Calculated Dose Distributions Research Articles

Detector response in the buildup region of small MV fields.

The model used to calculate dose distributions in a radiotherapy treatment plan relies on the data entered during beam commissioning. The quality of these data heavily depends on the detector choice made, especially in small fields and in the buildup region. Therefore, it is necessary to identify suitable detectors for measurements in the buildup region of small fields. To aid the understanding of a detector's limitations, several factors that influence the detector signal are to be analyzed, for example, the volume effect due to the detector size, the response to electron contamination, the signal dependence on the polarity used, and the effective point of measurement chosen. We tested the suitability of different small field detectors for measurements of depth dose curves with a special focus on the surface-near area of dose buildup for fields sized between 10×10 and 0.6×0.6cm2 . Depth dose curves were measured with 14 different detectors including plane-parallel chambers, thimble chambers of different types and sizes, shielded and unshielded diodes as well as a diamond detector. Those curves were compared with depth dose curves acquired on Gafchromic film. Additionally, the magnitude of geometric volume corrections was estimated from film profiles in different depths. Furthermore, a lead foil was inserted into the beam to reduce contaminating electrons and to study the resulting changes of the detector response. The role of the effective point of measurement was investigated by quantifying the changes occurring when shifting depth dose curves. Last, measurements for the small ionization chambers taken at opposing biasing voltages were compared to study polarity effects. Depth-dependent correction factors for relative depth dose curves with different detectors were derived. Film, the Farmer chamber FC23, a 0.13cm3 scanning chamber CC13 and a plane-parallel chamber PPC05 agree very well in fields sized 4×4 and 10×10cm2 . For most detectors and in smaller fields, depth dose curves differ from the film. In general, shielded diodes require larger corrections than unshielded diodes. Neither the geometric volume effect nor the electron contamination can account for the detector differences. The biggest uncertainty arises from the positioning of a detector with respect to the water surface and from the choice of the detector's effective point of measurement. Depth dose curves acquired with small ionization chambers differ by over 15% in the buildup region depending on sign of the biasing voltage used. A scanning chamber or a PPC40 chamber is suitable for fields larger than 4×4cm2 . Below that field size, the microDiamond or small ionization chambers perform best requiring the smallest corrections at depth as well as in the buildup region. Diode response changes considerably between the different types of detectors. The position of the effective point of measurement has a huge effect on the resulting curves, therefore detector specific rather than general shifts of half the inner radius of cylindrical ionization chambers for the effective point of measurement should be used. For small ionization chambers, averaging between both polarities is necessary for data obtained near the surface.

Intraoperative computed tomography imaging for dose calculation in intraoperative electron radiation therapy: Initial clinical observations.

In intraoperative electron radiation therapy (IOERT) the energy of the electron beam is selected under the conventional assumption of water-equivalent tissues at the applicator end. However, the treatment field can deviate from the theoretic flat irradiation surface, thus altering dose profiles. This patient-based study explored the feasibility of acquiring intraoperative computed tomography (CT) studies for calculating three-dimensional dose distributions with two factors not included in the conventional assumption, namely the air gap from the applicator end to the irradiation surface and tissue heterogeneity. In addition, dose distributions under the conventional assumption and from preoperative CT studies (both also updated with intraoperative data) were calculated to explore whether there are other alternatives to intraoperative CT studies that can provide similar dose distributions. The IOERT protocol was modified to incorporate the acquisition of intraoperative CT studies before radiation delivery in six patients. Three studies were not valid to calculate dose distributions due to the presence of metal artefacts. For the remaining three cases, the average gamma pass rates between the doses calculated from intraoperative CT studies and those obtained assuming water-equivalent tissues or from preoperative CT studies were 73.4% and 74.0% respectively. The agreement increased when the air gap was included in the conventional assumption (98.1%) or in the preoperative CT images (98.4%). Therefore, this factor was the one mostly influencing the dose distributions of this study. Our experience has shown that intraoperative CT studies are not recommended when the procedure includes the use of shielding discs or surgical retractors unless metal artefacts are removed. IOERT dose distributions calculated under the conventional assumption or from preoperative CT studies may be inaccurate unless the air gap (which depends on the surface irregularities of the irradiated volume and on the applicator pose) is included in the calculations.

