Verification of synchrotron microbeam radiation therapy using a purpose-built optical CT microscope

Abstract

This study presents an investigation of the use of 3-D dosimetry using optical computed tomography to provide verification of synchrotron microbeam radiation therapy (MRT). MRT is based on the remarkable tolerance of normal tissues to high doses of radiation when this dose is constrained to very narrow beams. At beamline ID17 of the European Synchrotron Radiation Facility, pre-clinical radiation therapy is delivered using an array of parallel microbeams of x-rays generated by a synchrotron-wiggler source. Measurement of the dose distribution around these microbeams requires a dosimeter with high spatial resolution, and the radiochromic plastic dosimeter PRESAGE™, used in conjunction with optical CT, is highly appropriate for this task. Two solid cylinders of 9.7 mm diameter PRESAGE™ were irradiated to create quality-assurance phantoms for the optical CT microscope using the dose-painting facilities at ID17. Images were analysed to ascertain the scanner linearity over the range 8 - 35 Gy and modulation-transfer function (MTF). With the initial scanner settings, MTF was found to be greater than 30% at 12 line pairs/mm and around 8% at 20.8 line pairs/mm, thus allowing individual lines of width 24 μm to be visualised. A further 9.7 mm PRESAGE™ sample was irradiated with a typical array of microbeams of FWHM 50 μm and centre-to-centre distance 400 μm. Results demonstrate how optical CT dosimetry may be capable (after further analysis) of making quantitative measurements of the peak-to-valley ratio of the microbeams. Finally, two samples of diameters 9.7 and 22 mm were irradiated from four directions using a typical MRT "cross-firing" pattern, and then imaged at two different image resolutions. The results show how optical CT dosimetry is able both to visualise the planned dose distribution and identify an incorrect treatment delivery. © 2010 IOP Publishing Ltd.