Heterogeneous Slab Phantoms Research Articles

Treatment planning system commissioning of the first clinical biology-guided radiotherapy machine.

PurposeThe RefleXion X1 is a novel radiotherapy machine designed for image‐guided radiotherapy (IGRT) and biology‐guided radiotherapy (BgRT). Its treatment planning system (TPS) generates IMRT and SBRT plans for a 6MV‐FFF beam delivered axially via 50 firing positions with the couch advancing every 2.1 mm. The purpose of this work is to report the TPS commissioning results for the first clinical installation of RefleXion™ X1.MethodsCT images of multiple phantoms were imported into the RefleXion TPS to evaluate the accuracy of data transfer, anatomical modeling, plan evaluation, and dose calculation. Comparisons were made between the X1, Eclipse™, and MIM™. Dosimetric parameters for open static fields were evaluated in water and heterogeneous slab phantoms. Representative clinical IMRT and SBRT cases were planned and verified with ion chamber, film, and ArcCHECK@ measurements. The agreement between TPS and measurements for various clinical plans was evaluated using Gamma analysis with a criterion of 3%/2 mm for ArcCHECK@ and film. End‐to‐end (E2E) testing was performed using anthropomorphic head and lung phantoms.ResultsThe average difference between the TPS‐reported and known HU values was −1.4 ± 6.0 HU. For static fields, the agreements between the TPS‐calculated and measured PDD10, crossline profiles, and inline profiles (FWHM) were within 1.5%, 1.3%, and 0.5 mm, respectively. Measured output factors agreed with the TPS within 1.3%. Measured and calculated dose for static fields in heterogeneous phantoms agreed within 2.5%. The ArcCHECK@ mean absolute Gamma passing rate was 96.4% ± 3.4% for TG 119 and TG 244 plans and 97.8% ± 3.6% for the 21 clinical plans. E2E film analysis showed 0.8 mm total targeting error for isocentric and 1.1 mm for off‐axis treatments.ConclusionsThe TPS commissioning results of the RefleXion X1 TPS were within the tolerances specified by AAPM TG 53, MPPG 5.a, TG 119, and TG 148. A subset of the commissioning tests has been identified as baseline data for an ongoing QA program.

Validating Acuros XB with heterogeneous phantoms: can film orientation have an impact?

The aim of this study was to measure doses using films placed parallel and perpendicular to a 6MV radiation beam in heterogeneous phantoms, and to compare these doses to those calculated using the dose calculation algorithms Acuros XB and Analytical Anisotropic Algorithm (AAA). A heterogeneous slab phantom composed of SB5 bone equivalent material sandwiched between WTe solid water, and another composed of cork between WTe solid water were studied. Dose measurements were carried out using Gafchromic EBT-3 and EBT-XD films placed parallel to the radiation beam through the centre of the phantom setup. Each film was then placed perpendicular to the beam, at four depths along the beam axis, with a similar method reported by Delbaere et al. Each phantom was CT scanned and a 3x3cm2 beam was calculated using Acuros XB dose-to-medium/ dose-to-water and AAA. The results showed agreement between the film measurements and dose calculations in the solid water and cork regions of the phantoms. However, deviations were observed in the bone where vertical film measurements agreed with doses calculated by Acuros dose-to-water, whereas horizontal film measurements agreed with AAA calculations. Horizontal and vertical film doses differed by up to 8%. Acuros dose-to-medium calculations were not in agreement with either film orientation. In conclusion, the orientation of the film when measuring the dose delivered by a radiation beam does have an impact on the measured dose. However, for the horizontal film measurements there were air gaps on either side of the film which may have affected the results.Reference: Delbaere A, Younes T, Vieillevigne L. On the conversion from dose-to-medium to dose-to-water in heterogeneous phantoms with Acuros XB and Monte Carlo calculations. Phys Med Biol. 2019 Oct;64.

