, GEO, and interplanetary missions.The total received dose on the Device-Under-Test (DUT) from secondary particles created during proton-material interactions at the first beam collimator and the beam dump never exceed 0.1% of the dose from primary protons. The METU-DBL is equiped with several measurement stations and services to the user teams. A secondary measurement station in a rotating drum that can hold multiple samples has been constructed next to the first collimator which provides neutrons for transmission experiments. At the target region, a robotic table is located, which provides mechanical and electrical mounting points to the samples and allows multiple samples to be tested in a row. A modular vacuum box can also be attached on the robotic table for any test that may require a vacuum environment. Power rails on the robotic table provide various outputs for the DUT. For the data acquisition, high-speed networking and a modular industrial PC are available at the target station. The design of the METU-DBL control software enables test users to integrate and optimize the data acquisition and controlling of the DUT.The beam properties at the target region are measured with the diamond, Timepix3, and fiber scintillator detectors mounted on the robotic table. With diamond and Timepix3 detectors, measurements are taken from the five different points (center and the four corners) of the test area to measure the proton flux and ensure that it is uniform across the full test area. Fiber scintillators on both axes (X and Y) scan the target area to cross-check the beam profile's uniformity. Secondary doses during the irradiation are measured by a Geiger-Müller tube sensitive to electrons and gammas above 0.1 MeV and by a neutron detector located at the entrance of the R&D room. The room cools down relatively fast after any irradiation (<1 hour).Accurate linear energy deposition rates and absorbed doses on the samples are calculated using MCNP6, FLUKA and Geant4 Monte Carlo simulations. Alanine dosimetry measurements that are calibrated against these simulations are also used to estimate the absorbed dose on the sample.

TUNING AND VALIDATION OF THE NEW RADCALC 3D MONTE CARLO BASED PRE-TREATMENT PLAN VERIFICATION TOOL

Verification procedures for patient-specific quality assurance (QA) in advanced radiotherapy are laborious and time-consuming. Moreover, it has been shown that these procedures cannot detect some inaccuracies for some particular complex cases due to tissue inhomogeneity and highly modulated plans. Secondary dose calculation verification of radiotherapy plans is an important aspect in patient-specific QA. A suitably optimized software, RadCalc equipped with 3D Monte Carlo Module (MC), was used to dosimetrically verify radiotherapy treatment plans where the measured dose distributions can be inaccurate due to the TPS dose calculation algorithm and/or treatment unit delivery uncertainties. MC Models were built using specific commissioning measurements and then the Additional Radiation to Light Field Offset parameter (Dosimetric Leaf Gap parameter) was tuned to achieve the best dose comparison agreement with phantom patient-specific QA measurements. The results showed a good agreement between the TPS and the simulations. RadCalc MC also allows to better estimate the plan doses in lung cancer patients and to detect the presence of possible inaccuracies due to tissue inhomogeneity, which is not estimable with a homogeneous phantoms.

Secondary radiation dose modeling in passive scattering and pencil beam scanning very high energy electron (VHEE) radiation therapy.

Electrons with kinetic energy up to a few hundred MeV, also called very high energy electrons (VHEE), are currently considered a promising technique for the future of radiation therapy (RT) and in particular ultra-high dose rate (UHDR) therapy. However, the feasibility of a clinical application is still being debated and VHEE therapy remains an active area of research for which the optimal conformal technique is also yet to be determined. In this work, we will apply two existing formalisms based on analytical Gaussian multiple-Coulomb scattering theory and Monte Carlo (MC) simulations to study and compare the electron and bremsstrahlung photon dose distributions arising from two beam delivery systems (passive scattering with or without a collimator or active scanning). We therefore tested the application of analytical and MC models to VHEE beams and assessed their performance and parameterization in the energy range of 6-200MeV. The optimized electron beam fluence, the bremsstrahlung, an estimation of central-axis and off-axis x-ray dose at the practical range and neutron contributions to the total dose, along with an extended parameterization for the photon dose model were developed, together with a comparison between double scattering (DS) and pencil beam scanning (PBS) techniques. MC simulations were performed with the TOPAS/Geant4 toolkit to verify the dose distributions predicted by the analytical calculations. The results for the clinical energy range (between 6 and 20MeV) as well as for higher energies (VHEE range between 20 and 200MeV) and for two treatment field sizes (5×5 and 10×10 cm2 ) are reported, showing a reasonable agreement with MC simulations with mean differences below 2.1%. The relative contributions of photons generated in the medium or by the scattering system along the central-axis (up to 50% of the total dose) are also illustrated, along with their relative variations with electron energy. The fast analytical models parametrized in this study allow an estimation of the amount of photons produced behind the practical range by a DS system with an accuracy lower than 3%, providing important information for the eventual design of a VHEE system. The results of this work could support future research on VHEE radiotherapy.