Effect of Relative Electron Density and Physical Density Adjustment in Patient-specific Quality Assurance Using PMMA Phantom

The purpose of this study was to improve the accuracy of dose-distribution calculations by understanding how the calculated dose varies with the change in the relative electron density replacing polymethyl methacrylate (PMMA) in patient-specific quality assurance. We calculated the relative electron density at which dose attenuation in each dose calculation algorithm coincides with the measured value of the dose attenuation of single-field irradiation. Next, the dose change was calculated by changing the relative electron density or physical electron density for substituting PMMA for each X-ray energy and calculation algorithm. Furthermore, using clinical plans, changes in point-dose verification and dose-distribution verification that occurred when the relative electron density or physical electron density was varied were investigated. The dose attenuation varies depending on the dose-calculation algorithm, and the optimum value of the electron density is different for each. After the electron density optimization, the point dose verification using the 97.1% to 98.3% (3%/3 mm), 90.0% to 94.3% (2%/3 mm) and gained a dominant improvement tendency (P<0.001). We clarified dose change accompanying relative electron density or physical electron density change. We concluded that the accuracy of dose-distribution calculation for verification improves by replacing PMMA with optimal relative electron density or physical electron density.

HUMAN BODY ANTHROPOMORPHIC PHANTOM UTILISATION FOR THE COMPLEX TESTING OF RADIATION THERAPY TECHNOLOGICAL PROCESS

The rapid development of technologies in the field of radiation therapy allows us nowadays to implement precision and most clinically effective radiotherapy techniques for oncological patient’s treatment to minimize the irradiation of normal tissues and improve local tumor control. An important condition for the implementation of the justification principle is strict compliance with the requirements for the accuracy of the dose delivered. High standards of radiation treatments performed are guaranteed by the development and strict compliance with the quality assurance (QA) program in the radiological department. However, due to QA programmes specificity, standardized and worldwide used tests included in the quality management system are trivial mechanical and dosimetric tests that can’t define the presence and magnitude of the integral error in the dose delivered to the patient, which arises as a result of the execution of sophisticated radiation therapy procedures, as well as to take into account the complexity of the implementation of modern methods of treatment. The aim of the work is to develop a method of complex dosimetric testing of the radiation therapy process (end-to-end audit), based on the utilization of the anthropomorphic phantom of the original design. The result of this work is the creation of the modified anthropomorphic phantom for precision dosimetric measurements, designed for testing the following technological procedures of the radiation therapy process: a computer tomography acquisition; a computerized treatment planning system, including a contouring module and dose distribution calculation algorithm; imaging systems integrated with radiation treatment units; dosimetric and technical characteristics of the radiation treatment units. Regular dosimetric testing of the radiation therapy technological process (end-to-end audit) with utilization of the technique proposed by the authors, based on the developed anthropomorphic phantom usage, will allow to assess the accuracy of dose distribution delivered to patients with all major malignant tumors localizations.

MLC parameters from static fields to VMAT plans: an evaluation in a RT-dedicated MC environment (PRIMO)

BackgroundPRIMO is a graphical environment based on PENELOPE Monte Carlo (MC) simulation of radiotherapy beams able to compute dose distribution in patients, from plans with different techniques. The dosimetric characteristics of an HD-120 MLC (Varian), simulated using PRIMO, were here compared with measurements, and also with Acuros calculations (in the Eclipse treatment planning system, Varian).Materials and methodsA 10 MV FFF beam from a Varian EDGE linac equipped with the HD-120 MLC was used for this work. Initially, the linac head was simulated inside PRIMO, and validated against measurements in a water phantom. Then, a series of different MLC patterns were established to assess the MLC dosimetric characteristics. Those tests included: i) static fields: output factors from MLC shaped fields (2 × 2 to 10 × 10 cm2), alternate open and closed leaf pattern, MLC transmitted dose; ii) dynamic fields: dosimetric leaf gap (DLG) evaluated with sweeping gaps, tongue and groove (TG) effect assessed with profiles across alternate open and closed leaves moving across the field. The doses in the different tests were simulated in PRIMO and then compared with EBT3 film measurements in solid water phantom, as well as with Acuros calculations. Finally, MC in PRIMO and Acuros were compared in some clinical cases, summarizing the clinical complexity in view of a possible use of PRIMO as an independent dose calculation check.ResultsStatic output factor MLC tests showed an agreement between MC calculated and measured OF of 0.5%. The dynamic tests presented DLG values of 0.033 ± 0.003 cm and 0.032 ± 0.006 cm for MC and measurements, respectively. Regarding the TG tests, a general agreement between the dose distributions of 1–2% was achieved, except for the extreme patterns (very small gaps/field sizes and high TG effect) were the agreement was about 4–5%. The analysis of the clinical cases, the Gamma agreement between MC in PRIMO and Acuros dose calculation in Eclipse was of 99.5 ± 0.2% for 3%/2 mm criteria of dose difference/distance to agreement.ConclusionsMC simulations in the PRIMO environment were in agreement with measurements for the HD-120 MLC in a 10 MV FFF beam from a Varian EDGE linac. This result allowed to consistently compare clinical cases, showing the possible use of PRIMO as an independent dose calculation check tool.

