Dose Distributions In Water Research Articles

Monte Carlo simulation of the Varian TrueBeam flattened-filtered beams using a surrogate geometry in PRIMO

BackgroundMonte Carlo simulation of radiation transport for medical linear accelerators (linacs) requires accurate knowledge of the geometrical description of the linac head. Since the geometry of Varian TrueBeam machines has not been disclosed, the manufacturer distributes phase-space files of the linac patient-independent part to allow researchers to compute absorbed dose distributions using the Monte Carlo method. This approach limits the possibility of achieving an arbitrarily small statistical uncertainty. This work investigates the use of the geometry of the Varian Clinac 2100, which is included in the Monte Carlo system PRIMO, as a surrogate.MethodsEnergy, radial and angular distributions extracted from the TrueBeam phase space files published by the manufacturer and from phase spaces tallied with PRIMO for the Clinac 2100 were compared for the 6, 8, 10 and 15 MV flattened-filtered beams. Dose distributions in water computed for the two sets of PSFs were compared with the Varian Representative Beam Data (RBD) for square fields with sides ranging from 3 to 30 cm. Output factors were calculated for square fields with sides ranging from 2 to 40 cm.ResultsExcellent agreement with the RBD was obtained for the simulations that employed the phase spaces distributed by Varian as well as for those that used the surrogate geometry, reaching in both cases Gamma (2%\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$2\\%$$\\end{document}, 2 mm) pass rates larger than 99%\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$99\\%$$\\end{document}, except for the 15 MV surrogate. This result supports previous investigations that suggest a change in the material composition of the TrueBeam 15 MV flattening filter. In order to get the said agreement, PRIMO simulations were run using enlarged transport parameters to compensate the discrepancies between the actual and surrogate geometries.ConclusionsThis work sustains the claim that the simulation of the 6, 8 and 10 MV flattening-filtered beams of the TrueBeam linac can be performed using the Clinac 2100 model of PRIMO without significant loss of accuracy.

Monte Carlo investigation of dose distribution of uniformly and non-uniformly loaded standard and notched eye plaques.

To investigate the effect of using non-uniform loading and notched plaques on dose distribution for eye plaques. Using EGSnrc Monte Carlo (MC) simulations, we investigate eye plaque dose distributions in water and in an anatomically representative eye phantom. Simulations were performed in accordance with TG43 formalism and compared against full MC simulations which account for inter-seed and inhomogeneity effects. For standard plaque configurations, uniformly and non-uniformly loaded plaque dose distributions in water showed virtually no difference between each other. For standard plaque, the MC calculated dose distribution in planes parallel to the plaque is narrower than the TG43 calculation due to attenuation at the periphery of the plaque by the modulay. MC calculated the dose behind the plaque is fully attenuated. Similar results were found for the notched plaque, with asymmetric attenuation along the plane of the notch. Cumulative dose volume histograms showed significant reductions in the calculated MC doses for both tumor and eye structures, compared to TG43 calculations. The effect was most pronounced for the notch plaque where the MC dose to the optic nerve was greatly attenuated by the modulay surrounding the optic nerve compared to the TG43. Thus, a reduction of optic nerve D95% from 14 to 0.2Gy was observed, when comparing the TG43 calculation to the MC result. The tumor D95% reduced from 89.2 to 79.95Gy for TG43 and MC calculations, respectively. TG43 calculations overestimate the absolute dose and the lateral dose distribution of both standard and notched eye plaques, leading to the dose overestimation for the target and organs at risk. The dose matching along the central axis for the non-uniformly loaded plaques to that of uniformly loaded ones was found to be sufficient for providing comparable coverage and can be clinically used in eye-cancer-busy centers.

Auto-commissioning of a Monte Carlo electron beam model with application to photon MLC shaped electron fields