Biological modeling of gadolinium-based nanoparticles radio-enhancement for kilovoltage photons: a Monte Carlo study

BackgroundGadolinium-based nanoparticles (GdNPs) are clinically used agents to increase the radiosensitivity of tumor cells. However, studies on the mechanisms and biological modeling of GdNP radio-enhancement are still preliminary. This study aims to investigate the mechanism of radio-enhancement of GdNPs for kilovoltage photons using Monte Carlo (MC) simulations, and to establish local effect model (LEM)-based biological model of GdNP radiosensitization.MethodsThe spectrum and yield of secondary electrons and dose enhancement around a single GdNP and clustered GdNPs were calculated in a water cube phantom by MC track-structure simulations using TOPAS code. We constructed a partial shell-like cell geometry model of pancreatic cancer cell based on transmission electron microscope (TEM) observations. LEM-based biological modeling of GdNP radiosensitization was established based on the MC-calculated nano-scale dose distributions in the cell model to predict the cell surviving fractions after irradiation.ResultsThe yield of secondary electrons for GdNP was 0.16% of the yield for gold nanoparticle (GNP), whereas the average electron energy was 12% higher. The majority of the dose enhancement came from the contribution of Auger electrons. GdNP clusters had a larger range and extent of dose enhancement than single GdNPs, although GdNP clustering reduced radial dose per interacting photon significantly. For the dose range between 0 and 8 Gy, the surviving fraction predicted using LEM-based biological model laid within one standard deviation of the published experimental results, and the deviations between them were all within 25%.ConclusionsThe mechanism of radio-enhancement of GdNPs for kilovoltage photons was investigated using MC simulations. The prediction results of the established LEM-based biological model for GdNP radiosensitization showed good agreement with published experimental results, although the deviation of simulation parameters can lead to large disparity in the results. To our knowledge, this was the first LEM-based biological model for GdNP radiosensitization.

Investigation of Dose Thresholds and Normalization Methods Effect on Gamma Index Analysis for SRT and SBRT Patients with a Monte Carlo Secondary Dose Check Software

Investigation of Dose Thresholds and Normalization Methods Effect on Gamma Index Analysis for SRT and SBRT Patients with a Monte Carlo Secondary Dose Check Software

PO-1729 Sensitivity and specificity of secondary dose calculation and phantom QA for head-and-neck plans

PO-1729 Sensitivity and specificity of secondary dose calculation and phantom QA for head-and-neck plans

PO-1717 Increased clinical utility of MC secondary dose calculation vs state-of-the-art analytical algorithm

PO-1717 Increased clinical utility of MC secondary dose calculation vs state-of-the-art analytical algorithm

MCNPX Estimation of Photoneutron Dose to Eye Voxel Anthropomorphic Phantom From 18MV Linear Accelerator.

The dose due to photoneutron contamination outside the field of irradiation can be significant when using high-energy linear accelerators. The eye is a radiation-sensitive organ, and this risk increases when high linear energy transfer neutron radiation is involved. This study aimed to provide a fast method to estimate photoneutron dose to the eye during radiotherapy. A typical high-energy linear accelerator operating at 18MV was simulated using the Monte Carlo N-Particle Transport Code System extended version (MCNPX 2.5.0). The latest International Atomic Energy Agency photonuclear data library release was integrated into the code, accounting for the most known elements and isotopes used in typical linear accelerator construction. The photoneutron flux from a 5 × 5cm2 field size was scored at the treatment table plane and used as a new source for estimating the absorbed dose in a high-resolution eye voxel anthropomorphic phantom. In addition, common shielding media were tested to reduce the photoneutron dose to the eye using common shielding materials. Introducing a 2cm thickness of common neutron shielding medium reduced the total dose received in the eye voxel anthropomorphic phantom by 54%. In conclusion, individualized treatment based on photoneutron dose assessment is essential to better estimate the secondary dose inside or outside the field of irradiation.