Validation of a Monte Carlo simulation for Microbeam Radiation Therapy on the Imaging and Medical Beamline at the Australian Synchrotron

Microbeam Radiation Therapy (MRT) is an emerging cancer treatment modality characterised by the use of high-intensity synchrotron-generated x-rays, spatially fractionated by a multi-slit collimator (MSC), to ablate target tumours. The implementation of an accurate treatment planning system, coupled with simulation tools that allow for independent verification of calculated dose distributions are required to ensure optimal treatment outcomes via reliable dose delivery. In this article we present data from the first Geant4 Monte Carlo radiation transport model of the Imaging and Medical Beamline at the Australian Synchrotron. We have developed the model for use as an independent verification tool for experiments in one of three MRT delivery rooms and therefore compare simulation results with equivalent experimental data. The normalised x-ray spectra produced by the Geant4 model and a previously validated analytical model, SPEC, showed very good agreement using wiggler magnetic field strengths of 2 and 3 T. However, the validity of absolute photon flux at the plane of the Phase Space File (PSF) for a fixed number of simulated electrons was unable to be established. This work shows a possible limitation of the G4SynchrotronRadiation process to model synchrotron radiation when using a variable magnetic field. To account for this limitation, experimentally derived normalisation factors for each wiggler field strength determined under reference conditions were implemented. Experimentally measured broadbeam and microbeam dose distributions within a Gammex RMI457 Solid Water® phantom were compared to simulated distributions generated by the Geant4 model. Simulated and measured broadbeam dose distributions agreed within 3% for all investigated configurations and measured depths. Agreement between the simulated and measured microbeam dose distributions agreed within 5% for all investigated configurations and measured depths.

Dynamic lung phantom commissioning for 4D dose assessment in proton therapy

Anthropomorphic phantoms mimicking organ and tumor motion of patients are essential for end-to-end testing of motion mitigation techniques in ion beam therapy. In this work a commissioning procedure developed with the in-house designed respiratory phantom ARDOS (Advanced Radiation DOSimetry system) is presented. The phantom was tested and benchmarked for 4D dose verification in proton therapy, which included: characterization of the tissue equivalent materials from computed tomography (CT) imaging, assessment of dose calculation accuracy in critical structures of the phantom, and testing various detectors for proton dosimetry in the ARDOS phantom.To prove the validity of the CT calibration curve, measured relative stopping powers (RSP) of the ARDOS materials were compared with values from CTs: original and overwritten with known material parameters. Override of rib- and soft-tissue phantom components improved RSP accuracy while inhomogeneous lung tissue, represented by the balsa wood, was better modelled by the CT Hounsfield units.Monte Carlo (MC) dose calculations were benchmarked against measurements with a reference Farmer chamber embedded in ARDOS material samples showing less than 3% relative dose difference. Differences between MC calculated dose distributions and those calculated by analytical algorithms for the ARDOS geometry were higher than 20% of the prescribed dose, depending on the position in the phantom.Pinpoint ionization chambers and thermoluminescence dosimeters showed differences of up to 5.5% compared to MC dose calculations for all lung setups in the static phantom. They were also able to detect dose distortions due to motion. EBT3 film dosimetry was shown to be suitable for 2D relative dose characterization, which could provide extended information on dose distributions in the penumbra area.The presented methodology and results can be used for drafting general recommendations for dynamic phantom commissioning, which is an essential step towards end-to-end evaluation of motion mitigation techniques in ion beam therapy.

Benchmarking a GATE/Geant4 Monte Carlo model for proton beams in magnetic fields.

