CIRMS Workshop

Joe McDonald
The Science & Technology Committee of the Council on Ionizing Radiation Measurements and Standards (CIRMS) sponsored a workshop held on April 7, 2000, at the National Institute of Standards and Technology (NIST) in Gaithersburg, MD, entitled "Computational Radiation Dosimetry: New Applications and Needs for Standards and Data". The workshop was organized by Joseph McDonald of the Pacific Northwest National Laboratories, Stephen Seltzer of the National Institute of Standards and Technology, and George Xu of Rensselaer Polytechnic Institute to assess the role of data and computation in the applications of importance to CIRMS. As the fidelity of computational methods and the underlying radiation interaction data has increased, and fast computers have become so easily available, there has been increasing recognition of the importance of radiation transport calculations in the radiological sciences, particularly Monte Carlo calculations. Such calculations now serve as an essential adjunct to experimental methods, providing information too difficult, expensive or even inaccessible by measurement. Reliance on calculations has increased throughout the wide spectrum of applications in the radiological sciences, such as diagnostic and therapeutic radiology, health physics, radiobiology, effects of internal and external emitters, radiation detector design, radiation hardness testing, and industrial radiation sterilization and process design. Recognizing the crosscutting importance of calculational methods, the workshop was organized to review to main computational tools used in radiation measurements by the CIRMS communities, to identify needs in critically evaluated standard reference data that support these calculations, and to consider benchmarking efforts for quality control in critical applications.

The workshop was organized into four sessions: I. Present status of physics and cross sections; II. Medical applications - Monte Carlo requirements from linacs to brachytherapy; III. Modeling and data needs in worker protection dosimetry; and IV. Modeling rad-hard electronics, products and process sterilization. In a series of eighteen presentations, the 70 attendees were brought up to date on some of the latest developments in cross-section database development, Monte Carlo and point-kernel code development, and issues in their various applications. In the area of cross section improvements, recent and current work was reviewed on elastic scattering of electrons and positrons, on photon Compton and Rayleigh scattering, and on the incorporation of improved electron-bremsstrahlung production cross sections and new medium-energy proton and neutron non-elastic nuclear cross sections into various versions of MCNP. The medical-applications session included talks on intensity-modulated radiation therapy and tomotherapy, photon Monte Carlo used for brachytherapy dose calculations, a new version of the EGS Monte Carlo code, fast Monte Carlo for radiation therapy dose calculations (including presentations on PEREGRINE and a new electron-photon code from the University of Michigan), and implementation issues for Monte Carlo radiotherapy planning software. In radiation protection dosimetry, the growing importance of Monte Carlo calculations was highlighted, particularly in the determination of fluence-to-dose coefficients and other factors important in the evaluation of operational quantities, as well as the use of new more-realistic anthropomorphic mathematical phantoms for the calculation of organ doses. Session IV included presentations on adjoint Monte Carlo applications for space radiation transport and the application of Monte Carlo and of point-kernel calculations for industrial radiation process optimization and design.

A discussion period at the end of the workshop produced a number of suggestions and preliminary recommendations. An opinion shared by many was that a more thorough understanding of radiation physics, cross sections, transport and effects, is important if we are to solve the many remaining problems in the radiological sciences. Some specific suggestions expressed, e.g., the need for critically evaluated cross sections for electron and photon interactions at low energies (below some few keV), for further efforts in the compilation of neutron and proton non-elastic nuclear cross sections and photonuclear cross sections, for additional neutron cross section measurements in the energy range of 20 to 200 MeV, and for critically evaluated heavy-ion stopping powers. Suggestions regarding quality control for radiation transport codes included recommendations that NIST take a lead in benchmarking efforts, particularly in view of the prediction that Monte Carlo calculations will be a part of every radiation therapy planning system perhaps within the next 5 years. Readers of this newsletter are invited to contribute their suggestions and recommendations on needs and priorities in data and software issues concerning radiation transport (send to s.seltzer@nist.gov).