Abstract
Monte Carlo (MC) computational techniques are widely used for applications in radiation medicine and have traditionally covered the areas of radiation dosimetry, shielding, radiotherapy treatment planning, and radiological imaging. Moreover, they have contributed to the improvement and understanding of the link between physical parameters of radiation delivery and therapy success. Recently, MC methods are being integrated with other technologies used in radiation therapy, such as inverse optimization, deformable image registration, and machine learning techniques for outcomes studies, etc.
The Fourth International Monte Carlo Workshop was held from 8–10 June, 2011 on the campus of McGill University, Montreal, Canada. The 2.5-day workshop was the fourth edition in a series that started at McGill University in 2001, proceedings of which have been published (Seuntjens and Mobit, 2002; Verhaegen and Seuntjens, 2005; Verhaegen and Seuntjens, 2008). These events have garnered interest across the world and were highly successful; parallel successful events have been organized by the European Workgroup on Monte Carlo treatment planning (Reynaerts, 2007; Spezi, 2010).
The fourth edition of the workshop was co-organized by McGill University (Medical Physics) and Université Laval. In what turned out to be sunny and pleasant Canadian spring days, the status of MC techniques was examined as related to radiation medicine, but in a broader sense in terms of techniques and applicability compared to previous editions. Developers of MC technologies were brought together with users from clinical, industrial and academic backgrounds. The topics discussed at the workshop included code development, variance reduction, clinical implementation and evaluation, parallel processing, GPUs, planning and correlation studies, applications in adaptive radiation therapy, imaging and dosimetry as well as integration of MC techniques with non-MC related applications involving machine learning and radiation biology outcome modeling. The workshop was attended by 106 participants from nine countries. 43 oral presentations were given and the poster session contained 13 presentations discussed integrally in the workshop program.
Selected papers from the workshop comprise the content of this special section and the papers were peer-reviewed in accordance with the typical high standards of Physics in Medicine and Biology (PMB). In broad strokes, representative papers from almost all sessions can be found in the section.
The clinical impetus of accurate dose calculations in radiation therapy is often linked to specific applications, such as lung cancer radiation therapy: the need for accurate dose calculations in heterogeneous clinical treatment planning and delivery verification scenarios. Radiation medicine benefits from MC calculations for a variety of other applications that we will briefly discuss.
To arrive at accurate dose calculations, one needs to examine the basics of underlying physics; models, implementations and geometric inputs play fundamental roles. MC codes used in radiation medicine applications frequently undergo changes and updates that affect their accuracy. In the first session, recent developments in stopping power models for Penelope, low energy photon cross-sections in EGSnrc, new algorithms and input geometries with Geant4 were presented. The paper by Sawkey et al (2012) discusses electron scattering algorithms in Geant4 and compares them to detailed experiments in other papers in this section.
Source modeling plays a very important role in radiotherapy applications, whether the algorithm is MC-based or not. Beam modeling was introduced in an excellent plenary presentation by Kawrakow and accuracy of these models was further discussed by Faddegon. An upcoming radiation therapy application that requires accurate beam modeling is MERT (modulated electron beam radiation therapy), which represents the use of a large number of intensity and energy modulated radiation fields. In this section, Connell et al (2012) investigate the benefit of scattering foil-free beams for the application of MERT using experimental and MC techniques.
Brachytherapy is an application that has been receiving increased 'MC attention' due to the importance of low energies of the radiation involved as well as successes of certain brachytherapy treatments in terms of local control (e.g., Vuong et al 2002). Detailed source modeling and approaches for fast treatment planning-type calculations are being developed to replace approximate methods currently used in clinical applications. Afsharpour et al (2012) discuss the development and performance of the ALGEBRA algorithm based on Geant4 and its interfacing to clinical TP stations through DICOM-RT.