Abstract ID: 111 Monte Carlo modeling of Orthovoltage treatment fields

Computational power and anthropomorphous phantoms advances in the last years have extended greatly the number of potential applications of Monte Carlo methods in the medical field. Aiming at an orthovoltage treatment plan definition, the results of Monte Carlo simulations performed with the MCNP6 code on two different typologies of computational models have been compared: Unstructured Mesh (UM) model and Voxel model. The capability to simulate the particle transport directly on the unstructured mesh model geometry has been recently introduced in the release 6 of MCNP [1] , which has been chosen for this study. These kind of models overcome the insurgence of voxel effects [2] and provide a more accurate description of volumes and complex surfaces. The simulations were performed on anthropomorphous models based on anonymous CT scans. Firstly, the UM model was obtained from the CT scans thanks to the SCAN-IP mesh tools (Simpleware), and secondly the CT was voxelized with different voxel sizes in order to compare and gauge the voxel effects. The radiation source simulates the Orthovoltage X-Ray tube in use at IRCCS-ASMN in Reggio Emilia, tested in different configurations and tube voltages. The computational source has been validated with experimental measures on the real one applied on heterogeneous slab phantoms. The validation showed good agreement of the simulated setups with the experiments, with deviations varying accordingly to the source configuration, but always below 5–6%. The comparison between the two models showed how, with decreasing voxel sizes, the voxel models results partially converge to the UM models values. Furthermore, it was highlighted that the major deviations between the models resulted where the dose gradient is higher, along the field borders and when tissue heterogeneity is more and more considered. The study shows how the UM model can be implemented for evaluating the dose deposition within anthropomorphous phantoms and, in perspective, be used in treatment planning.

Monte Carlo dose calculations for high-dose–rate brachytherapy using GPU-accelerated processing

PurposeCurrent clinical brachytherapy dose calculations are typically based on the Association of American Physicists in Medicine Task Group report 43 (TG-43) guidelines, which approximate patient geometry as an infinitely large water phantom. This ignores patient and applicator geometries and heterogeneities, causing dosimetric errors. Although Monte Carlo (MC) dose calculation is commonly recognized as the most accurate method, its associated long computational time is a major bottleneck for routine clinical applications. This article presents our recent developments of a fast MC dose calculation package for high-dose–rate (HDR) brachytherapy, gBMC, built on a graphics processing unit (GPU) platform. Methods and MaterialsgBMC-simulated photon transport in voxelized geometry with physics in 192Ir HDR brachytherapy energy range considered. A phase-space file was used as a source model. GPU-based parallel computation was used to simultaneously transport multiple photons, one on a GPU thread. We validated gBMC by comparing the dose calculation results in water with that computed TG-43. We also studied heterogeneous phantom cases and a patient case and compared gBMC results with Acuros BV results. ResultsRadial dose function in water calculated by gBMC showed <0.6% relative difference from that of the TG-43 data. Difference in anisotropy function was <1%. In two heterogeneous slab phantoms and one shielded cylinder applicator case, average dose discrepancy between gBMC and Acuros BV was <0.87%. For a tandem and ovoid patient case, good agreement between gBMC and Acruos BV results was observed in both isodose lines and dose-volume histograms. In terms of the efficiency, it took ∼47.5 seconds for gBMC to reach 0.15% statistical uncertainty within the 5% isodose line for the patient case. ConclusionsThe accuracy and efficiency of a new GPU-based MC dose calculation package, gBMC, for HDR brachytherapy make it attractive for clinical applications.

Dosimetry audit of radiotherapy treatment planning systems.

In radiotherapy Treatment Planning Systems (TPS) various calculation algorithms are used. The accuracy of dose calculations has to be verified. Numerous phantom types, detectors and measurement methodologies are proposed to verify the TPS calculations with dosimetric measurements. A heterogeneous slab phantom has been designed within a Coordinated Research Project (CRP) of the IAEA. The heterogeneous phantom was developed in the frame of the IAEA CRP. The phantom consists of frame slabs made with polystyrene and exchangeable inhomogeneity slabs equivalent to bone or lung tissue. Special inserts allow to position thermoluminescent dosimeters (TLD) capsules within the polystyrene slabs below the bone or lung equivalent slabs and also within the lung equivalent material. Additionally, there are inserts that allow to position films or ionisation chamber in the phantom. Ten Polish radiotherapy centres (of 30 in total) were audited during on-site visits. Six different TPSs and five calculation algorithms were examined in the presence of inhomogeneities. Generally, most of the results from TLD were within 5 % tolerance. Differences between doses calculated by TPSs and measured with TLD did not exceed 4 % for bone and polystyrene equivalent materials. Under the lung equivalent material, on the beam axis the differences were lower than 5 %, whereas inside the lung equivalent material, off the beam axis, in some cases they were of around 7 %. The TLD results were confirmed with the ionisation chamber measurements. The comparison results of the calculations and the measurements allow to detect limitations of TPS calculation algorithms. The audits performed with the use of heterogeneous phantom and TLD seem to be an effective tool for detecting the limitations in the TPS performance or beam configuration errors at audited radiotherapy departments.

