Physics study of microbeam radiation therapy with PSI‐version of Monte Carlo code GEANT as a new computational tool

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Abstract
Microbeam radiation therapy (MRT) is a currently experimental method of radiotherapy which is mediated by an array of parallel microbeams of synchrotron‐wiggler‐generated x‐rays. Suitably selected, nominally supralethal doses of x‐rays delivered to parallel microslices of tumor‐bearing tissues in rats can be either palliative or curative while causing little or no serious damage to contiguous normal tissues. Although the pathogenesis of MRT‐mediated tumor regression is not understood, as in all radiotherapy such understanding will be based ultimately on our understanding of the relationships among the following three factors: (1) microdosimetry, (2) damage to normal tissues, and (3) therapeutic efficacy. Although physical microdosimetry is feasible, published information on MRT microdosimetry to date is computational. This report describes Monte Carlo‐based computational MRT microdosimetry using photon and/or electron scattering and photoionization cross‐section data in the 1 eV through 100 GeV range distributed publicly by the U.S. Lawrence Livermore National Laboratory (LLNL) in the 1990s. These are compared with Monte Carlo‐based microdosimetric computations using a code and physical data available in the 1980s. With the aim of using the PSI‐version of GEANT Monte Carlo code for future macro‐ and micro/nano‐dosimetric studies of Microbeam Radiation Therapy (MRT) a comparison of this code is made with the INHOM(EGS4) (version 1990), Dilmanian‐CPE and Persliden‐CPE Monte Carlo photon–electron codes (both version 1990) with which the absorbed dose distributions were calculated in 1990 and 1991 considering, (a) a single cylindrical microbeam, (b) multiple cylindrical microbeams in an orthogonal square bundle, and (c) multiple planar microbeams. It is shown that the PSI‐version of GEANT can potentially deliver more accurate results (a) using presently the most advanced atomic data, and especially (b) employing “Single‐collision” electron transport instead of only the “Condensed‐history” electron transport as in code INHOM(EGS4). In contrast Dilmanian‐CPE and Persliden‐CPE codes deposit the electron energy locally instead of transporting it to the correct position.