PurposeMagnetic resonance guidance in proton therapy (MRPT) is expected to improve its current performance. The combination of magnetic fields with clinical proton beam lines poses several challenges for dosimetry, treatment planning and dose delivery. Proton beams are deflected by magnetic fields causing considerable changes in beam trajectories and also a retraction of the Bragg peak positions. A proper prediction and compensation of these effects is essential to ensure accurate dose calculations. This work aims to develop and benchmark a Monte Carlo (MC) beam model for dose calculation of MRPT for static magnetic fields up to 1 T.MethodsProton beam interactions with magnetic fields were simulated using the GATE/Geant4 toolkit. The transport of charged particle in custom 3D magnetic field maps was implemented for the first time in GATE. Validation experiments were done using a horizontal proton pencil beam scanning system with energies between 62.4 and 252.7 MeV and a large gap dipole magnet (B = 0–1 T), positioned at the isocenter and creating magnetic fields transverse to the beam direction. Dose was measured with Gafchromic EBT3 films within a homogeneous PMMA phantom without and with bone and tissue equivalent material slab inserts. Linear energy transfer (LET) quenching of EBT3 films was corrected using a linear model on dose‐averaged LET method to ensure a realistic dosimetric comparison between simulations and experiments. Planar dose distributions were measured with the films in two different configurations: parallel and transverse to the beam direction using single energy fields and spread‐out Bragg peaks. The MC model was benchmarked against lateral deflections and spot sizes in air of single beams measured with a Lynx PT detector, as well as dose distributions using EBT3 films. Experimental and calculated dose distributions were compared to test the accuracy of the model.ResultsMeasured proton beam deflections in air at distances of 465, 665, and 1155 mm behind the isocenter after passing the magnetic field region agreed with MC‐predicted values within 4 mm. Differences between calculated and measured beam full width at half maximum (FWHM) were lower than 2 mm. For the homogeneous phantom, measured and simulated in‐depth dose profiles showed range and average dose differences below 0.2 mm and 1.2%, respectively. Simulated central beam positions and widths differed <1 mm to the measurements with films. For both heterogenous phantoms, differences within 1 mm between measured and simulated central beam positions and widths were obtained, confirming a good agreement of the MC model.ConclusionsA GATE/Geant4 beam model for protons interacting with magnetic fields up to 1 T was developed and benchmarked to experimental data. For the first time, the GATE/Geant4 model was successfully validated not only for single energy beams, but for SOBP, in homogeneous and heterogeneous phantoms. EBT3 film dosimetry demonstrated to be a powerful dosimetric tool, once the film response function is LET corrected, for measurements in‐line and transverse to the beam direction in magnetic fields. The proposed MC beam model is foreseen to support treatment planning and quality assurance (QA) activities toward MRPT.

Calculation of clinical dose distributions in proton therapy from microdosimetry.

To introduce a new algorithm-MicroCalc-for dose calculation by modeling microdosimetric energy depositions and the spectral fluence at each point of a particle beam. Proton beams are considered as a particular case of the general methodology. By comparing the results obtained against Monte Carlo computations, we aim to validate the microdosimetric formalism presented here and in previous works. In previous works, we developed a function on the energy for the average energy imparted to a microdosimetric site per event and a model to compute the energetic spectrum at each point of the patient image. The number of events in a voxel is estimated assuming a model in which the voxel is completely filled by microdosimetric sites. Then, dose at every voxel is computed by integrating the average energy imparted per event multiplied by the number of events per energy beam of the spectral distribution within the voxel. Our method is compared with the proton convolution superposition (PCS) algorithm implemented in Eclipse™ and the fast Monte Carlo code MCsquare, which is here considered the benchmark, for in-water calculations, using in both cases clinically validated beam data. Two clinical cases are considered: a brain and a prostate case. For a SOBP beam in water, the mean difference at the central axis found for MicroCalc is of 0.86% against 1.03% for PCS. Three-dimensional gamma analyses in the PTVs compared with MCsquare for criterion (3%, 3mm) provide gamma index of 95.07% with MicroCalc vs 94.50% with PCS for the brain case and 99.90% vs 100.00%, respectively, for the prostate case. For selected organs at risk in each case (brainstem and rectum), mean and maximum difference with respect to MCsquare dose are analyzed. In the brainstem, mean differences are 0.25Gy (MicroCalc) vs 0.56Gy (PCS), whereas for the rectum, these values are 0.05Gy (MicroCalc) vs 0.07Gy (PCS). The accuracy of MicroCalc seems to be, at least, not inferior to PCS, showing similar or better agreement with MCsquare in the considered cases. Additionally, the algorithm enables simultaneous computation of other quantities of interest. These results seem to validate the microdosimetric methodology in which the algorithm is based on.

A study on the comparison of the dose distribution of low kV X-ray radiation in generically modulated bone and CT scanned bone

The INTRABEAM system is a device that uses low kV X-ray radiation in the treatment of cancers. During Kypho-IORT the INTRABEAM delivers a single high dose of radiation to a metastasis in a spinal vertebra, sterilising the metastasis, before the vertebra is stabilised with PMMA. However, much remains uncertain about the dose distribution of low kV X-ray radiation in heterogeneous tissue, such as bone. Current generation planning CTs do not have sufficient resolution to resolve the trabecular architecture. Voxel averaging therefore affects the calculation of bone density values which are typically underestimated. This subsequently affects dose calculations. In this study the GEometry ANd Tracking 4 (GEANT4) code was used to develop a generic model of a spinal vertebra that includes trabecular architecture. The dose distribution of low kV X-ray radiation within this architecture was modelled using a Virtual Source Model (VSM) of the INTRABEAM and compared to the dose distribution of the same radiation in simulated CT scanned bone. The results show that the dose distribution of low kV X-ray radiation in trabecular bone is more complex than previously suggested. A spherical dose distribution is modulated by the trabecular architecture such that the energy deposition in the trabecular walls can reach five times higher than expected, while the energy deposition in the trabecular openings is reduced by upwards of 50%. With further development, this code may be incorporated into future treatment planning software resulting in improvements in the dose distribution calculations for low kV treatments such as Kypho-IORT.

