MOSFET dosimetry for microbeam radiation therapy at the European Synchrotron Radiation Facility

Preclinical experiments are carried out with ~20–30 μm wide, ~10 mm high parallel microbeams of hard, broad-‘‘white’’-spectrum x rays (~50–600 keV) to investigate microbeam radiation therapy (MRT) of brain tumors in infants for whom other kinds of radiotherapy are inadequate and/or unsafe. Novel physical microdosimetry (implemented with MOSFET chips in the ‘‘edge-on’’ mode) and Monte Carlo computer-simulated dosimetry are described here for selected points in the peak and valley regions of a microbeam-irradiated tissue-equivalent phantom. Such microbeam irradiation causes minimal damage to normal tissues, possible because of rapid repair of their microscopic lesions. Radiation damage from an array of parallel microbeams tends to correlate with the range of peak-valley dose ratios (PVDR). This paper summarizes comparisons of our dosimetric MOSFET measurements with Monte Carlo calculations. Peak doses at depths \u3c22 mm are 18% less than Monte Carlo values, whereas those depths \u3e22 mm and valley doses at all depths investigated (2 mm–62 mm) are within 2–13% of the Monte Carlo values. These results lend credence to the use of MOSFET detector systems in edge-on mode for microplanar irradiation dosimetry

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

Microbeam Radiation Therapy (MRT) is a radiation treatment technique under development for inoperable brain tumors. MRT is based on the use of a synchrotron generated X-ray beam with an extremely high dose rate ( ~ 20 kGy/sec), striated into an array of X-ray micro-blades. In order to advance to clinical trials, a real-time dosimeter with excellent spatial resolution must be developed for absolute dosimetry. The design of a real-time dosimeter for such a radiation scenario represents a significant challenge due to the high photon flux and vertically striated radiation field, leading to very steep lateral dose gradients. This article analyses the striated radiation field in the context of the requirements for temporal dosimetric measurements and presents the architecture of a new dosimetry system based on the use of silicon detectors and fast data acquisition electronic interface. The combined system demonstrates micrometer spatial resolution and microsecond real time readout with accurate sensitivity and linearity over five orders of magnitude of input signal. The system will therefore be suitable patient treatment plan verification and may also be expanded for in-vivo beam monitoring for patient safety during the treatment

Experimental dosimetry for Microbeam Radiation Therapy

The thesis gives an overview on the preclinical results in Microbeam Radiation Therapy (MRT), a novel radiation therapy using microscopically small beams. In the first chapter preclinical results and biological observations after Microbeam Radiation Therapy are presented, in particular the normal tissue tolerance is highlighted. A chapter based on theoretical Monte Carlo dose calculations is summarizing a set of data on peak to valley dose ratios (PVDR) and relative dose distributions for various parameter settings, providing some guideline for preclinical studies. The main part of the thesis is focusing on the experimental dosimetry, on one side to measure the high dose rate in the homogenous field proposing the necessary corrections to be applied for absolute dose measurements and on the other side, to measure peak and valley dose. For the high resolution dose measurements of the spatially fractionated beam, results using several types of detectors are presented and discussed. Various results using Gafchromic film dosimetry in combination with a microdensitometer show slightly higher (~10-15 %) valley dose than the MC calculated values. Results of theoretical calculations of output factors and their experimental verification are in very good agreement. The great potential of interlaced Microbeams in an anthropomorphic phantom with one single high dose delivery is discussed, including the technical challenges to be mastered in the future

Faster and safer? FLASH ultra-high dose rate in radiotherapy

Recent results from the Franco-Swiss team of Institute Curie and Centre Hospitalier Universitaire Vaudois demonstrate a remarkable sparing of normal tissue after irradiation at ultra-high dose rate (>40 Gy/s). The “FLASH” radiotherapy maintains tumor control level, suggesting that ultra-high dose rate can substantially enhance the therapeutic window in radiotherapy. The results have been obtained so far only with 4-6 MeV electrons in lung and brain mouse model. Nevertheless, they have attracted a great attention for the potential clinical applications. Oxygen depletion had been discussed many years ago as a possible mechanism for reduction of the damage after exposure to ultra-high dose-rate. However, the mechanism underlying the effect observed in the FLASH radiotherapy remains to be elucidated

Feasibility study of online high-spatial-resolution MOSFET dosimetry in static and pulsed x-ray radiation fields