Clinical evaluation of inter-fractional organ motion using 3D ultrasound image-guided radiotherapy for positioning prostate cancer patients

Clinical evaluation of inter-fractional organ motion using 3D ultrasound image-guided radiotherapy for positioning prostate cancer patients

Dose measurement for Acuros XB (AXB) verification for 6 MV photon beams

Purpose: To determine the accuracy of dose calculations performed by the novel Acuros XB (AXB) dose calculation algorithm in dosimetrically challenging heterogeneous slab phantoms.

Evaluation of a novel susceptibility weighted imaging MRI sequence for neonate brain scanning

Evaluation of a novel susceptibility weighted imaging MRI sequence for neonate brain scanning

SU-E-T-137: Dosimetric Validation for Pinnacle, Acuros, AAA, and Brainlab Algorithms with Induced Inhomogenieties

Purpose: To compare the dosimetric accuracy of the Eclipse 11.0 Acuros XB and Anisotropic Analytical Algorithm (AAA), Pinnacle-3 9.2 Collapsed Cone Convolution, and the iPlan 4.1 Monte Carlo (MC) and Pencil Beam (PB) algorithms using measurement as the gold standard. Methods: Ion chamber and diode measurements were taken for 6, 10, and 18 MV beams in a phantom made up of slab densities corresponding to solid water, lung, and bone. The phantom was setup at source-to-surface distance of 100 cm, and the field sizes were 3.0 × 3.0, 5.0 × 5.0, and 10.0 × 10.0 cm2. Data from the planning systems were computed along the central axis of the beam. The measurements were taken using a pinpoint chamber and edge diode for interface regions. Results: The best agreement between data from the algorithms and our measurements occurs away from the slab interfaces. For the 6 MV beam, iPlan 4.1 MC software performs the best with 1.7% absolute average percent difference from measurement. For the 10 MV beam, iPlan 4.1 PB performs the best with 2.7% absolute average percent difference from measurement. For the 18 MV beam, Acuros performs the best with 2.0% absolute average percent difference from measurement. It is interesting to note that the steepest drop in dose occurred the at lung heterogeneity-solid water interface of the18 MV, 3.0 × 3.0 cm2 field size setup. In this situation, Acuros and AAA performed best with an average percent difference within −1.1% of measurement, followed by iPlan 4.1 MC, which was within 4.9%. Conclusion: This study shows that all of the algorithms perform reasonably well in computing dose in a heterogeneous slab phantom. Moreover, Acuros and AAA perform particularly well at the lung-solid water interfaces for higher energy beams and small field sizes.

SU‐E‐I‐01: A Fast, Analytical Pencil Beam Based Method for First Order X‐Ray Scatter Estimation of Kilovoltage Cone Beam X‐Rays

Purpose:To analytically estimate first‐order x‐ray scatter for kV cone beam x‐ray imaging with high computational efficiency.Methods:In calculating first‐order scatter using the Klein‐Nishina formula, we found that by integrating the point‐to‐point scatter along an interaction line, a “pencil‐beam” scatter kernel (BSK) can be approximated to a quartic expression when the imaging field is small. This BSK model for monoenergetic, 100keV x‐rays has been verified on homogeneous cube and cylinder water phantoms by comparing with the exact implementation of KN formula. For heterogeneous medium, the water‐equivalent length of a BSK was acquired with an improved Siddon's ray‐tracing algorithm, which was also used in calculating pre‐ and post‐ scattering attenuation. To include the electron binding effect for scattering of low‐kV photons, the mean corresponding scattering angle is determined from the effective point of scattered photons of a BSK. The behavior of polyenergetic x‐rays was also investigated for 120kV x‐rays incident to a sandwiched infinite heterogeneous slab phantom, with the electron binding effect incorporated. Exact computation and Monte Carlo simulations were performed for comparisons, using the EGSnrc code package.Results:By reducing the 3D volumetric target (o(n3)) to 2D pencil‐beams (o(n2)), the computation expense can be generally lowered by n times, which our experience verifies. The scatter distribution on a flat detector shows high agreement between the analytic BSK model and exact calculations. The pixel‐to‐pixel differences are within (‐2%, 2%) for the homogeneous cube and cylinder phantoms and within (0, 6%) for the heterogeneous slab phantom. However, the Monte Carlo simulation shows increased deviation of the BSK model toward detector periphery.Conclusion:The proposed BSK model, accommodating polyenergetic x‐rays and electron binding effect at low kV, shows great potential in efficiently estimating the first‐order scatter from small imaging fields. We are investigating more thoroughly to improve performance and explore applications.