New calculation method for 3D dose distribution in tetrahedral-mesh phantoms in Geant4

The tetrahedral-mesh (TM) geometry, which is a very promising geometry for computational human phantoms, has a limitation in 3D dose distribution calculation for medical applications. Even though Geant4 provides the read-out geometry for calculating 3D dose distribution in the TM geometry, this method significantly slows down the computation speed. In the present study, we developed a new method, called Moving Voxel-based Dose-Distribution Calculator (MVDDC), to rapidly calculate a 3D dose distribution in a TM geometry. To evaluate the performance of the MVDDC method, a simple TM cubic phantom and a human phantom were implemented in Geant4. Subsequently, the phantoms were irradiated with proton spot beams under various conditions, and the obtained results were compared with those of the read-out geometry method. The results show that there is no significant difference between the dose distributions calculated using the new method and the read-out geometry method. With respect to the computational performance, the speeds of simulations using the MVDDC were approximately 1.4–2.7 times faster than those of the simulations using the read-out geometry method.

Potentials and limits of a novel CT reconstruction algorithm (DirectDensity™) developed for radiotherapy treatment planning

Purpose: Computed tomography (CT) represents the standard of treatment planning in radiotherapy. Calculation of dose distribution requires proper energy-dependent curves to convert Hounsfield Units into relative to water electron densities (rED). To possibly reduce the patient dose while maintaining optimal image quality, it could be useful to modulate x-ray tube potential (kVp) based on patient size, but this practice requires the use of several conversion curves in the treatment planning system (TPS). The DirectDensityTM (DD) algorithm, recently developed by Siemens Healthcare, uses projection data to reconstruct images directly proportional to rEDs, regardless of the used kVp. The purpose of this study was to evaluate this algorithm for clinical use in radiotherapy. Methods: Image quality was evaluated on Catphan images reconstructed with standard and DD algorithms. The CIRS Electron Density Phantom, with its several tissue-equivalent inserts, was then used to assess the accuracy of reconstructed rEDs against those certified at different tube voltages. Finally, the treatment plans calculated on clinical images reconstructed with standard and DD algorithms were compared. Results: DD showed an image quality comparable to standard algorithms except for the contrast: DD causes a loss in contrast (up to 69%) for materials densities greater than 1.1 g cm−3. CIRS images analysis proved the rEDs reconstruction accuracy except for the metal inserts: titanium rED was underestimated up to 40% when kVp was reduced. Nevertheless, minimal dosimetric differences were found between treatment plans calculated in different body regions. Conclusions: The DD algorithm allows to plan radiotherapy treatments on CT images acquired at tube potentials optimized both for the anatomical region and patient size, without complicating the clinical workflow with tube potential specific calibration procedures. However, the presence of contrast medium and metal implants can limit the applications of this algorithm in clinical practice.

Validation of the RayStation Monte Carlo dose calculation algorithm using realistic animal tissue phantoms.

PurposeThe aim of this study is to validate the RayStation Monte Carlo (MC) dose algorithm using animal tissue neck phantoms and a water breast phantom.MethodsThree anthropomorphic phantoms were used in a clinical setting to test the RayStation MC dose algorithm. We used two real animal necks that were cut to a workable shape while frozen and then thawed before being CT scanned. Secondly, we made a patient breast phantom using a breast prosthesis filled with water and placed on a flat surface. Dose distributions in the animal and breast phantoms were measured using the MatriXX PT device.ResultsThe measured doses to the neck and breast phantoms compared exceptionally well with doses calculated by the analytical pencil beam (APB) and MC algorithms. The comparisons between APB and MC dose calculations and MatriXX PT measurements yielded an average depth difference for best gamma agreement of <1 mm for the neck phantoms. For the breast phantom better average gamma pass rates between measured and calculated dose distributions were observed for the MC than for the APB algorithms.ConclusionsThe MC dose calculations are more accurate than the APB calculations for the static phantoms conditions we evaluated, especially in areas where significant inhomogeneous interfaces are traversed by the beam.

Variation in accumulated dose of volumetric-modulated arc therapy for pancreatic cancer due to different beam starting phases.