A new approach to calculating the ratio of the compton to total, mass attenuation coefficient

X-rays and gamma rays are high-energy radiation that ionize atoms of matter as they pass through them. These rays interact with matter in such a way that secondary rays with energy equal to or lower than the incident photon are produced. The buildup factor is used to estimate the intensity of these secondary rays. One of the methods to calculate the buildup factor is the Geometric Progression formula. To use this formula, the equivalent atomic number must be determined. The ratio of the Compton mass attenuation coefficient to the total mass attenuation coefficient (R) is used to calculate the equivalent atomic number for each compound, element, or mixture. In this research, an equation was introduced to calculate the ratio of the Compton mass attenuation coefficient to the total mass attenuation coefficient (R) for X and gamma rays. This equation was the first of its kind and had good accuracy. It was a two-variable function of energy and atomic number that covered the energy range of 0.015–15 MeV and the range of 4–92 for atomic number. The highest SSE and RMSE fitting errors for the energy component for all calculated elements were equal to 0.000921 and 0.0076, respectively, and the same calculation errors for the atomic number component parameters were equal to 9.5561 and 0.7092, respectively. The maximum relative error of the calculated value with the actual value of water as a composition was less than 6%, and the amount of the same error for lead as an element was less than 9%. This equation can be used in other nuclear codes, as the functions used in it are very straightforward. It reduces the speed of calculations and the volume of calculations for calculating the secondary radiation dose.

A novel Monte Carlo (MC) dose model for small MLC fields of the cyberknife® M6TM radiosurgery system using the EGSnrc.

The multi-leaf collimator (MLC)-equipped CyberKnife® M6 radiosurgery system (CKM6) (Accuray Inc., Sunnyvale, CA) has been increasingly employed for stereotactic radiosurgery (SRS) to treat relatively small lesions. However, achieving an accurate dose distribution in such cases is usually challenging due to the combination of numerous small fields≤(30×30)mm2 . In this study, we developed a new Monte Carlo (MC) dose model for the CKM6 system using the EGSnrc to investigate dose variations in the small fields. The dose model was verified for the static MLC fields ranging from (53.8×53.9) to (7.6×7.7) mm2 at 800mm source to axis distance in a water phantom, based on the computed doses of Accuray Precision® (Accuray Inc.) treatment planning system (TPS). We achieved a statistical uncertainty of ≤4% by simulating 30-50 million incident particles/histories. Then, the treatment plans were created for the same fields in the TPS, and the corresponding measurements were performed with MapCHECK2 (Sun Nuclear Corporation), a standard device for patient-specific quality assurance (PSQA). Results of the MC simulations, TPS, and MapCHECK2 measurements were inter-compared. An overall difference in dosimetric parameters such as profiles, tissue maximum ratio (TMR), and output factors (OF) between the MC simulations and the TPS results was found ≤3% for (53.8×53.9-15.4×15.4) mm2 MLC fields, and it rose to 4.5% for the smallest (7.6mm×7.7mm) MLC field. The MapCHECK2 results showed a deviation ranging from -1.5% to + 4.5% compared to the TPS results, whereas the deviation was within±2.5% compared with the MC results. Overall, our MC dose model for the CKM6 system showed better agreement with measurements and it could serve as a secondary dose verification tool for the patient-specific QA in small fields.

Preliminary evaluation of a novel secondary check tool for intensity modulated radiotherapy treatment planning

PurposeTo evaluate the dosimetric accuracy of the Delta4 Insight (DI) secondary-check dosimetry system. MethodsAbsolute dosimetry in reference conditions, output factors, percent depth doses normalized and off-axis dose profiles for different field sizes calculated by DI were compared with measurements. Dose calculations for 20 clinical IMRT/VMAT plans generated in the TPS using both AAA or AcurosXB algorithms were compared with measurements. The average difference between calculated and measured point dose in high-dose region was calculated for all cases. 3D dose measurements were performed in Delta4 Phantom+ and a comparison between calculated and measured dose distributions was performed by means of the gamma analysis with 3 %/2 mm criteria. The dose distributions calculated by DI for 20 IMRT/VMAT plans were compared with those calculated by the TPS. ResultsThe absolute dosimetry computed by DI showed dose value in agreement with the measured one within 0.3 %. The average differences between measured and calculated output factors were less than 2.5 %. The average PDD differences were less than 0.6 %. An excellent agreement between calculations and off-axis measurements is found. The point doses calculated for the 20 recalculated plan showed good agreement with measurements with average differences less than 0.5 %. The average gamma pass rate values for the Delta4 Phantom + 3D dose analysis was greater than 97.%. The comparison of DI with theTPS showed good agreement for the used metrics. ConclusionsDelta4 Insight may provide a useful independent secondary dose verification system for IMRT/VMAT plans, complementing the traditional global QA protocols.