Marc-André Renaud

Direct aperture optimization for FLEC‐based MERT and its application in mixed beam radiotherapy

PURPOSE Despite promising research in modulated electron radiotherapy (MERT), an applicator to produce modulated electron beams and associated treatment planning software is still not commercially available. This work investigated an optimization process in treatment planning for the McGill few leaf electron collimator (FLEC) MERT delivery device. In addition, the possibility of combining MERT with photon fields was examined to investigate mixed beam radiotherapy. METHODS A FLEC direct aperture optimization (DAO) method, in which FLEC apertures and weights were iteratively optimized was created. The authors evaluated the performance of DAO against our previous technique for generating FLEC plans and with commercially available photon beam optimization algorithms using a basic target and organ at risk geometry. The authors applied the DAO technique on a sarcoma treatment to evaluate clinical parameters. Finally, the authors examined the merit of mixing the DAO generated FLEC electron fields with photon fields to improve the dosimetry of the sarcoma treatment. RESULTS In relation to the alternative plans, the DAO generated sarcoma MERT plan was competitive in its ability to reduce the dose to OAR but weaker in its ability to highly conform the dose to the target volume. The addition of photon fields improved the quality of the MERT plan in terms of OAR sparing and target conformality. CONCLUSIONS The DAO approach yielded deliverable FLEC-based MERT plans with a limited number of fields. The approach combined with photon optimization added flexibility, where the mutual benefits of each radiation type was used in unison to improve plan quality.

On mixed electron–photon radiation therapy optimization using the column generation approach

Purpose: Despite considerable increase in the number of degrees of freedom handled by recent radiotherapy optimisation algorithms, treatments are still typically delivered using a single modality. Column generation is an iterative method for solving large optimisation problems. It is well suited for mixed‐modality (e.g., photon–electron) optimisation as the aperture shaping and modality selection problem can be solved rapidly, and the performance of the algorithm scales favourably with increasing degrees of freedom. We demonstrate that the column generation method applied to mixed photon–electron planning can efficiently generate treatment plans and investigate its behaviour under different aperture addition schemes. Materials and methods: Column generation was applied to the problem of mixed‐modality treatment planning for a chest wall case and a leg sarcoma case. 6 MV beamlets (100 cm SAD) were generated for the photon components along with 5 energies for electron beamlets (6, 9, 12, 16 and 20 MeV), simulated as shortened‐SAD (80 cm) beams collimated with a photon MLC. For the chest wall case, IMRT‐only, modulated electron radiation therapy (MERT)‐only, and mixed electron–photon (MBRT) treatment plans were created using the same planning criteria. For the sarcoma case, MBRT and MERT plans were created to study the behaviour of the algorithm under two different sets of planning criteria designed to favour specific modalities. Finally, the efficiency and plan quality of four different aperture addition schemes was analysed by creating chest wall MBRT treatment plans which incorporate more than a single aperture per iteration of the column generation loop based on a heuristic aperture ranking scheme. Results: MBRT plans produced superior target coverage and homogeneity relative to IMRT and MERT plans created using the same optimisation criteria, all the while preserving the normal tissue‐sparing advantages of electron therapy. Adjusting the planning criteria to favour a specific modality in the sarcoma case resulted in the algorithm correctly emphasizing the appropriate modality. As expected, adding a single aperture per iteration yielded the lowest (best) cost function value per aperture included in the treatment plan. However, a greedier scheme was able to converge to approximately the same cost function after 125 apertures in one third of the running time. Electron apertures were on average 50–100% larger than photon apertures for all aperture addition schemes. The distribution of intensities among the available modalities followed a similar trend for all schemes, with the dominant modalities being 6 MV photons along with 6, 9 and 20 MeV electrons. Conclusion: The column generation method applied to mixed modality treatment planning was able to produce clinically realistic treatment plans and combined the advantages of photon and electron radiotherapy. The running time of the algorithm depended heavily on the choice of mixing scheme. Adding the highest ranked aperture for each modality provided the best trade‐off between running time and plan quality for a fixed number of apertures. This work contributes an efficient methodology for the planning of mixed electron–photon treatments.

SU‐D‐BRB‐03: All‐Inclusive DOSXYZnrc Source for Monte Carlo QA of External Beam Radiotherapy

Purpose: To extend the versatility and simplicity of external beam Monte Carlo(MC) methods within BEAMnrc/DOSXYZnrc with the introduction of a new universal DOSXYZnrc source for multi‐field simulations. Methods and Materials: A single DOSXYZnrc source, capable of simulating all types of external beam radiotherapy, was developed as an extension to the standard set of sources. The source incorporates dynamic movement of components such as MLC leaves, jaws, gantry, collimator and couch. This was achieved by coupling the field index between the BEAMnrc components with the field index of the DOSXYZnrc components through the LATCH variable, transferred from BEAMnrc to DOSXYZnrc via the phase space information. This allows the state of the accelerator (such as MLC leaves, jaw positions and angles of incidence) to be changed for each history, effectively simulating coupled time‐dependent geometries. The process for RapidArc is as follows. RapidArc fields are defined within the control points of the DICOM plan file. Each point defines a MLC pattern and gantry angle. Within the MC simulation, the MLC pattern was defined in BEAMnrc, while the gantry angles were defined in the new DOSXYZnrc source. Control points for Tomotherapy are defined with the XML and sinogram files. In addition, IMRT and conformal treatments such as SBRT, which are comprised of 1–7 beams, can be merged into a single simulation. For practical considerations, it is more efficient to run a single MC simulation than to manage the submission of multiple beams and recombining them afterwards. We tested the new source within the context of the MMCTP system. Results and Conclusion: This source has enabled MC plan verification of RapidArc and Tomotherapy and simplified the MC calculation of IMRT and SBRT into a single simulation for efficient QA. The new source adds much needed functionality to the existing set of DOSXYZnrc sources.