PurposeTo assess the effects of different beam starting phases on dosimetric variations in the clinical target volume (CTV) and organs at risk (OARs), and to identify the relationship between plan complexity and the dosimetric impact of interplay effects in volumetric‐modulated arc therapy (VMAT) plans for pancreatic cancer.MethodsSingle and double full‐arc VMAT plans were generated for 11 patients. A dose of 50.4 Gy in 28 fractions was prescribed to cover 50% of the planning target volume. Patient‐specific Digital Imaging and Communications in Medicine–Radiation Therapy plan files were divided into 10 files based on the respiratory phases in four‐dimensional computed tomography (4DCT) simulations. The phase‐divided VMAT plans were calculated in consideration of the beam starting phase for each arc and were then combined in the mid‐ventilation phase of 4DCT (4D plans). The dose‐volumetric parameters were compared with the calculated dose distributions without consideration of the interplay effects (3D plans). Additionally, relationships among plan parameters such as modulation complexity scores, monitor units (MUs), and dose‐volumetric parameters were evaluated.ResultsDosimetric differences in the median values associated with different beam starting phases were within ± 1.0% and ± 0.2% for the CTV and ± 0.5% and ± 0.9% for the OARs during single and double full‐arc VMAT, respectively. Significant differences caused by variations in the beam starting phases were observed only for the dose‐volumetric parameters of the CTV during single full‐arc VMAT (P < 0.05), associated with moderate or strong correlations between the MUs and the dosimetric differences between the 4D and 3D plans.ConclusionsThe beam starting phase affected CTV dosimetric variations of single full‐arc VMAT. The use of double full‐arc VMAT mitigated this problem. However, variation in the dose delivered to OARs was not dependent on the beam starting phase, even for single full‐arc VMAT.

Enhanced GEANT4 Monte Carlo simulations of the space radiation effects on the International Space Station and Apollo missions using high-performance computing environment

A significant challenge to current and future manned and/or unmanned space missions is due to deep space radiation. An improvement in more realistic (more accurate) simulation models in predicting the effects of radiation within the spacecraft is required, especially to better predict dose to astronauts, energy deposition within sensitive electronics, and effectiveness of radiation shielding for long-term space missions. The International Space Station provides an invaluable resource for long-term measurements of the radiation environment in Low Earth Orbit (LEO); however, the only manned missions with dosimetry data available beyond LEO are the Apollo missions. Thus the physiological effects and dosimetry for deep space missions are not well understood in planning extended missions. GEANT4, a Monte Carlo method, represents a powerful physics simulation tool to assess the effects of radiation transport through spacecraft. The newest version of GEANT4 supports multithreading and Message Passing Interface (MPI) allowing for much faster distributive processing of simulations, using a high-performance computing environment. This paper introduces a new simulation model and application using GEANT4 that greatly reduces computational time to hours instead of weeks without any post simulation processing based on high-performance computing. This paper also introduces a new set of GEANT4 computational detectors for calculating dose distribution, besides the historically used International Commission of Radiation Units simulation spheres. The computational detectors include a thermoluminescent detector, tissue equivalent proportional counter, and human phantom, along with additional new scorers to calculate dose equivalence based on the International Commission of Radiation Protection standards. This study presents GEANT4 simulations of the dose deposition for the International Space Station and the Apollo 11 and 14 missions, which replicate well the dose measurements during these missions. The simulations of both Apollo missions show consistent doses from galactic cosmic rays and radiation belts with a small variation in dose distribution across the Apollo capsule. The greatest contributor to radiation dose for both Apollo missions in the simulations came from galactic cosmic rays. Simulations of historical solar particle events during an Apollo missions show a solar particle event would not be fatal and below mission limits. These GEANT4 models also provides the values of the dose deposition and dose equivalent for various organs within a human phantom in the International Space Station and Apollo command module, which are developed for the first time using this GEANT4 based application.

Benchmarking Monte-Carlo dose calculation for MLC CyberKnife treatments

BackgroundVendor-independent Monte Carlo (MC) dose calculation (IDC) for patient-specific quality assurance of multi-leaf collimator (MLC) based CyberKnife treatments is used to benchmark and validate the commercial MC dose calculation engine for MLC based treatments built into the CyberKnife treatment planning system (Precision MC).MethodsThe benchmark included dose profiles in water in 15 mm depth and depth dose curves of rectangular MLC shaped fields ranging from 7.6 mm × 7.7 mm to 115.0 mm × 100.1 mm, which were compared between IDC, Precision MC and measurements in terms of dose difference and distance to agreement. Dose distributions of three phantom cases and seven clinical lung cases were calculated using both IDC and Precision MC. The lung PTVs ranged from 14 cm3 to 93 cm3. Quantitative comparison of these dose distributions was performed using dose-volume parameters and 3D gamma analysis with 2% global dose difference and 1 mm distance criteria and a global 10% dose threshold. Time to calculate dose distributions was recorded and efficiency was assessed.ResultsAbsolute dose profiles in 15 mm depth in water showed agreement between Precision MC and IDC within 3.1% or 1 mm. Depth dose curves agreed within 2.3% / 1 mm. For the phantom and clinical lung cases, mean PTV doses differed from − 1.0 to + 2.3% between IDC and Precision MC and gamma passing rates were > =98.1% for all multiple beam treatment plans. For the lung cases, lung V20 agreed within ±1.5%. Calculation times ranged from 2.2 min (for 39 cm3 PTV at 1.0 × 1.0 × 2.5 mm3 native CT resolution) to 8.1 min (93 cm3 at 1.1 × 1.1 × 1.0 mm3), at 2% uncertainty for Precision MC for the 7 examined lung cases and 4–6 h for IDC, which, however, is not optimized for efficiency but used as a gold standard for accuracy.ConclusionsBoth accuracy and efficiency of Precision MC in the context of MLC based planning for the CyberKnife M6 system were benchmarked against MC based IDC framework. Precision MC is used in clinical practice at our institute.