Objective. Presently electron beam treatments are delivered using dedicated applicators. An alternative is the usage of the already installed photon multileaf collimator (pMLC) enabling efficient electron treatments. Currently, the commissioning of beam models is a manual and time-consuming process. In this work an auto-commissioning procedure for the Monte Carlo (MC) beam model part representing the beam above the pMLC is developed for TrueBeam systems with electron energies from 6 to 22 MeV. Approach. The analytical part of the electron beam model includes a main source representing the primary beam and a jaw source representing the head scatter contribution each consisting of an electron and a photon component, while MC radiation transport is performed for the pMLC. The auto-commissioning of this analytical part relies on information pre-determined from MC simulations, in-air dose profiles and absolute dose measurements in water for different field sizes and source to surface distances (SSDs). For validation calculated and measured dose distributions in water were compared for different field sizes, SSDs and beam energies for eight TrueBeam systems. Furthermore, a sternum case in an anthropomorphic phantom was considered and calculated and measured dose distributions were compared at different SSDs. Main results. Instead of the manual commissioning taking up to several days of calculation time and several hours of user time, the auto-commissioning is carried out in a few minutes. Measured and calculated dose distributions agree generally within 3% of maximum dose or 2 mm. The gamma passing rates for the sternum case ranged from 96% to 99% (3% (global)/2 mm criteria, 10% threshold). Significance. The auto-commissioning procedure was successfully implemented and applied to eight TrueBeam systems. The newly developed user-friendly auto-commissioning procedure allows an efficient commissioning of an MC electron beam model and eases the usage of advanced electron radiotherapy utilizing the pMLC for beam shaping.

3D range-modulators for proton therapy: near field simulations with FLUKA and comparison with film measurements

The 3D range-modulator is a device used in particle delivery systems that can create a highly conformal and homogeneous dose distribution in the target volume with mono-energetic beams, providing an option for high dose-rate FLASH therapy. In the normal case, the modulators are positioned at a typical distance of 30-50 cm in front of the target in order to avoid the fluence ripples resulting from the periodic structure of the modulators. FLUKA Monte Carlo simulation package was used to investigate the fluence distributions of protons penetrating through the 2D range-modulator, the simplified version of the 3D range-modulator, and to determine the minimum distance at which the fluence is homogeneous enough for the treatment. To implement the complex geometry of the modulator in FLUKA, a dedicated FLUKA user routine was developed for the simulation of the periodic pin structures. The highest fluence ripple occurred at a few centimetres behind the modulators and then faded away as the distance increased, which can be described by the edge-scattering effect and later by the blur-out of the overlapping contributions from the pins. Moreover, the dose distribution in water was investigated, particularly for small distances between the modulators and the water phantom. Furthermore, the Monte Carlo results were compared with radiochromic film measurements irradiated with a 3D-printed range modulator and showed a good qualitative agreement. Prospectively, for low modulator-to-target distances, the strong dose inhomogeneities which appear in the proximal part of the target, could introduce additionally a kind of ‘mini beam’ normal-tissue sparing by the 3D range-modulators.

Sharp dose profiles for high precision proton therapy using strongly focused proton beams

The main objective of radiotherapy is to exploit the curative potential of ionizing radiation while inflicting minimal radiation-induced damage to healthy tissue and sensitive organs. Proton beam therapy has been developed to irradiate the tumor with higher precision and dose conformity compared to conventional X-ray irradiation. The dose conformity of this treatment modality may be further improved if narrower proton beams are used. Still, this is limited by multiple Coulomb scattering of protons through tissue. The primary aim of this work was to develop techniques to produce narrow proton beams and investigate the resulting dose profiles. We introduced and assessed three different proton beam shaping techniques: (1) metal collimators (100/150 MeV), (2) focusing of conventional- (100/150 MeV), and (3) focusing of high-energy (350 MeV, shoot-through) proton beams. Focusing was governed by the initial value of the Twiss parameter alpha (alpha _0), and can be implemented with magnetic particle accelerator optics. The dose distributions in water were calculated by Monte Carlo simulations using Geant4, and evaluated by target to surface dose ratio (TSDR) in addition to the transverse beam size (sigma _T) at the target. The target was defined as the location of the Bragg peak or the focal point. The different techniques showed greatly differing dose profiles, where focusing gave pronouncedly higher relative target dose and efficient use of primary protons. Metal collimators with radii <,2~mathrm{mm} gave low TSDRs (<~0.7) and large sigma _T(>~3.6~hbox {mm}). In contrast, a focused beam of conventional (150~hbox {MeV}) energy produced a very high TSDR (>~80) with similar sigma _T as a collimated beam. High-energy focused beams were able to produce TSDRs >~100 and sigma _T around 1.5 mm. From this study, it appears very attractive to implement magnetically focused proton beams in radiotherapy of small lesions or tumors in close vicinity to healthy organs at risk. This can also lead to a paradigm change in spatially fractionated radiotherapy. Magnetic focusing would facilitate FLASH irradiation due to low losses of primary protons.

