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Microbeam radiation therapy (MRT) is a new oncology method currently under development for the treatment of inoperable pediatric brain tumors. Monte Carlo simulation, or the computational study of radiation transport in matter, is often used in radiotherapy to theoretically estimate the dose required for treatment. However, its potential use in MRT dose planning systems is currently hindered by the significant discrepancies that have been observed between measured and theoretical dose and the PVDR (peak to valley dose ratio). The need to resolve these discrepancies is driven by the desirability of making MRT available to humans in the next few years. This article aims to resolve some of the discrepancies by examining the simplifications adopted in previous MRT Monte Carlo studies, such as the common practice of commencing microbeam transport on the surface of the target which neglects the influence of the distributed synchrotron source, multislit collimator, and the beam divergence between them. This article uses PENELOPE Monte Carlo simulation to investigate the influence of these beamline components upstream of the target on the lateral dose profiles and PVDRs of an array of 25 microbeams. It also compares the dose profiles and PVDRs of a microbeam array produced from a single simulation (full array) to those produced from the superposition of a single microbeam profile (sup array). The effect of modeling the distributed source and the beam divergence was an increase in the absorbed dose in the penumbral and valley regions of the microbeam profiles. Inclusion of the multislit collimator resulted in differences of up to 5 microm in the FWHM of microbeam profiles across the array, which led to minor variations in the corresponding PVDR yields.

Secondary students' ideas about scientific concepts underlying climate change Secondary students' ideas about scientific concepts underlying climate change

Abstract-The dosimetry of X-ray microbeams using MOSFETs results in an asymmetrical beam profile due to a lack of lateral charged particle equilibrium. Monte Carlo simulations were carried out using PENELOPE and GEANT4 codes to study this effect and a MOSFET on a micropositioner was scanned in the microbeam. Based on the simulations a new method of microbeam dosimetry is proposed. The proposed edge-on face-to-face (EOFF) MOSFET detector, a die arrangement proposed here for the first time, should alleviate the asymmetry. Further improvement is possible by thinning the silicon body of the MOSFET.Index Terms-Charged-particle equilibrium (CPE), dose enhancement effects (DEEs), dosimetry, microbeam, MOSFET, radiotherapy, synchrotron.

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