RapidBrachyMCTPS: a Monte Carlo-based treatment planning system for brachytherapy applications

Despite being considered the gold standard for brachytherapy dosimetry, Monte Carlo (MC) has yet to be implemented into a software for brachytherapy treatment planning. The purpose of this work is to present RapidBrachyMCTPS, a novel treatment planning system (TPS) for brachytherapy applications equipped with a graphical user interface (GUI), optimization tools and a Geant4-based MC dose calculation engine, RapidBrachyMC. Brachytherapy sources and applicators were implemented in RapidBrachyMC and made available to the user via a source and applicator library in the GUI. To benchmark RapidBrachyMC, TG-43 parameters were calculated for the microSelectron v2 (192Ir) and SelectSeed (125I) source models and were compared against previously validated MC brachytherapy codes. The performance of RapidBrachyMC was evaluated for a prostate high dose rate case. To assess the accuracy of RapidBrachyMC in a heterogeneous setup, dose distributions with a cylindrical shielded/unshielded applicator were validated against film measurements in a Solid WaterTM phantom. TG-43 parameters calculated using RapidBrachyMC generally agreed within 1%-2% of the results obtained in previously published work. For the prostate case, clinical dosimetric indices showed general agreement with Oncentra TPS within 1%. Simulation times were on the order of minutes on a single core to achieve uncertainties below 2% in voxels within the prostate. The calculation time was decreased further using the multithreading features of Geant4. In the comparison between MC-calculated and film-measured dose distributions, at least 95% of points passed the 3%/3 mm gamma index criteria in all but one case. RapidBrachyMCTPS can be used as a post-implant dosimetry toolkit, as well as for MC-based brachytherapy treatment planning. This software is especially well suited for the development of new source and applicator models.

TU-AB-BRC-07: Efficiency of An IAEA Phase-Space Source for a Low Energy X-Ray Tube Using Egs++

PURPOSE To extend the capability of the EGSnrc C++ class library (egs++) to write and read IAEA phase-space files as a particle source, and to assess the relative efficiency gain in dose calculation using an IAEA phase-space source for modelling a miniature low energy x-ray source. METHODS We created a new ausgab object to score particles exiting a user-defined geometry and write them to an IAEA phase-space file. A new particle source was created to read from IAEA phase-space data. With these tools, a phase-space file was generated for particles exiting a miniature 50 kVp x-ray tube (The INTRABEAM System, Carl Zeiss). The phase-space source was validated by comparing calculated PDDs with a full electron source simulation of the INTRABEAM. The dose calculation efficiency gain of the phase-space source was determined relative to the full simulation. The efficiency gain as a function of i) depth in water, and ii) job parallelization was investigated. RESULTS The phase-space and electron source PDDs were found to agree to 0.5% RMS, comparable to statistical uncertainties. The use of a phase-space source for the INTRABEAM led to a relative efficiency gain of greater than 20 over the full electron source simulation, with an increase of up to a factor of 196. The efficiency gain was found to decrease with depth in water, due to the influence of scattering. Job parallelization (across 2 to 256 cores) was not found to have any detrimental effect on efficiency gain. CONCLUSION A set of tools has been developed for writing and reading IAEA phase-space files, which can be used with any egs++ user code. For simulation of a low energy x-ray tube, the use of a phase-space source was found to increase the relative dose calculation efficiency by factor of up to 196. The authors acknowledge partial support by the CREATE Medical Physics Research Training Network grant of the Natural Sciences and Engineering Research Council (Grant No. 432290).

TU-AB-201-08: Rotating Shield High Dose Rate Brachytherapy with 153Gd and 75Se Isotopes