Friday, June 17, 20222:30 PM - 3:30 PM PHSPP01 Presentation Time: 2:30 PM: Dosimetry for Beta Emitting Brachytherapy Sources

<h3>Purpose</h3> Dosimetry for photon-emitting brachytherapy sources is well established, to the point of commercial software for clinical treatment planning to account for material heterogeneities and patient-specific radiation scatter conditions. However, dosimetry standards for beta sources are simpler and have not been recently investigated. While the AAPM TG-149 and ICRU 72 Reports issue clinical guidance and reference data, a modern evaluation of beta-source dosimetry is missing. This investigation strives to identify the potential role for beta-emitting radionuclides as well as to examine theoretical sources for a deeper understanding of beta dosimetry. <h3>Materials and Methods</h3> Available beta-emitting brachytherapy sources include P-32, Sr/Y-90, and Ru/Rh-106 with Y-90 commonly used for selective internal radiation therapy (SIRT) of cancers in the liver. Energy spectra of these sources were taken from ICRU 72 with dose simulations performed using the MCNP v6.2 Monte Carlo radiation transport code. From ICRU 72, the maximum energies for P-32, Sr/Y-90, Y-90, and Ru/Rh-106 were 1.71, 2.28, 2.28, and 3.54 MeV, respectively. Monoenergetic sources of 0.5, 1, 1.5, 2, 3, and 4 MeV were also simulated to extend the energy range and scrutinize dose-distribution dependence on electron energy. Since 2D dose distribution are highly dependent on the design of a source, point sources were simulated to evaluate the depth-dose distribution as a function of radius, <i>r</i>, as well radial dose functions that removed the inverse-square effect. Dose was sampled in 15 cm diameter spheres comprised of water, tissue (ICRU 44 skeletal muscle), or bone (ICRU 44 cortical bone). To mass density effect was quantified in water for 0.5, 1, and 2 g/cm³ with 1 and 2 g/cm³ for tissue and bone, respectively. Observed dose differences were then ascribed to medium differences and not confounded by small differences in mass density. Dose was sampled with radial increments from 0.001 to 0.02 cm for direct comparisons to ICRU 72 results and to evaluate the effect of volume averaging. While only electron sources were considered, dose contributions from bremsstrahlung photons were quantified through comparing results of electron- and electron:photon-transport. Energy deposition was calculated using the MCNP *F8 pulse-height tally, then dividing by the voxel mass to obtain absorbed dose per electron history. With 10⁷ source electrons, statistical uncertainties (<i>k</i>=1) were <0.1% for <i>r</i><0.7 CSDA. When studying bremsstrahlung, 10⁸ source electrons produced 4% statistical uncertainties for <i>r</i>>1.2 CSDA. <h3>Results</h3> In the high dose-gradient region for radionuclide-based beta emitters, the calculated dose distributions in water were within 20% of those from the ICRU 72 Report, which used binned monoenergetic sources to approximate the continuous beta spectra. The radionuclide-based sources exhibited radial dose functions in water that diminished much more steeply than Pd-103, similar to a <0.01 MeV photon source. Sr/Y-90 had a bimodal distribution. Radial dose functions for radionuclide-based sources were inversely proportional to changes in mass density, differing from the scaling-factor models by Cross (1967) and the ICRU 56 Report over the large range of media and densities examined herein. Further, radial dose functions for monoenergetic sources demonstrated a Bragg peak with dose enhancement at <i>r</i>∼0.7 CSDA for all studied energies and media. A sampling of results for radionuclides and monoenergetic sources in water is given in the figure. <h3>Conclusions</h3> Beta dosimetry is largely governed by the mass density with medium composition being a much lesser effect, unlike for low-energy photons where the photoelectric effect is highly sensitive to <i>Z</i>_eff. Unlike for high-energy photon dosimetry, beta dosimetry is negligibly impacted by radiation scatter conditions due to their short range. While Ru/Rh-106 and Sr/Y-90 are proven for shallow plaque brachytherapy and Y-90 for SIRT, beta-emitting radionuclides do not have adequate range for interstitial implants like prostate brachytherapy since dose uniformity is much worse than with HDR Ir-192 or LDR Pd-103 brachytherapy.

Toward 3D dose verification of an electronic brachytherapy source with a plastic scintillation detector.