SU-E-T-556: Verification and Evaluation of Acuros XB Dose Calculations for Stereotactic Ablative Radiotherapy of the Thoracic Spine

Purpose: To evaluate the Eclipse Acuros XB (AXB) dose calculation engine for Stereotactic Ablative Radiotherapy (SAbR) of the thoracic spine using step-and-shoot IMRT and RapidArc. Methods: The accuracy of the AXB dose calculation engine was first verified by comparing with film and ion chamber measurements for open beams with field sizes from 2×2cm2-10×10cm2, delivered to a heterogeneous phantom of a slab-geometry. Acquired images of an anthropomorphic thoracic phantom, composed of tissue equivalent lungs, vertebrae, and soft tissue, were then used to create step-and-shoot IMRT and RapidArc treatment plans. Point dose measurements in vertebral bone and spinal cord were compared to Acuros calculations. Finally, step-and-shoot IMRT and dual-arc RapidArc plans were created using the Analytical Anisotropic Algorithm (AAA) for five patients with thoracic spinal metastases following our institutional protocol. All treatment plans were then recalculated with AXB while keeping beam parameters the same. Clinically significant dosimetric parameters, such as GTV coverage and spinal cord dose were compared. Results: Excellent agreement was achieved between point dose and planar dose measurements and the AXB calculations in the heterogeneous slab and anthropomorphic phantoms. Patient IMRT and RapidArc plans had GTV D90 and mean dose values 0.2%–2.6% and 0.2%–2.1% lower, respectively, for AXB calculations compared with AAA. The maximum cord dose was also lower for AXB compared with AAA (−2.7%±2.4% for IMRT; −2.6%±1.2% for RapidArc). AXB dose calculations were 4−6 times faster than AAA for RapidArc plans, but 7−9 times slower for IMRT plans. Conclusion: AXB calculations were shown to be dosimetrically accurate for heterogeneous media. This work demonstrates that AXB calculates a reduced dose compared with AAA in both the target and spinal cord for thoracic spine SAbR. The dosimetric differences should be considered when using AAA for thoracic spine treatment planning.

A generalized 2D pencil beam scaling algorithm for proton dose calculation in heterogeneous slab geometries.

Pencil beam algorithms are commonly used for proton therapy dose calculations. Szymanowski and Oelfke ["Two-dimensional pencil beam scaling: An improved proton dose algorithm for heterogeneous media," Phys. Med. Biol. 47, 3313-3330 (2002)] developed a two-dimensional (2D) scaling algorithm which accurately models the radial pencil beam width as a function of depth in heterogeneous slab geometries using a scaled expression for the radial kernel width in water as a function of depth and kinetic energy. However, an assumption made in the derivation of the technique limits its range of validity to cases where the input expression for the radial kernel width in water is derived from a local scattering power model. The goal of this work is to derive a generalized form of 2D pencil beam scaling that is independent of the scattering power model and appropriate for use with any expression for the radial kernel width in water as a function of depth. Using Fermi-Eyges transport theory, the authors derive an expression for the radial pencil beam width in heterogeneous slab geometries which is independent of the proton scattering power and related quantities. The authors then perform test calculations in homogeneous and heterogeneous slab phantoms using both the original 2D scaling model and the new model with expressions for the radial kernel width in water computed from both local and nonlocal scattering power models, as well as a nonlocal parameterization of Molière scattering theory. In addition to kernel width calculations, dose calculations are also performed for a narrow Gaussian proton beam. Pencil beam width calculations indicate that both 2D scaling formalisms perform well when the radial kernel width in water is derived from a local scattering power model. Computing the radial kernel width from a nonlocal scattering model results in the local 2D scaling formula under-predicting the pencil beam width by as much as 1.4 mm (21%) at the depth of the Bragg peak for a 220 MeV proton beam in homogeneous water. This translates into a 32% dose discrepancy for a 5 mm Gaussian proton beam. Similar trends were observed for calculations made in heterogeneous slab phantoms where it was also noted that errors tend to increase with greater beam penetration. The generalized 2D scaling model performs well in all situations, with a maximum dose error of 0.3% at the Bragg peak in a heterogeneous phantom containing 3 cm of hard bone. The authors have derived a generalized form of 2D pencil beam scaling which is independent of the proton scattering power model and robust to the functional form of the radial kernel width in water used for the calculations. Sample calculations made with this model show excellent agreement with expected values in both homogeneous water and heterogeneous phantoms.