Calculation of beta radiation dose of a circular Y-90 skin patch: Analytical and simulation methods

Beta radiation therapy of superficial skin tumors using a radioactive patch can be an interesting treatment, mainly when the lesion is near sensitive targets, for example, close to the eye or bone. Due to their short range and rapid dose fall-off, beta radiation from a skin patch deposits its energy within the first layers of the skin. This property may lead to the protection of sensitive structures located right beneath the skin lesion during the radiation therapy procedure. To create and use individual and personalized radiation patches the distribution of the beta radiation dose needs to be known. An accurate and realistic calculation is needed to determine the amount of radioisotope required and the duration of the radiation therapy procedure to deliver the prescribed dose in a certain depth from the surface. In this paper, the theoretical model for a dose distribution calculation from a radioactive skin patch loaded with Yttrium-90 (Y-90) in different depths of skin tissue is presented. The calculation was based on the Loevinger beta-point source dose function, which is a numerical method for dosimetry of beta-emitting isotopes. Results of the calculation were also compared with that of Monte Carlo simulation methods to assess the reliability of the application of the Loevinger method in dosimetry of a beta-emitting radioactive patch.

Clinical Evaluation of a Rotational Set-up Correction Device for Multiple Brain Metastases Using Single-Isocenter Stereotactic Radiation Therapy

Stereotactic radiation therapy (SRT) using a single plan with one isocenter by helical tomotherapy (HT) can simultaneously treat multiple brain metastases with highly conformal dose distributions. This may reduce treatment time and intra-fractional motion. However, rotational set-up errors in the pitch and yaw angles cannot be corrected in the HT system. Therefore, increases in rotational set-up error and distance of the target-to-rotational-center may be associated with significant dosimetric uncertainties with this irradiation technique. Previous research shows that when rotational errors are increased to 2.0°, the prescription doses which cover 95% of the planning target volume were only 63% of all tested cases. Here, we developed a new head supporting device that allows for a highly-accurate correction of rotational errors during image-guided radiation therapy. In this study, we investigated the amounts of rotational correction required prior to irradiation, and evaluated the dosimetric efficacy of rotational correction for SRT, using HT unit to treat multiple brain metastases. A novel rotational set-up correction device was constructed from carbon material and conjected with the commonly-used head shell system on the top of our device. We were able to rotate in 0.1° increments for pitch and yaw angles using two screws. We retrospectively analyzed data from three patients irradiated by single-isocenter SRT for multiple brain metastases using the HT unit. The SRT plans were created with 30 Gy in three fractions prescribed for D99% to clinical target volumes. The required rotational correction values were determined by mega-voltage computed tomography (MVCT) before irradiation. Residual correction values were quantified after correction by the developed device. Moreover, dosimetric efficacy was evaluated using the calculated dose distribution with MVCT after rotational correction. Dose distribution was compared between 4D (three horizontal axes + roll) and 6D correction. We clarified efficacy of rotational correction depending on the distance of the target-to-rotational-center. The rotational correction for pitch was 1.2° ± 1.7°, roll: 0.9° ± 1.2°, and yaw: 1.3° ± 1.6°. The maximum correction value in pitch and yaw was 3.0°. The residual correction value after rotational correction was 0.2° ± 0.3° in both pitch and yaw angles. The difference in target dose between 4D and 6D corrections was improved by 2.5% in coverage index, and 1.0% on D99%. Dosimetric efficacy of rotational correction did not depend on the distance of the target-to-rotational-center. This suggests that irradiated doses for brain metastases in any location can be improved. Novel rotational set-up correction device can be corrected to within 0.5° in pitch and yaw angle error, and benefited rotational correction and irradiated dose distribution for single-isocenter SRT for multiple brain metastases.

Is Monte Carlo-Based Dose Calculation Algorithm Required for All Lung Tumors Treated with Stereotactic Body Radiotherapy?