BackgroundElectronic brachytherapy (eBT) is considered a safe treatment with good outcomes. However, eBT lacks standardized and independent dose verification, which could impede future use.PurposeTo validate the 3D dose‐to‐water distribution of an electronic brachytherapy (eBT) source using a small‐volume plastic scintillation detector (PSD).MethodsThe relative dose distribution of a Papillon 50 (P50) (Ariane Medical Systems, UK) eBT source was measured in water with a PSD consisting of a cylindrical scintillating BCF‐12 fiber (length: 0.5 mm, Ø: 1 mm) coupled to a photodetector via an optical fiber. The measurements were performed with the PSD mounted on a motorized stage in a water phantom (MP3) (PTW, Germany). This allowed the sensitive volume of the PSD to be moved to predetermined positions relative to the P50 applicator, which pointed vertically downward while just breaching the water surface. The percentage depth‐dose (PDD) was measured from 0 to 50 mm source‐to‐detector distance (SDD) in 1–3 mm steps. Dose profiles were measured along two perpendicular axes at five different SDDs with step sizes down to 0.5 mm. Characterization of the PSD consisted of determining the energy correction through Monte Carlo (MC) simulation and by measuring the stability and dose rate linearity using a well‐type ionization chamber as a reference. The measured PDD and profiles were validated with corresponding MC simulations.ResultsThe measured and simulated PDD curves agreed within 2% (except at 0 mm and 43 mm depth) after the PSD measurements were corrected for energy dependency. The absorbed dose decreased by a factor of 2 at 7 mm depth and by a factor of 10 at 26 mm depth. The measured dose profiles showed dose gradients at the profile edges of more than 50%/mm at 5 mm depth and 15%/mm at 50 mm depth. The measured profile widths increased 0.66 mm per 1 mm depth, while the simulated profile widths increased 0.74 mm per 1 mm depth. An azimuthal dependency of > 10% was observed in the dose at 10 mm distance from the beam center. The total uncertainty of the measured relative dose is < 2.5% with a positional uncertainty of 0.4 mm. The measurements for a full 3D dose characterization (PDD and profiles) can be carried out within 8 h, the limiting factor being cooling of the P50.ConclusionThe PSD and MP3 water phantoms provided a method to independently verify the relative 3D dose distribution in water of an eBT source.

Characterization of small active detectors for electronic brachytherapy dosimetry

Three active detectors, Exradin A26 chamber, IBA RAZOR Nano chamber, and PTW 60019 microDiamond, were characterized for their use in electronic BrachyTherapy (eBT) dosimetry. The detectors were calibrated in terms of air kerma using the ISO “N” (narrow) and “TW” (therapy) X-rays series from 7.5 kV to 100 kV. The responses to mono-energetic photons and their uncertainties were determined with Bayesian parameter estimation, assuming a model that incorporated smoothness via a spline function. The response functions obtained this way are consistent with 18 calibration qualities simultaneously. This approach improves on the traditional procedure of associating the response to the mean energy of the corresponding spectra. The energy responses (with uncertainties) were obtained in 0.25 keV energy steps from 6 keV to 70 keV. With differences in magnitude due to their sizes and the nature of their active volumes, the energy responses of the 3 detectors follow a similar relative behaviour. Even when the response is far from flat at low energies, i.e. below about 20 keV, the determination of reliable energy dependence curves enables the use of these detectors for dosimetry in the vicinity of eBT units. The angular dependence of the three detectors with respect to beam incidence was also measured in air in a 180° range in steps of 10° using the Zeiss INTRABEAM system (50 kV). For both energy and angular response characterization, the detectors axis were aligned in parallel with the beam axis (end-on), since this is the expected orientation in further measurements of absorbed dose distribution in water around eBT sources at PTB. This work is an effort to provide traceability for detectors and measurement procedures for the determination of 3D dose distributions as part of the ongoing European EMPIR project “Primary standards and traceable measurement methods for X-ray emitting electronic brachytherapy devices” (PRISM-eBT)”.

Preliminary tests of dosimetric quality and projected therapeutic outcomes of multi-phase 4D radiotherapy with proton and carbon ion beams