Application of the pencil-beam redefinition algorithm in heterogeneous media for proton beam therapy

In proton beam therapy, changes in the proton range due to lateral heterogeneity may cause serious errors in the dose distribution. In the present study, the pencil-beam redefinition algorithm (PBRA) was applied to proton beam therapy to address the problem of lateral density heterogeneity. In the calculation, the phase-space parameters were characterized for multiple range (i.e. proton energy) bins for given pencil beams. The particles that were included in each pencil beam were transported and redefined periodically until they had stopped. The redefined beams formed a detouring path that was different from that of the non-redefined pencil beams, and the path of each redefined beam was straight. The results calculated by the PBRA were compared with measured proton dose distributions in a heterogeneous slab phantom and an anthropomorphic phantom. Through the beam redefinition process, the PBRA was able to predict the measured proton-detouring effects. Therefore, the PBRA may allow improved calculation accuracy when dealing with lateral heterogeneities in proton therapy applications.

Verification and Dosimetric Impact of Acuros XB Algorithm for Stereotactic Body Radiation Therapy (SBRT) and RapidArc Planning for Non-Small-Cell Lung Cancer (NSCLC) Patients

Purpose: The experimental verification of the Acuros XB (AXB) algorithm was conducted in a heterogeneous rectangular slab phantom, and compared to the Anisotropic Analytical Algorithm (AAA). The dosimetric impact of the AXB for stereotactic body radiation therapy (SBRT) and RapidArc planning for 16 non-small-cell lung cancer (NSCLC) patients was assessed due to the dose recalculation from the AAA to the AXB. Methods: The calculated central axis percentage depth doses (PDD) in a heterogeneous slab phantom for an open field size of 3 ×3 cm2 were compared against the PDD measured by an ionization chamber. For 16 NSCLC patients, the dose-volume parameters from the treatment plans calculated by the AXB and the AAA were compared using identical jaw settings, leaf positions, and monitor units (MUs). Results: The results from the heterogeneous slab phantom study showed that the AXB was more accurate than the AAA; however, the dose underestimation by the AXB (up to ?3.9%) and AAA (up to ?13.5%) was observed. For a planning target volume (PTV) in the NSCLC patients, in comparison to the AAA, the AXB predicted lower mean and minimum doses by average 0.3% and 4.3% respectively, but a higher maximum dose by average 2.3%. The averaged maximum doses to the heart and spinal cord predicted by the AXB were lower by 1.3% and 2.6% respectively; whereas the doses to the lungs predicted by the AXB were higher by up to 0.5% compared to the AAA. The percentage of ipsilateral lung volume receiving at least 20 and 5 Gy (V20 and V5 respectively) were higher in the AXB plans than in the AAA plans by average 1.1% and 2.8% respectively. The AXB plans produced higher target heterogeneity by average 4.5% and lower plan conformity by average 5.8% compared to the AAA plans. Using the AXB, the PTV coverage (95% of the PTV covered by the 100% of the prescribed dose) was reduced by average 8.2% than using the AAA. The AXB plans required about 2.3% increment in the number of MUs in order to achieve the same PTV coverage as in the AAA plans. Conclusion: The AXB is more accurate to use for the dose calculations in SBRT lung plans created with a RapidArc technique; however, one should also note the reduced PTV coverage due to the dose recalculation from the AAA to the AXB.