Improvements have been made in the measurement of dose profiles in several types of X-ray beams. These include 120-kVp X-ray beams from an orthovoltage X-ray machine, 6-MV Bremsstrahlung from a medical LINAC in conformal mode and the 50-200 keV energy spectrum of microbeams produced at the medical beamline station of the European Synchrotron Radiation Facility. Using a quadruple metal-oxide-semiconductor field-effect transistor (MOSFET) sensor chip in edge on mode together with a newly developed sensor readout system, the feasibility of online scanning of the profiles of quasi-static and pulsed radiation beams was demonstrated. Measurements of synchrotron pulsed microbeams showed that a micrometer-scale spatial resolution was achievable. The use of several MOSFETs on the same chip gave rise to the correction of misalignments of the oxide films of the sensor with respect to the microbeam, ensuring that the excellent spatial resolution of the MOSFET used in edge-on mode was fully utilized

MOSFET dosimetry with high spatial resolution in intense synchrotron-generated x-ray microbeams

Various dosimeters have been tested for assessing absorbed doses with microscopic spatial resolution in targets irradiated by high-flux, synchrotron-generated, low-energy (∼30–300 keV) x-ray microbeams. A MOSFET detector has been used for this study since its radio sensitive element, which is extraordinarily narrow (∼1 μm), suits the main applications of interest, microbeam radiation biology and microbeam radiation therapy (MRT). In MRT, micrometer-wide, centimeter-high, and vertically oriented swaths of tissue are irradiated by arrays of rectangular x-ray microbeams produced by a multislit collimator (MSC). We used MOSFETs to measure the dose distribution, produced by arrays of x-ray microbeams shaped by two different MSCs, in a tissue-equivalent phantom. Doses were measured near the center of the arrays and maximum/minimum (peak/valley) dose ratios (PVDRs) were calculated to determine how variations in heights and in widths of the microbeams influenced this for the therapy, potentially important parameter. Monte Carlo (MC) simulations of the absorbed dose distribution in the phantom were also performed. The results show that when the heights of the irradiated swaths were below those applicable to clinical therapy (\u3c1 mm) the MC simulations produce estimates of PVDRs that are up to a factor of 3 higher than the measured values. For arrays of higher microbeams (i.e., 25 μm×1 cm instead of 25×500 μm2), this difference between measured and simulated PVDRs becomes less than 50%. Closer agreement was observed between the measured and simulated PVDRs for the Tecomet® MSC (current collimator design) than for the Archer MSC. Sources of discrepancies between measured and simulated doses are discussed, of which the energy dependent response of the MOSFET was shown to be among the most important

MOSFET dosimetry with high spatial resolution in intense synchrotron-generated xray microbeams

Various dosimeters have been tested for assessing absorbed doseswith microscopic spatial resolution in targets irradiated by high-flux, synchrotron-generated,low-energy (~30–300 keV) x-ray microbeams. A MOSFET detector has been usedfor this study since its radio sensitive element, which isextraordinarily narrow (~1 µm), suits the main applications of interest, microbeamradiation biology and microbeam radiation therapy (MRT). In MRT, micrometer-wide,centimeter-high, and vertically oriented swaths of tissue are irradiated byarrays of rectangular x-ray microbeams produced by a multislit collimator(MSC). We used MOSFETs to measure the dose distribution, producedby arrays of x-ray microbeams shaped by two different MSCs,in a tissue-equivalent phantom. Doses were measured near the centerof the arrays and maximum/minimum (peak/valley) dose ratios (PVDRs) werecalculated to determine how variations in heights and in widthsof the microbeams influenced this for the therapy, potentially importantparameter. Monte Carlo (MC) simulations of the absorbed dose distributionin the phantom were also performed. The results show thatwhen the heights of the irradiated swaths were below thoseapplicable to clinical therapy (\u3c1 \u3emm) the MC simulations produce estimatesof PVDRs that are up to a factor of 3higher than the measured values. For arrays of higher microbeams(i.e., 25 µm×1 cm instead of 25×500 µm2), this difference between measured andsimulated PVDRs becomes less than 50%. Closer agreement was observedbetween the measured and simulated PVDRs for the Tecomet® MSC(current collimator design) than for the Archer MSC. Sources ofdiscrepancies between measured and simulated doses are discussed, of whichthe energy dependent response of the MOSFET was shown tobe among the most important