Objective. The purpose of this study was to perform preliminary pre-clinical tests to compare the dosimetric quality of two approaches to treating moving tumors with ion beams: synchronously delivering the beam with the motion of a moving planning target volume (PTV) using the recently developed multi-phase 4D dose delivery (MP4D) approach, and asynchronously delivering the ion beam to a motion-encompassing internal tumor volume (ITV) combined with rescanning. Approach. We created 4D optimized treatment plans with proton and carbon ion beams for two patients who had previously received treatment for non-small cell lung cancer. For each patient, we created several treatment plans, using approaches with and without motion mitigation: MP4D, ITV with rescanning, static deliveries to a stationary PTV, and deliveries to a moving tumor without motion compensation. Two sets of plans were optimized with margins or robust uncertainty scenarios. Each treatment plan was delivered using a recently-developed motion-synchronized dose delivery system (M-DDS); dose distributions in water were compared to measurements using gamma index analysis to confirm the accuracy of the calculations. Reconstructed dose distributions on the patient CT were analyzed to assess the dosimetric quality of the deliveries (conformity, uniformity, tumor coverage, and extent of hotspots). Main results. Gamma index analysis pass rates confirmed the accuracy of dose calculations. Dose coverage was >95% for all static and MP4D treatments. The best conformity and the lowest lung doses were achieved with MP4D deliveries. Robust optimization led to higher lung doses compared to conventional optimization for ITV deliveries, but not for MP4D deliveries. Significance. We compared dosimetric quality for two approaches to treating moving tumors with ion beams. Our findings suggest that the MP4D approach, using an M-DDS, provides conformal motion mitigation, with full target coverage and lower OAR doses.

Monte Carlo calculation of the relative TG-43 dosimetry parameters for the INTRABEAM electronic brachytherapy source

The INTRABEAM system (Carl Zeiss Meditec AG, Jena, Germany) is an electronic brachytherapy (eBT) device designed for intraoperative radiotherapy applications. To date, the INTRABEAM x-ray source has not been characterized according to the AAPM TG-43 specifications for brachytherapy sources. This restricts its modelling in commercial treatment planning systems (TPSs), with the consequence that the doses to organs at risk are unknown. The aim of this work is to characterize the INTRABEAM source according to the TG-43 brachytherapy dosimetry protocol. The dose distribution in water around the source was determined with Monte Carlo (MC) calculations. For the validation of the MC model, depth dose calculations along the source longitudinal axis were compared with measurements using a soft x-ray ionization chamber (PTW 34013) and two synthetic diamond detectors (microDiamond PTW TN60019). In our results, the measurements in water agreed with the MC model calculations within uncertainties. The use of the microDiamond detector yielded better agreement with MC calculations, within estimated uncertainties, compared to the ionization chamber at points of steeper dose gradients. The radial dose function showed a steep fall-off close to the INTRABEAM source (10 mm) with a gradient higher than that of commonly used brachytherapy radionuclides (192Ir, 125I and 103Pd), with values of 2.510, 1.645 and 1.232 at 4, 6 and 8 mm, respectively. The radial dose function partially flattens at larger distances with a fall-off comparable to that of the Xoft Axxent® (iCAD, Inc., Nashua, NH) eBT system. The simulated 2D polar anisotropy close to the bare probe walls showed deviations from unity of up to 55% at 10 mm and 155°. This work presents the MC calculated TG-43 parameters for the INTRABEAM, which constitute the necessary data for the characterization of the source as required by a TPS used in clinical dose calculations.

Prediction of dose distribution from luminescence image of water using a deep convolutional neural network for particle therapy.

We recently obtained nearly the same depth profiles of luminescence images of water as dose for protons by subtracting the Cerenkov light component emitted by secondary electrons of prompt gamma photons. However, estimating the distribution of Cerenkov light with this correction method is time-consuming, depending on the irradiated energy of protons by Monte Carlo simulation. Therefore, we proposed a method of estimating dose distributions from the measured luminescence images of water using a deep convolutional neural network (DCNN). In this study, we adopted the U-Net architectures as the DCNN. To prepare a large amount of image data for DCNN training, we calculated the training data pairs of two-dimensional (2D) dose distributions and luminescence images of water by Monte Carlo simulation for protons and carbon ions. After training the U-Net model for protons or carbon ions using these dose distributions and luminescence images calculated by Monte Carlo simulation, we predicted the dose distributions from the calculated and measured luminescence images of water using the trained U-Net model. All of the U-Net model's predicted images were in good agreement with the MC-calculated dose distributions and showed lower values of the root mean square percentage error (RSMPE) and higher values in the structural similarity index (SSIM) in comparison with these values for calculated or measured luminescence images. We confirmed that the DCNN effectively predicts dose distributions in water from the measured as well as calculated luminescence images of water for particle therapy.

A GEMPix-based integrated system for measurements of 3D dose distributions in water for carbon ion scanning beam radiotherapy.