Sci-Thur AM: Planning - 08: Validation of a commercial Monte Carlo code used for stereotactic radiosurgery and stereotactic body radiation therapy.

Our project consisted of validating the BrainLab iPlan Monte Carlo algorithm, used in conjunction with the stereotactic radiosurgery (SRS) mode of the Varian Novalis TX linear accelerator, for clinical use. Our approach was to "benchmark" the iPlan algorithm by comparing dose distributions with those obtained using a BEAMnrc model of the Novalis SRS mode. The BEAMnrc model was obtained by modifying an existing accelerator model to include the SRS flattening filter and source characteristics of the Novalis TX, and by reprogramming a component module to model the high definition 120-leaf multi-leaf collimator. The free parameters of interleaf air gap and leaf density were adjusted by matching to interleaf leakage profiles measured with EBT2 film. The BEAMnrc model was used to perform comparisons of depth dose curves and planar distributions for fields in homogeneous and heterogeneous slab phantoms between both MC codes and film. The source parameters of electron beam energy, size and angular spread were determined to be 6.6 MeV, 0.7 mm and 0.8 mm (cross and in-plane), and 1.27°, respectively. Comparisons between iPlan and EGSnrc MC codes show agreement within 2% for PDD curves, and a high pass rate (>98%) on gamma analysis (3%/3mm) for planar distributions, when the scored quantity is dose to medium. Discrepancies between both MC codes and film measurements were seen near bone inhomogeneities, where the film trend agrees somewhat with iPlan MC reporting dose-to-water. Further work is being performed to understand these differences and how film is used to measure dose near bone.

Experimental evaluation of a spatial resampling technique to improve the accuracy of pencil-beam dose calculation in proton therapy

In proton therapy, pencil-beam algorithms (PBAs) are the most widely used dose calculation methods. However, the PB calculations that employ one-dimensional density scaling neglect the effects of lateral density heterogeneity on the dose distributions, whereas some particles included in such pencil beams could overextend beyond the interface of the density heterogeneity. We have simplified a pencil-beam redefinition algorithm (PBRA), which was proposed for electron therapy, by a spatial resampling technique toward an application for proton therapy. The purpose of this study is to evaluate the calculation results of the spatial resampling technique in terms of lateral density heterogeneity by comparison with the dose distributions that were measured in heterogeneous slab phantoms. The pencil beams are characterized for multiple residual-range (i.e., proton energy) bins. To simplify the PBRA, the given pencil beams are resampled on one or two transport planes, in which smaller sub-beams that are parallel to each other are generated. We addressed the problem of lateral density heterogeneity comparing the calculation results to the dose distributions measured at different depths in heterogeneous slab phantoms using a two-dimensional detector. Two heterogeneity slab phantoms, namely, phantoms A and B, were designed for the measurements and calculations. In phantom A, the heterogeneity slab was placed close to the surface. On the other hand, in phantom B, it was placed close to the Bragg peak in the mono-energetic proton beam. In measurements, lateral dose profiles showed a dose reduction and increment in the vicinity of x = 0 mm in both phantoms at depths z = 142 and 161 mm due to lateral particle disequilibrium. In phantom B, these dose reduction∕increment effects were higher∕lower, respectively, than those in phantom A. This is because a longer distance from the surface to the heterogeneous slab increases the strength of proton scattering. Sub-beams, which were generated from the resampling plane, formed a detouring∕overextending path that was different from that of elemental pencil beams. Therefore, when the spatial resampling was implemented at the surface and immediately upstream of the lateral heterogeneity, the calculation could predict these dose reduction∕increment effects. Without the resampling procedure, these dose reduction∕increment effects could not be predicted in both phantoms owing to the blurring of the pencil beam. We found that the PBA with the spatial resampling technique predicted the dose reduction∕increment at the dose profiles in both phantoms when the sampling plane was defined immediately upstream of the heterogeneous slab. We have demonstrated the implementation of a spatial resampling technique for pencil-beam calculation to address the problem of lateral density heterogeneity. While further validation is required for clinical use, this study suggests that the spatial resampling technique can make a significant contribution to proton therapy.

