Determination of ion recombination for absolute dosimetry in microbeam radiation therapy

Abstract

Introduction Microbeam Radiation Therapy (MRT) uses a 3rd generation synchrotron X-ray source. MRT is based on the dose volume effect by applying an array of microbeams 25–100 lm wide with a center-to-center spacing of 100–400 lm. The synchrotron X-ray beam is minimally divergent over distances on a scale of 1–2 m, and is very intense, with in-beam dose rates as high as 13 kGy/s. These physical properties lead to sharp beam penumbras in tissue using an energy spectrum between 50 to 350 keV (as exists at the ID17 Biomedical beamline at the ESRF). Such dose rates make the dosimetry and in particular the determination the ion recombination correction factor to be applied to the cylindrical ionization chamber readings a challenging task. Material and methods We used three different approaches for the determination of the ion recombination correction factor: 1. the ‘‘two-voltage” method, 2. a cross-calibration with Alanine dosimeters, and 3. a storage ring current (mA) ramping method. This last method involves quantifying the decrease in ionization chamber response while increasing the storage ring current, which is attributable to the lack of ion recombination correction. The MRT beam has a 352 MHz fine structure, which can be considered, from the point of view of the charge collection process in the ionisation chamber, as a continuous beam. Consequently, among the different ‘‘two-voltage” methods used the approach of a continuous beam was applied. Results The ‘‘two-voltage” method resulted in a mean ion recombination correction Alanine dosimeter, an average ion recombination of 5.5% was found. The current ramping method gave a correction of 4.6% at a ring current of 200 mA. Conclusion The current ramping method is the most suitable approach to ion recombination correction, if we wish to use an ionization chamber measurement as the standard dosimetric reference. Indeed, with the current ramping method, the ion recombination correction can be determined reliably at a given current in the machine, assuming negligible ion recombination at very low current and a linear relationship between the ring current and the dose rate.