Purpose: To introduce rotating shield brachytherapy (RSBT) for different cancer sites with 1⁵3Gd and ⁷⁵Se isotopes. RSBT is a form of intensity modulated brachytherapy, using shielded rotating catheters to provide a better dose distribution in the tumour while protecting healthy tissue. Methods: BrachySource, a Geant4-based Monte Carlo dose planning system was developed for investigation of RSBT with 1⁵3Gd and ⁷⁵Se for different cancer sites. Dose distributions from 1⁵3Gd, ⁷⁵Se and 1 9 2Ir isotopes were calculated in a 40 cm radius water phantom by using the microSelectron-v2 source model. The source was placed inside a cylindrical platinum shield with 1.3 mm diameter. An emission window coinciding with the active core of the source was created by removing half (180°) of the wall of the shield. Relative dose rate distributions of the three isotopes were simulated. As a proof of concept, a breast cancer patient originally treated with Mammosite was re-simulated with unshielded 1 9 2Ir and shielded 1⁵3Gd. Results: The source with the lowest energy, 1⁵3Gd, decreased the dose on the shielded side by 91%, followed by ⁷⁵Se and 1 9 2Ir with 36% and 16% reduction at 1 cm from the source. The breast cancer patient simulation showed the ability of shielded 1⁵3Gd to spare the chest wall by a 90% dose reduction when only one emission window angle is considered. In this case, fully covering the PTV would require more delivery angles and the chest wall dose reduction would be less, however, the simulation demonstrates the potential of shielded 1⁵3Gd to selectively isolate organs at risk. Conclusion: Introducing 1⁵3Gd and ⁷⁵Se sources combined with RSBT will allow escalation of dose in the target volume while maintaining low doses in radiation sensitive healthy tissue. Tailoring treatments to each individual patient by treating all parts of the tumour without over-irradiation of normal tissues will be possible. The author acknowledges partial support by the CREATE Medical Physics Research Training Network grant of the Natural Sciences and Engineering Research Council (Grant number: 432290), and the Quebec Fonds de recherche Nature et Technologies.

SU‐E‐T‐498: Implementation of Clinical Monte Carlo Dose Calculation for CyberKnife On a Web‐Based Treatment Planning System WebTPS

Purpose: The scope of this study is to implement an accurate Cyberknife model on a web‐based tool (WebTPS), which uses the EGSnrc Monte Carlo dose calculation engine. WebTPS will be mostly used as a reference to evaluate clinical treatment plans in highly heterogeneous phantoms. Methods: The WebTPS dose calculation module is linked to the user code DOSxyznrc. WebTPS automatically converts CyberKnife clinical plans to DOSxyznrc input files. Phantoms are created using a tissue segmentation method from HU‐ED calibrated curves and materials are assigned based on CT data and contours performed by radiation oncologists. Parallel computation is run on a high‐performance cluster (Compute Canada) to achieve reasonable simulation time. The CyberKnife model is built on the BEAMnrc system using manufacturers specifications. Simulated and experimental data are compared to estimate the optimal electron beam parameters. The beam energy estimation is based on percent depth dose (PDD) data comparison, while the spot size is validated using output factor (OF) and off‐axis ratio (OAR) data. An egs_chamber model of a PTW60012 diode is used to simulate OF experimental measurements for different collimator sizes. Results: A preliminary linac model optimization yields a 0.5% agreement between experimental and simulation PDD data; a 0.5% or 1 mm agreement for OAR data and a 2% agreement for OF data. Full treatment plan simulations are achieved with the CyberKnife model using patient heterogeneous phantoms. Uncertainties under 1% are achieved for less than 2 hours of CPU time. Conclusion: This work aims to develop a suitable model for reference plan dose calculation. WebTPS will be used in several clinical and research applications where the CyberKnife embedded ray‐tracing algorithm show significant limitations. Further improvements are yet to be achieved to match experimental data to a level of 1%.

SU‐E‐T‐698: Clinical Evaluation of the McGill Monte Carlo Treatment Planning System (MMCTP) for Fixed‐Field IMRT and RapidArc Plans

Purpose: To investigate the clinical application of a Monte Carlotreatment planning system for verification of fixed‐field IMRT and RapidArc® treatment plans. Methods: The McGill Monte Carlotreatment planning (MMCTP) system was used to calculate dose distributions from fixed‐field IMRT and RapidArc plans that had been generated in Eclipse v8.6 using the AAA dose calculation algorithm. Planar dose measurements were performed using the MapCHECK 2 diode array in three different measurement geometries. The first measurement geometry collapsed all of the gantry angles to 0° for the treatment delivery, and was only possible for the fixed‐field IMRT plans. The second measurement geometry used an Isocentric Mounting Fixture (IMF) that rotated the MapCHECK 2 array while the gantry rotated during RapidArc delivery. The third measurement geometry placed the MapCHECK 2 array between two 6 cm Solid Water® slabs with the detector remaining stationary on the treatment couch while the gantry rotated during RapidArc delivery. Results: A benchmark head and neck fixed‐field IMRT plan demonstrated excellent agreement between the Eclipse calculations, MMCTP calculations, and the MapCHECK 2 measurements for the collapsed gantry angle geometry. The Eclipse and MMCTP calculations were generally in good agreement for the RapidArc plans, but differences of up to 7% were seen in high dose gradient regions. For the RapidArc plans, the MapCHECK 2 results agreed well with the calculated distributions for stationary measurements between the Solid Water slabs but had poorer agreement when the IMF delivery was used. Conclusions: This work demonstrates that MMCTP can be used as an independent verification of fixed‐field IMRT and RapidArc treatment plans. MapCHECK 2 measurements between the Solid Water slabs agreed well with the calculated results and were the preferred QA method since the measured distributions best simulated the intended dose distributions in the patients treatment plan.