Monte Carlo‐based treatment planning system calculation engine for microbeam radiation therapy

Microbeam radiation therapy (MRT) is a synchrotron radiotherapy technique that explores the limits of the dose-volume effect. Preclinical studies have shown that MRT irradiations (arrays of 25-75-μm-wide microbeams spaced by 200-400 μm) are able to eradicate highly aggressive animal tumor models while healthy tissue is preserved. These promising results have provided the basis for the forthcoming clinical trials at the ID17 Biomedical Beamline of the European Synchrotron Radiation Facility (ESRF). The first step includes irradiation of pets (cats and dogs) as a milestone before treatment of human patients. Within this context, accurate dose calculations are required. The distinct features of both beam generation and irradiation geometry in MRT with respect to conventional techniques require the development of a specific MRT treatment planning system (TPS). In particular, a Monte Carlo (MC)-based calculation engine for the MRT TPS has been developed in this work. Experimental verification in heterogeneous phantoms and optimization of the computation time have also been performed. The penelope/penEasy MC code was used to compute dose distributions from a realistic beam source model. Experimental verification was carried out by means of radiochromic films placed within heterogeneous slab-phantoms. Once validation was completed, dose computations in a virtual model of a patient, reconstructed from computed tomography (CT) images, were performed. To this end, decoupling of the CT image voxel grid (a few cubic millimeter volume) to the dose bin grid, which has micrometer dimensions in the transversal direction of the microbeams, was performed. Optimization of the simulation parameters, the use of variance-reduction (VR) techniques, and other methods, such as the parallelization of the simulations, were applied in order to speed up the dose computation. Good agreement between MC simulations and experimental results was achieved, even at the interfaces between two different media. Optimization of the simulation parameters and the use of VR techniques saved a significant amount of computation time. Finally, parallelization of the simulations improved even further the calculation time, which reached 1 day for a typical irradiation case envisaged in the forthcoming clinical trials in MRT. An example of MRT treatment in a dog's head is presented, showing the performance of the calculation engine. The development of the first MC-based calculation engine for the future TPS devoted to MRT has been accomplished. This will constitute an essential tool for the future clinical trials on pets at the ESRF. The MC engine is able to calculate dose distributions in micrometer-sized bins in complex voxelized CT structures in a reasonable amount of time. Minimization of the computation time by using several approaches has led to timings that are adequate for pet radiotherapy at synchrotron facilities. The next step will consist in its integration into a user-friendly graphical front-end.

Implementation of EPID transit dosimetry based on a through-air dosimetry algorithm

A method to perform transit dosimetry with an electronic portal imaging device (EPID) by extending the commercial implementation of a published through-air portal dose image (PDI) prediction algorithm Van Esch et al. [Radiother. Oncol. 71, 223-234 (2004)] is proposed and validated. A detailed characterization of the attenuation, scattering, and EPID response behind objects in the beam path is used to convert through-air PDIs into transit PDIs. The EPID detector response beyond a range of water equivalent thicknesses (0-35 cm) and field sizes (3×3 to 22.2×29.6 cm(2)) was analyzed. A constant air gap between the phantom exit surface and the EPID was utilized. A model was constructed that accounts for the beam's attenuation along the central axis, the presence of phantom scattered radiation, the detector's energy dependent response, and the difference in EPID off-axis pixel response relative to the central pixel. The efficacy of the algorithm was verified by comparing predicted and measured PDIs for IMRT fields delivered through phantoms of increasing complexity. The expression that converts a through-air PDI to a transit PDI is dependent on the object's thickness, the irradiated field size, and the EPID pixel position. Monte Carlo derived narrow-beam linear attenuation coefficients are used to model the decrease in primary fluence incident upon the EPID due to the object's presence in the beam. This term is multiplied by a factor that accounts for the broad beam scatter geometry of the linac-phantom-EPID system and the detector's response to the incident beam quality. A 2D Gaussian function that models the nonuniformity of pixel response across the EPID detector plane is developed. For algorithmic verification, 49 IMRT fields were repeatedly delivered to homogeneous slab phantoms in 5 cm increments. Over the entire set of measurements, the average area passing a 3%∕3mm gamma criteria slowly decreased from 98% for no material in the beam to 96.7% for 35 cm of material in the beam. The same 49 fields were delivered to a heterogeneous slab phantom and on average, 97.1% of the pixels passed the gamma criteria. Finally, a total of 33 IMRT fields were delivered to the anthropomorphic phantom and on average, 98.1% of the pixels passed. The likelihood of good matches was independent of anatomical site. A prediction of the transit PDI behind a phantom or patient can be created for the purposes of treatment verification via an extension of the Van Esch through-air PDI algorithm. The results of the verification measurements through phantoms indicate that further investigation through patients during their treatments is warranted.