PurposeCommercially available systems for ion beam reference dosimetry in water are mainly based on ionization chambers. In those systems, a large number of small detectors are typically arranged in a two‐dimensional (2D) array or matrix to achieve high spatial resolution (order of several millimeters) and large field coverage at the same time. The goal of this work was to investigate the reliability of a detector of superior spatial resolution to perform three‐dimensional (3D) ionization measurements in carbon ion pencil beams.MethodsThe GEMPix is a small gaseous detector with a highly pixelated readout, consisting of a drift region (with 2.8 cm3 × 2.8 cm3 × 0.3 cm3 volume), three gas electron multipliers (GEMs) for signal amplification and four Timepix ASICs with 55 µm pixel pitch and a total of 262,144 pixels. An integrated system was designed and built, which consists of a commercial water phantom with a three‐axis motorized arm, a reference large‐area ionization chamber for signal normalization to the beam output and the GEMPix itself. Measurements at different depths in water have been performed at the Italian National Centre for Oncological Hadrontherapy (CNAO) with three carbon ion beam energies. Lateral beam profiles measured with the GEMPix at the shallowest depth were compared to those measured with radiochromic EBT3 films in air in the position of the reference ionization chamber. The Timepix readout was calibrated in energy by using one independent depth scan with carbon ions of 150 mm range. Bragg peak curves were also simulated using the Monte Carlo FLUKA code as a reference.ResultsBeam profiles measured with the GEMPix were smooth and showed similar shape and full width at half maximum when compared to those measured with radiochromic EBT3 films. Smooth, reproducible Bragg curves were obtained with statistical uncertainties of about 2%, matching FLUKA simulations of the Bragg curves within 15% for most data points. This difference is partially explained for the measurement with carbon ions of 150 mm range by a saturation effect in the GEMs. The high granularity of the readout allowed to produce 2D images of the deposited dose at different depths, as well as 3D data distributions.ConclusionsThis paper demonstrates the capability of the GEMPix detector to measure the 3D dose distribution of carbon ions in water for a clinical pencil beam reliably. In the future, the detector area will be increased to cover fields of scanned beams. Measurements at higher beam intensities and with protons are planned.

Dosimetric characterization of a novel 90Y source for use in the conformal superficial brachytherapy device

PurposeTo characterize the dose distribution in water of a novel beta-emitting brachytherapy source for use in a Conformal Superficial Brachytherapy (CSBT) device. Methods and materialsYttrium-90 (90Y) sources were designed for use with a uniquely designed CSBT device. Depth dose and planar dose measurements were performed for bare sources and sources housed within a 3D printed source holder. Monte Carlo simulated dose rate distributions were compared to film-based measurements. Gamma analysis was performed to compare simulated and measured dose rates from seven 90Y sources placed simultaneously using the CSBT device. ResultsThe film-based maximum measured surface dose rate for a bare source in contact with the surface was 3.35 × 10–7 cGy s−1 Bq−1. When placed in the source holder, the maximum measured dose rate was 1.41 × 10–7 cGy s−1 Bq−1. The Monte Carlo simulated depth dose rates were within 10% or 0.02 cm of the measured dose rates for each depth of measurement. The maximum film surface dose rate measured using a seven-source configuration within the CSBT device was 1.78 × 10−7 cGy s−1 Bq−1. Measured and simulated dose rate distribution of the seven-source configuration were compared by gamma analysis and yielded a passing rate of 94.08%. The gamma criteria were 3% for dose-difference and 0.07056 cm for distance-to-agreement. The estimated measured dose rate uncertainty was 5.34%. Conclusions90Y is a unique source that can be optimally designed for a customized CSBT device. The rapid dose falloff provided a high dose gradient, ideal for treatment of superficial lesions. The dose rate uncertainty of the 90Y-based CSBT device was within acceptable brachytherapy standards and warrants further investigation.

Numerical Model Simulating the Therapeutic Electron Beam of a Clinical Linear Accelerator

We present here the results of developing a numerical model of therapeutic electron beams from a clinical accelerator, which is required to solve the task of defining the transverse profile of the fields intended for radiotherapy procedures. We describe a model of an electron source with typical main elements of medical accelerators, including a standard applicator used for forming the beam profile. The model was shown to be sufficiently accurate by comparing the calculated data for the electron beam depth dose distribution in water with the results of experiments obtained using a medical accelerator.

