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.

SU-E-T-491: A FLUKA Monte Carlo Computational Model of a Scanning Proton Beam Therapy Nozzle at IU Proton Therapy Center.

PURPOSE: Charged particle therapy, especially proton therapy is a growing treatment modality worldwide. Monte Carlo (MC) simulation of the interactions of proton beam with equipment, devices and patient is a highly efficient tool that can substitute measurements for complex and unrealistic experiments. The purpose of this study is to design a MC model of a treatment nozzle to characterize the proton scanning beam and commissioning the model for the Indiana University Health Proton Therapy Center (IUHPTC. METHODS: The general purpose Monte Carlo code FLUKA was used for simulation of the proton beam passage through the elements of the treatment nozzle design. The geometry of the nozzle was extracted from the design blueprints. The initial parameters for beam simulation were determined from calculations of beam optics design to derive a semi-empirical model to describe the initial parameters of the beam entering the nozzle. The lateral fluence and energy distribution of the beam entering the nozzle is defined as a function of the requested range. The uniform scanning model at the IUHPTC is implemented. The results of simulation with the beam and nozzle model are compared and verified with measurements. RESULTS: The lateral particle distribution and energy spectra of the proton beam entering the nozzle were compared with measurements in the interval of energies from 70 MeV to 204.8 MeV. The accuracy of the description of the proton beam by MC simulation is better than 2% compared with measurements, providing confidence for complex simulation in phantom and patient dosimetry with the MC simulated nozzle and the uniform scanning proton beam. CONCLUSIONS: The treatment nozzle and beam model was accurately implemented in the FLUKA Monte Carlo code and suitable for the research purpose to simulate the scanning beam at IUHPTC.

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.

Clinical characterization of a proton beam continuous uniform scanning system with dose layer stacking.

A proton beam delivery system on a gantry with continuous uniform scanning and dose layer stacking at the Midwest Proton Radiotherapy Institute has been commissioned and accepted for clinical use. This paper was motivated by a lack of guidance on the testing and characterization for clinical uniform scanning systems. As such, it describes how these tasks were performed with a uniform scanning beam delivery system. This paper reports the methods used and important dosimetric characteristics of radiation fields produced by the system. The commissioning data include the transverse and longitudinal dose distributions, penumbra, and absolute dose values. Using a 208 MeV cyclotron's proton beam, the system provides field sizes up to 20 and 30 cm in diameter for proton ranges in water up to 27 and 20 cm, respectively. The dose layer stacking method allows for the flexible construction of spread-out Bragg peaks with uniform modulation of up to 15 cm in water, at typical dose rates of 1-3 Gy/min. For measuring relative dose distributions, multielement ion chamber arrays, small-volume ion chambers, and radiographic films were employed. Measurements during the clinical commissioning of the system have shown that the lateral and longitudinal dose uniformity of 2.5% or better can be achieved for all clinically important field sizes and ranges. The measured transverse penumbra widths offer a slight improvement in comparison to those achieved with a double scattering beam spreading technique at the facility. Absolute dose measurements were done using calibrated ion chambers, thermoluminescent and alanine detectors. Dose intercomparisons conducted using various types of detectors traceable to a national standards laboratory indicate that the measured dosimetry data agree with each other within 5%.

Measurement of the activity of 11C and 22Na sources using a 4pi-beta-gamma coincidence system.

Many radiation applications, including positron emission tomography (PET) studies and activation dosimetry, require the knowledge of the activity of short-lived radionuclide samples, whereas relative measurements may be hampered by the absence of a reference source. Using 11C radionuclide as an example, an analytical model based on a probabilistic approach has been set up to predict the activity of a pure positron emitter measured using the 4pi-beta-gamma coincidence technique. The model has been extended to describe the measurement on a 22Na source used to test the measurement technique. Comparison of the modeled results with the measurements confirms the general validity of the model. The model has also been studied for the effect of the variation of key measurement conditions, such as nominal source activity, detector efficiency, detector background levels, and coincidence resolving time. The 4pi-beta-gamma coincidence technique and the results of modeling allow the activity measurements on 22Na and 11C sources with an estimated relative standard uncertainly on the order of one percent.

A compact ridge filter for spread out Bragg peak production in pulsed proton clinical beams.

A number of designs have been proposed for ridge filters and range modulators used in proton therapy to modify the beam in order to spread out the Bragg peak. Despite the variety of solutions, no simple design capable of providing large fields and easy variation of the spread out Bragg peak (SOBP) length in a pulsed beam has been developed. We propose a compact ridge filter that can be used in a proton beam of any time structure. It allows the production of depth dose distributions that meet the requirements of therapy dose fields.

Investigation of applicability of alanine and radiochromic detectors to dosimetry of proton clinical beams.

Cancer therapy studies using proton accelerators are underway in several major medical centers in the U.S., Russia, Japan and elsewhere. To facilitate dosimetry intercomparisons between these laboratories, alanine-based detectors produced at the National Institute of Standards and Technology and commercially available radiochromic films were studied for their possible use as passive transfer dosimeters for clinical proton beams. Evaluation of characteristics of these instruments, including the LET dependence of their response of proton energy, was carried out at the Institute of Theoretical and Experimental Physics. Results of absolute dose measurements were regarded as a preliminary step of dose intercomparison between ITEP and NIST. Measurements made in a number of experiments showed average agreement between the ITEP and NIST dosimetry standards to 2.5%.

Relative biological effectiveness of proton medical beam at Moscow synchrotron determined by the Chinese hamster cells assay.

PURPOSE: Assessment of relative biological effectiveness (RBE) of the proton medical beam at Moscow synchrotron. METHODS AND MATERIALS: The study was performed at Moscow proton medical facility (Institute for Theoretical and Experimental Physics). Relative biological effectiveness of the synchrotron proton beam was assessed at the entry of the unmodulated 179 MeV beam and in the center of spread out Bragg peak (SOBP), from measurements of the survival of Chinese hamster cells (clone 431). Gamma-radiation of 60Co was used as a reference source. RESULTS: According to the linear regression model, mean RBE values at 37% survival level were found to be 1.10 +/- 0.04 at the beam entry and 1.14 +/- 0.05 in the center of SOBP. Values of RBE obtained using the linear-quadratic model for 10% and 37% survival levels were 1.09 and 1.07, respectively, at the beam entry and 1.07 and 1.08, respectively, in the center of SOBP. CONCLUSIONS: The data obtained indicate that (a) the RBE values at the entry of the unmodulated beam and at the center of the SOBP are in close agreement, with an average of about 1.10; (b) protons are radiobiologically somewhat more effective than 60Co gamma rays; and, (c) high pulse dose rate of the medical beam does not significantly affect biological effects of the beam.