Potential High Resolution Dosimeters For MRT

Abstract

Abstract. Microbeam Radiation Therapy (MRT) uses highly collimated, quasi-parallel arrays of X-ray microbeams of 50-600 keV, produced by 2 nd and 3 rd generation synchrotron sources, such as the National Synchrotron Light Source (NSLS) in the U.S., and the European Synchrotron Radiation Facility (ESRF) in France, respectively. High dose rates are necessary to deliver therapeutic doses in microscopic volumes, to avoid spreading of the microbeams by cardiosynchronous movement of the tissues. A small beam divergence and a filtered white beam spectrum in the energy range between 30 and 250 keV results in the advantage of steep dose gradients with a sharper penumbra than that produced in conventional radiotherapy. MRT research over the past 20 years has allowed a vast number of results from preclinical trials on different animal models, including mice, rats, piglets and rabbits. Microbeams in the range between 10 and 100 micron width show an unprecedented sparing of normal radiosensitive tissues as well as preferential damage to malignant tumor tissues. Typically, MRT uses arrays of narrow (~25-100 micron-wide) microplanar beams separated by wider (100-400 microns centre-to-centre, c-t-c) microplanar spaces. We note that thicker microbeams of 0.1-0.68 mm used by investigators at the NSLS are still called microbeams, although some invesigators in the community prefer to call them minibeams. This report, however, limits it discussion to 25-100 µm microbeams. Peak entrance doses of several hundreds of Gy are surprisingly well tolerated by normal tissues. High resolution dosimetry has been developed over the last two decades, but typical dose ranges are adapted to dose delivery in conventional Radiation Therapy (RT). Spatial resolution in the sub-millimetric range has been achieved, which is currently required for quality assurance measurements in Gamma-knife RT. Most typical commercially available detectors are not suitable for MRT applications at a dose rate of 16000 Gy/s, micron resolution and a dose range over several orders of magnitude. This paper will give an overview of all dosimeters tested in the past at the ESRF with their advantages and drawbacks. These detectors comprise: Ionization chambers, Alanine Dosimeters, MOSFET detectors, Gafchromic ® films, Radiochromic polymers, TLDs, Polymer gels, Fluorescent Nuclear Track Detectors (Al 2 O 3 :C, Mg single crystal detectors), OSL detectors and Floating Gate-based dosimetry system. The aim of such a comparison shall help with a decision on which of these approaches is most suitable for high resolution dose measurements in MRT. The principle of these detectors will be presented including a comparison for some dosimeters exposed with the same irradiation geometry, namely a 1×1 cm 2 field size with microbeam exposures at the surface, 0.1 cm and 1 cm in depth of a PMMA phantom. For these test exposures, the most relevant irradiation parameters for future clinical trials have been chosen: 50 micron FWHM and 400 micron c-t-c distance. The experimental data are compared w...

Micrometer‐resolved film dosimetry using a microscope in microbeam radiation therapy

“…At the ESRF, a Joyce Loebl microdensitometer which has a much higher spatial resolution is used to resolve the microbeams. 2 Nevertheless, its resolution is also limited. The modulation transfer function (MTF) of the device was measured and the resolution estimated to be around 25 µm.…”

“…For a future clinical application of MRT, doses need to be measured within 3% accuracy. Bräuer-Krisch et al 2 present several means of dosimetry and analyzed their strengths and weaknesses. Among others, solid state detectors are discussed such as flash memory MOSFET detectors, fluorescent nuclear track detectors (FNTD), thermoluminescent dosimeters (TLDs), and silicon strip detectors.…”

Medical physics aspects of the synchrotron radiation therapies: Microbeam radiation therapy (MRT) and synchrotron stereotactic radiotherapy (SSRT)

“…The relativeedose profiles at depth can be determined using several types of detectors, all with their individual advantages and disadvantages [34]. The most promising results so far were obtained using Gafchromic films either in combination with a microdensitometer [35] or a modified Zeiss Axio Vert.A1 microscope.…”

“…After several variable MSCs (Archer collimator [17]), Tecomet MSC [33] the advanced ESRF MSC (EMSC) produced from a solid tungsten carbide piece using new wire cutting techniques [34] produces perfectly regular beams of 50 micron with a deviation of ±1 micron. This was an important step to be able to assume identical sizes in FWHM of these microbeams as input data into the MC calculations for realistic dose computation.…”

X-Tream: a novel dosimetry system for Synchrotron Microbeam Radiation Therapy

“…Ionising chambers provide realtime dose measurements but do not have the necessary spatial resolution for profiling such narrow -1 - X-ray beams and therefore cannot be used in the real treatment configuration of MRT. A full account of other dosimetric techniques used in MRT is given in Brauer-Krisch [7]. The Centre for Medical Radiation Physics (CMRP) has been involved in the dosimetry of MRT since 1996.…”