Characterization of an x-ray source with a partitioned diamond-tungsten target for electronic brachytherapy with 3D beam directionality

The Sculptura™ is a new high-dose-rate electronic brachytherapy system developed by Sensus Healthcare. By combining a steerable electron beam with a partitioned diamond-tungsten x-ray target, the x-ray source of the Sculptura™ is capable of producing highly anisotropic dose distributions, thus achieving true 3D beam directionality. This article reports the spectral and dosimetric characterization of the Sculptura™ x-ray source through a combination of measurements and Monte Carlo simulations for operating points between 50–100 kV. Excellent agreement (~5% discrepancy) between the simulations and measurements was obtained for in-air dose rate characterization. The validated simulations were then used to calculate the dose distribution in water. Dose rates of >2 cGy/min/μA can be produced at 100 kV, thus delivering 10 Gy in 1 min for typical operating conditions. The dose distributions are sharply peaked, with a full-width at half-maximum azimuth of about 100°.

Beam commissioning of the first compact proton therapy system with spot scanning and dynamic field collimation.

To describe the measurements and to present the results of the beam commissioning and the beam model validation of a compact, gantry-mounted, spot scanning proton accelerator system with dynamic layer-by-layer field collimation. We performed measurements of depth dose distributions in water, spot and scanned field size in air at different positions from the isocenter plane, spot position over the 20 × 20 cm2 scanned area, beam monitor calibration in terms of absorbed dose to water and specific field collimation measurements at different gantry angles to commission the system. To validate the beam model in the treatment planning system (TPS), we measured spot profiles in water at different depths, absolute dose in water of single energy layers of different field sizes and inversely optimised spread-out Bragg peaks (SOBP) under normal and oblique beam incidence, field size and penumbra in water of SOBPs, and patient treatment specific quality assurance in homogeneous and heterogeneous phantoms. Energy range, spot size, spot position and dose output were consistent at all gantry angles with 0.3 mm, 0.4 mm, 0.6 mm and 0.5% maximum deviations, respectively. Uncollimated spot size (one sigma) in air with an air-gap of 10 cm ranged from 4.1 to 16.4 mm covering a range from 32.2 to 1.9 cm in water, respectively. Absolute dose measurements were within 3% when comparing TPS and experimental data. Gamma pass rates >98% and >96% at 3%/3 mm were obtained when performing 2D dose measurements in homogeneous and in heterogeneous media, respectively. Leaf position was within ±1 mm at all gantry angles and nozzle positions. Beam characterisation and machine commissioning results, and the exhaustive end-to-end tests performed to assess the proper functionality of the system, confirm that it is safe and accurate to treat patients. This is the first paper addressing the beam commissioning and the beam validation of a compact, gantry-mounted, pencil beam scanning proton accelerator system with dynamic layer-by-layer multileaf collimation.

Development of EPID-based dosimetry for FFF-beam verification in radiation therapy

The purpose of this study was to develop Electronic Portal Imaging Devices (EPID)-based dosimetry for Flattening-Filter-Free (FFF) beam verification. All radiation measurements were performed with source to imager distances (SID) of 150 cm to reduce saturation effect. EPID images were converted to radiation absorbed dose with our algorithm including four parameters: linearity of dose response with Monitor Unit (MU), beam profile correction, collimator scatter, and scatter kernel. The Calibration Units (CU) of image were scaled to dose (Gy) by using linearity of dose response with MU. Off-axis response differences between EPID and water were reduced with beam profile correction. Scatter kernel was applied to EPID images to reduce the residual error. The algorithm accuracy was validated with 12 arcs of Volumetric Modulated Arc Therapy (VMAT) plans by using gamma analysis comparing between EPID-based dosimetry and a plane dose distribution of Treatment Planning Systems (TPS). Gamma Passing Rates (GPR) were used to determine the dose agreements with criteria of 2%, 2 mm and 3%, 3 mm. The mean of GPR was 97.91%, and 99.62% for criteria of 2%,2 mm and, 3%, 3 mm, respectively. Our EPID-based dosimetry showed good agreement with plane dose distribution in water. These results indicated that our EPID-based dosimetry can perform FFF-beam verification.

Determining the energy spectrum of clinical linear accelerator using an optimized photon beam transmission protocol.

