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

Microbeam radiation therapy (MRT) is a currently experimental method of rad iotherapy which is mediated by an array of parallel microbeams of synchrotr on-wiggler-generated x-rays. Suitably selected. nominally supralethal doses of x-rays delivered to parallel microslices of tumor-bearing tissues in ra ts can be either palliative or curative while causing little or no serious damage to contiguous normal tissues. Although the pathogenesis of MRT-media ted tumor regression is not understood, as in all radiotherapy such underst anding will be based ultimately on our understanding of the relationships a mong 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 computatio nal. This report describes Monte Carlo-based computational MRT microdosimet ry using photon and/or electron scattering and photoionization cross-sectio n data in the 1 eV through 100 GeV range distributed publicly by the U.S. L awrence Livermore National Laboratory (LLNL) in the 1990s. These are compar ed with Monte Carlo-based microdosimetric computations using a code and phy sical 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 wit h the INHOM(EGS4) (version 1990), Dilmanian-CPE and Persliden-CPE Monte Car lo photon-electron codes (both version 1990) with which the absorbed dose d istributions were calculated in 1990 and 1991 considering, (a) a single cyl indrical microbeam, (b) multiple cylindrical microbeams in an orthogonal sq uare bundle, and (c) multiple planar microbeams. it is shown that the PSI-v ersion of GEANT can potentially deliver more accurate results (a) using pre sently the most advanced atomic data, and especially (b) employing "Single- collision" electron transport instead of only the "Condensed-history'' elec tron transport as in code INHOM(EGS4). In contrast Dilmanian-CPE and Persli den-CPE codes deposit the electron energy locally instead of transporting i t to the correct position. (C) 2000 American Association of Physicists in M edicine. [S0094-2405(00)01106-8].