The primary objective of this study is to compare the accuracy of a Boltzmann equation solvers algorithm against a Monte Carlo based dose calculation algorithm for island and peripheral lung tumors treated with Stereotactic Body Radiotherapy (SBRT). Ten lung SBRT patients considered for this retrospective study, five patients with an island lung tumor and five patients with a chest wall or lateral tumor . For each patient, a simulation CT scan and a 10-phase 4DCT scans were acquired. The initial delivered plans were planned with dynamic conformal arc technique using a planning system that utilizes the x-ray voxel Monte Carlo (XVMC) algorithm. The patients’ treatment plans were transferred to a treatment planning system utilizes the Acuros AXB algorithm. These plans were retrospectively re-calculated keeping the initial arc angles and multi leaf collimator (MLC) segments. Validation plans were generated by projecting the plans into a 4D detector/cylindrical diode detector array quality assurance (QA) phantom. Quality assurance plans were delivered using a 6X SRS beam. Measured dose distributions were compared with calculated dose distributions using 3% 3mm gamma analysis. Dose-volumetric data including the maximum dose, minimum dose, D95% for internal tumor volume (ITV) and planning tumor volume (PTV) were compared for the two algorithms using the Wilcoxon Rank Sum test. All analyses were performed using data and decision management software with two-tailed p-values reported and a p-value of <0.05 considered significant. The table below summarizes the results for dose-volumetric data for all 10 patients. For the island tumors the ITV minimum dose and the PTV minimum dose showed a trend for a difference between the AXB and XVMC algorithms (p=0.06, p=0.06, respectively). There was no difference observed between the ITV max, or PTV max . For the peripheral tumors there was no difference observed for any of the dose volumetric data. Passing rate for both algorithms is above 90% with (3mm, 3%) gamma analysis. The XVMC requires 16% more monitor unit than the AXB algorithm. The dosimetric performance of the AXB is comparable to the XVMC algorithm. This indicates that a Monte Carlo based dose calculation algorithm is not the only algorithm that can be used for lung tumors treated with SBRT. The AXB algorithm can be used clinically even for island type lung tumor where high heterogeneity is present. Slight differences in minimum doses were noted for island tumors; however, these differences are usually clinically acceptable.Abstract 3828; Table 1CriteriaXVMCAXBITV Max dose (%)131.2±5.6130.5±7.1ITV Min dose (%)106.7 ±6.2108.2 ±9.9PTV Max dose (%)131.2 ±6.2130.6 ±6.7PTV Min dose (%)83.1±5.386.0±3.9D95%101.4±1.9100.6±1.7Gamma (3%, 3mm)95.0 ± 3.093.7 ± 2.0 Open table in a new tab

Pre-clinical and small field dosimetry

Preclinical in vivo studies have drastically improved over the past decade with the development of cone beam computed tomography (CBCT) image-guided small animal irradiation systems. Such systems produce 220 or 225 kV x-rays with square and circular field sizes ranging from 0.5 to 10 mm. The dosimetry of such equipment involving kilovoltage small-field dosimetry has not received as much attention as the megavoltage small-field dosimetry. The dosimetry of megavoltage small fields can be challenging due to lateral charged particle disequilibrium, detector volume averaging effect, and high dose gradients. Clinically there has been a rapid increase in the use of small fields in modern radiotherapy techniques such as stereotactic radiosurgery (SRS) and stereotactic radiotherapy (SBRT). This study presents dosimetric properties of image-guided small animal irradiation systems. Both EBT Gafchromic films and 3-D PRESAGE measured beam data were presented and compared with the calculated dose distribution from a commissioned planning system. For megavoltage small-field dosimetry, EBT3 films and PRESAGE dosimeters were used to measure the dose distributions for MLC-delimited 6 x 6 to 20 x 20 mm2 square fields, and for selected IMRT and VMAT plans with small field sizes or segments of 1-4 cm. A single-beam optical CT scanner was used as the readout mechanism of the radiation-induced 3-D information in the PRESAGE phantoms. Measured data sets were compared with calculated results from Eclipse Acuros XB. The results for kilovoltage small animal irradiator showed that PDD data measured from EBT films and PRESAGE dosimeters are in agreement within 3%; profiles and 2-D dose distributions measured from PRESAGE present a larger penumbra compared with those from EBT films. Discrepancies between measured beam data and treatment planning data were identified. For megavoltage small fields, the measured data percent depth dose, beam profiles, and dose distributions were found to agree within experimental uncertainties for EBT films and PRESAGE dosimeters. Beam profiles for MLC-delimited field sizes less than 10 mm reveal some discrepancies in the penumbra region between measured data and calculated results. Dose distributions from EBT film and PRESAGE measurements demonstrate that 3-D dosimetry measurement for small-field IMRT and VMAT QA may be necessary to ensure a complete verification of dose delivery and accuracy.