The complex beam delivery techniques for patient treatment using a clinical linear accelerator (linac) may result in variations in the photon spectra, which can lead to dosimetric differences in patients that cannot be accounted for by current treatment planning systems (TPSs). Therefore, precise knowledge of the fluence and energy spectrum (ES) of the therapeutic beam is very important. However, owing to the high energy and flux of the beam, the ES cannot be measured directly, and validation of the spectrum modeled in the TPS is difficult. The aim of this study is to develop an efficient beam transmission measurement procedure for accurately reconstructing the ES of a therapeutic x-ray beam generated by a clinical linac. The attenuation of a 6MV photon beam from an Elekta Synergy Platform clinical linac through different thicknesses of graphite and lead was measured using an ion chamber. The response of the ion chamber as a function of photon energy was obtained using the Monte Carlo (MC) method in the Geant4 simulation code. Using the curves obtained in the photon beam transmission measurements and the ion chamber energy response, the ES was reconstructed using an iterative algorithm based on a mathematical model of the spectrum. To evaluate the accuracy of the spectrum reconstruction method, the reconstructed ES (ESrecon ) was compared to that determined by the MC simulation (ESMC ). The ion chamber model in the Geant4 simulation was well validated by comparing the ion chamber perturbation factors determined by the TRS-398 calibration protocol and EGSnrc; the differences were within 0.57%. The number of transmission measurements was optimized to 10 for efficient spectrum reconstruction according to the rate of increase in the spectrum reconstruction accuracy. The distribution of ESrecon obtained using the measured transmission curves was clearly similar to the reference, ESMC , and the dose distributions in water calculated using ESrecon and ESMC were similar within a 2% local difference. However, in a heterogeneous medium, the dose discrepancy between them was >5% when a complex beam delivery technique composed of 171 control points was used. The proposed measurement procedure required a total time of approximately 1h to obtain and analyze 20 transmission measurements. In addition, it was confirmed that the transmission curve of high-Z materials influences the accuracy of spectrum reconstruction more than that of low-Z materials. A well-designed transmission measurement protocol suitable for clinical environments could be an essential tool for better dosimetric accuracy in patient treatment and for periodic verification of the beam quality.

Monte Carlo dosimetry for a new 32P brachytherapy source using FLUKA code.

PurposeDosimetric characterization of a new 32P brachytherapy source was studied and the validity of the FLUKA code to reproduce the dosimetric parameters in a water phantom was evaluated. In addition, dose rate distributions around the 32P source sheathed by a catheter and unsheathed source were investigated in different tissue phantoms.Material and methodsThe new 32P source was modeled using FLUKA Monte Carlo code. According to the AAPM TG-60 recommendations, reference of absorbed dose rate, radial dose function, anisotropy function, and an away-along table for quality assurance purposes inside water phantom were calculated. Moreover, the results of the radial dose function and dose rate were obtained for the sheathed source and unsheathed sources at radial distances in different tissue phantoms: liver, fat tissue, 9-component soft tissue, and 4-component soft tissue.ResultsThe calculated dosimetric parameters of the new 32P source by FLUKA code in water phantom agreed well with that of the GEANT4 calculation. The 2D away-along dose results were similar to the GEANT4 simulation for distances less than 0.25 cm, small differences were apparent at long distances from the source. Dose rate evaluation for the sheathed source shows that the presence of a catheter increases the dose values up to 2.11% in comparison with the unsheathed source in water phantom. Our results show that the radial dose function calculated in water, as generalized by AAPM TG-60, differed in tissue, especially at large distances from the source.ConclusionsThis work fully characterizes dosimetric parameters of the sheathed and unsheathed new 32P brachytherapy sources in water and different tissue phantoms by using FLUKA code. The results demonstrate that the dose distribution in water differed from the calculated ones in tissue phantoms due to the densities and atomic composition for tissues that are not taken account by the TG-60 formalism.

Prototype modulated orthovoltage stereotactic radiosurgery cones

This work sought create a set of prototype modulated orthovoltage stereotactic radiosurgery (SRS) cones, and to perform a dosimetric characterization of the prototypes. Four radiosurgical cone collimators (with cone diameters of 5 mm, 6 mm, 8 mm, and 10 mm) were built for use with an orthovoltage unit, along with epoxy-infiltrated bonded tungsten filters designed to shape the resulting dose distributions. Dosimetry measurements were performed using radiochromic film in a water phantom for both filtered and unfiltered cones at depths of 2.5 cm, 5.0 cm, and 7.5 cm. Films were scanned using a flatbed scanner and the beam profiles were analyzed. Extracted beam profiles validated the ability to optimize dosimetric results based on desired dose distributions, where for this work the goal distributions were rectangular functions. Radiochromic film measurements of dose distributions in water confirmed that the prototypes were able to achieve distributions approaching rectangular functions at depth, as determined by penumbra and flatness statistics. A prototype set of novel, modulated orthovoltage SRS filtered cones was successfully constructed, and a full dosimetric characterization was completed in water. In all configurations, filtered, optimized cones were able to achieve distributions more closely approaching the goal distributions compared to open cones.