Dosimetry protocol for the preclinical trials in white‐beam minibeam radiation therapy

In the quest of a curative radiotherapy treatment for gliomas, new delivery modes are being explored. At the Biomedical Beamline of the European Synchrotron Radiation Facility, a new spatially fractionated technique, called minibeam radiation therapy (MBRT), is under development. The aims of this work were to assess different dosimetric aspects and to establish a dosimetry protocol to be applied in the forthcoming animal (rat) studies in order to evaluate the therapeutic index of this new radiotherapy approach. Absolute dosimetry was performed with a thimble ionization chamber (PTW semiflex 31010) whose center was positioned at 2 g cm(-2) depth. To translate the dose measured in broad beam configuration to the dose deposited with a minibeam, the scatter factors were used. Those were assessed by using the Monte Carlo simulations and verified experimentally with Gafchromic films and a Bragg Peak chamber. The comparison of the theoretical and experimental data were used to benchmark the calculations. Finally, the dose distributions in a rat phantom were evaluated by using the validated Monte Carlo calculations. The absolute dosimetry in broad beam configuration was measured in reference conditions. The dose rate was in the range between 168 and 224 Gy∕min, depending on the storage ring current. A scatter factor of 0.80 ± 0.04 was obtained. Percentage depth dose and lateral profiles were evaluated both in homogenous and heterogeneous slab phantoms. The general good agreement between Monte Carlo simulations and experimental data permitted the benchmark of the calculations. Finally, the peak doses in the rat head phantom were assessed from the measurements in reference conditions. In addition, the peak-to-valley dose ratio values as a function of depth in the rat head were evaluated. A new promising radiotherapy approach is being explored at the ESRF: Minibeam Radiation Therapy. To assess the therapeutic index of this new modality, in vivo experiments are being planned, for which an accurate knowledge of the dosimetry is essential. For that purpose, a complete set of measurements and Monte Carlo simulations was performed. The first dosimetry protocol for preclinical trials in minibeam radiation therapy was established. This protocol allows to have reproducibility in terms of dose for the different biological studies.

SU-E-T-721: Spatial Re-Sampling of Pencil Beams to Improve the Dose-Calculation Accuracy in Proton Therapy

Purpose: We have developed a novel dose calculation algorithm, a spatial re‐sampling pencil beam algorithm (SR‐PBA), to improve the pencil‐beam dose accuracy in heterogeneous regions of a patient's body. Methods: The SR‐PBA employs more sub‐beams and splitting compared with previous methods. Sampling map analysis, which is another important concept of the SR‐PBA, is preprocessing to determine the physical parameters of re‐sampled sub‐beams at sampling plane in order to avoid a time‐consuming problem. We verified the superiority of the SR‐PBA method to the conventional PBA by comparing their calculation results with the experimentally measured dose distributions in the heterogeneous slab phantoms. In order to evaluate the depth dependence of the accuracy in the slab, we designed an L‐shaped range compensator and two phantoms (A and B) in which low‐density regions were placed. Results: In a phantom A, the density interface was located at a shallow region, which corresponds to the plateau of the depth‐dose curve. On the other hand, in a phantom B, the interface was located at a deep region, which is close to the proton range. The lateral dose distributions were measured using a two‐dimensional detector. In both phantoms, the lateral‐dose profiles showed the dose reduction in the vicinity of × = 0 mm. In the phantom A, the PBA could not reproduce this dose reduction with +10.7 % at × = 0 while the SR‐PBA could within 1.5 % when the setup error of 1 mm was taken into account. In the phantom B, only the SR‐PBA could reproduce this dose reduction with 0.4 % at × = 0. We found that the SR‐PBA reproduced the dose reduction/increment at the dose profiles of the heterogeneous slab regardless of the depth dependence. Conclusions: This study suggests that our proposed algorithm is feasible for simulating proton dose distributions in the practical proton therapy.