Computational acceleration has always been an important consideration for the more widespread deployment of MC techniques in different areas including radiation therapy. A computer architecture that received significant attention in recent years is the Graphics Processing Unit (GPU) technology in conjunction with MC and other modeling methods in radiation therapy. At the workshop Jiang presented a summary of the efforts by the San Diego group in this regard. Complementary to the role of GPUs and other hardware acceleration platforms, alternative grid-based solvers such as deterministic transport methods have made their entrée into the world of clinical treatment planning. In this context, Wareing discussed the background and strengths of the Acuros deterministic solver of the Boltzmann equation. One of the time consuming components in MC transport is ray tracing or the navigation and the handling of boundary crossing. Schümann et al (2012) discuss optimal techniques for voxel navigation through patient Cartesian grids in Geant4. The explicit simulation of time-dependencies with MC techniques, known as 4D MC, is of importance in a variety of studies including anatomical variations, source modulation as well as correlative outcome studies. 4D MC options have become part of the most radiation therapy-related packages including BEAMnrc/EGSnrc, Geant4 and Penelope. Shin et al (2012) study a modular framework for the simulation of time-dependent quantities in the Geant4 toolkit TOPAS.
Another topic of significant interest is nuclear medicine and molecular imaging. Zaidi reviewed the role of MC techniques in positron emission tomography, which is very relevant to recent developments in imaging technologies, such as PET-MR.
The efficacy of proton therapy and its potential in clinical treatment is affected by accurate knowledge of particle range uncertainties in tissues, which is influenced by fundamental as well as clinical issues. In this section, Paganetti (2012) presents a topical review of the role of MC in proton therapy and estimation of its range uncertainties.
The application of radiation therapy to a patient provokes a response of a 'system' that depends not only on an accurate description of the manner in which energy is absorbed in the patient tissues but, more importantly, on radiation biological and clinical aspects, whereas the role of MC techniques is witnessing increased interest on this front. The workshop devoted a dedicated session to applications of MC in radiation biology and treatment outcomes, which has been reviewed in this section by El Naqa et al (2012) from macroscopic and microscopic perspectives. Incerti presented the progress of the Geant4-DNA track structure project whereas Lee et al (2012) discussed analytical modeling of radiation therapy lung late effects and local MC dose over time. Chow et al (2012) applied MC techniques to quantify dose enhancement effects by gold nanoparticles in electron beams. Significant discussions took place about suitable ways for dose-to-medium reporting for clinical applications such as brachytherapy and Gadolinium neutron capture therapy.
The workshop concluded with two sessions involving dosimetry starting with the impact of tissue characterization from different imaging technologies on accuracy of dose calculations in a presentation by Verhaegen. Dosimetry involving absorbed dose detectors is an area in which MC techniques have a longstanding, established role. Bouchard (2012) discussed a formal cavity theory suitable for MC implementation and Kairn et al (2012) presented a comparison between gel dosimetry and MC dosimetry in a stereotactic application.
The role of MC techniques as a radiation physics modeling approach in the optimization of efficacy and success of radiation therapy is expanding. We have witnessed its role in radiation therapy physics as an invaluable and accurate technique to understand basic measurement dosimetry. It has become the ultimate treatment planning dose calculation algorithm and it will play both a fundamental as well as a technical role in improving the understanding of a variety of effects and processes that are part of the patient system response to radiation therapy. This evolution will be fueled with the ever-improving computer hardware and algorithmic developments. There may be a day where one can contemplate in-silico clinical trials as a result of these developments.
We would like to gratefully acknowledge the financial support from the Research Institute, the Department of Radiation Oncology and the Cancer Mission of the McGill University Health Centre, the Department of Oncology of McGill University, the Canadian Organization of Medical Physicists (COMP), Association Québequoise des physiciens médicaux (AQPMC) and the following corporate sponsors: Nucletron, BV (official sponsor of the day for 8 June, 2011); Varian Medical Systems (official sponsor of the day for 9 June, 2011) and Elekta/CMS Inc. (official sponsor of the day for 10 June, 2011). The workshop was also endorsed by the International Atomic Energy Agency. A final word of thanks goes out to all of those who contributed to the successful workshop: our local medical physics students and staff, our administrative assistants Tatjana Nisic and Margery Knewstubb, and all of our colleagues who helped with the reviewing process of the papers presented in this special section, the IOP Publishing staff, and the authors who generated new and exciting work.
Jan Seuntjens, Luc Beaulieu, Issam El Naqa and Philippe Després Guest Editors
References
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