A gas scintillator detector for 2D dose profile monitoring in pencil beam scanning and pulsed beam proton radiotherapy treatments.

In order to address dosimetry demands during proton therapy treatments utilizing pencil beam scanning and/or pulsed beam accelerators, we have developed a xenon-filled gas scintillation detector (GSD) that can monitor delivered dose and 2D beam centroid position pulse-by-pulse in real time, with high response linearity up to high instantaneous dose rates. We present design considerations for the GSD and results of beam tests carried out at operating proton therapy clinics. In addition to demonstrating spatial resolution with σ of a few hundred microns in each transverse dimension and relative dose precision better than 1% over large treatment areas, the test beam results also reveal the dependence of the GSD dose normalization on dose rate, beam energy, and gas impurities. The results demonstrate the promise of the GSD technology to provide an important addition to dosimetry approaches for next-generation ion beam therapy.

Characterization of a GEM-based scintillation detector with He-CF4 gas mixture in clinical proton beams.

Accurate, high-spatial resolution dosimetry in proton therapy is a time consuming task, and may be challenging in the case of small fields, due to the lack of adequate instrumentation. The purpose of this work is to develop a novel dose imaging detector with high spatial resolution and tissue equivalent response to dose in the Bragg peak, suitable for beam commissioning and quality assurance measurements. A scintillation gas electron multiplier (GEM) detector based on a double GEM amplification structure with optical readout was filled with a He/CF4 gas mixture and evaluated in pristine and modulated proton beams of several penetration ranges. The detector's performance was characterized in terms of linearity in dose rate, spatial resolution, short- and long-term stability and tissue-equivalence of response at different energies. Depth-dose profiles measured with the GEM detector in the 115-205 MeV energy range were compared with the profiles measured under similar conditions using the PinPoint 3D small-volume ion chamber. The GEM detector filled with a He-based mixture has a nearly tissue equivalent response in the proton beam and may become an attractive and efficient tool for high-resolution 2D and 3D dose imaging in proton dosimetry, and especially in small-field applications.

Dose Imaging Detectors for Radiotherapy Based on Gas Electron Multipliers.

New techniques in charged particle therapy and widespread use of modern dynamic beam delivery systems demand new beam monitoring devices as well as accurate 2D dosimetry systems to verify the delivered dose distribution. We are developing dose imaging detectors based on gas electron multipliers (GEM) with the goal of improving dose measurement linearity, position and timing resolution, and to ultimately allow pre-treatment verification of dose distributions and dose delivery monitoring employing scanning beam technology. A prototype 10×10 cm(2) double-GEM detector has been tested in the 205 MeV proton beam using electronic and optical readout modes. Preliminary results with electronic cross-strip readout demonstrate fast response and single-pixel (4 mm) position resolution. In optical readout mode, the line spread function of the detector was found to have σ=0.7 mm. In both readout modes, the detector response was linear up to dose rates of 50 Gy/min, with adequate representation of the Bragg peak in depth-dose profile measurements.

Measurement of the absolute n p scattering differential cross section at 194 MeV.

We describe a double-scattering experiment with a novel tagged neutron beam to measure differential cross sections for np backscattering to better than +/-2% absolute precision. The measurement focuses on angles and energies where the cross section magnitude and angle dependence constrain the charged pion-nucleon coupling constant, but existing data show serious discrepancies among themselves and with energy-dependent partial-wave analyses. The present results are in good accord with the partial-wave analyses, but deviate systematically from other recent measurements.