Scan pattern optimization for uniform proton beam scanning.

Magnetic beam scanning allows one to spread proton beam over the desired radiation field area, improving beam utilization and conformity to the target area. This article discusses generic scan forms for generating uniform circular and rectangular fields and establishes criteria that can be applied to optimize selected scan patterns. During construction of the Midwest Proton Radiotherapy Institute (MPRI), Indiana University developed a magnetically scanned beam spreading system for the 3 m long gantry nozzle. Based on the commissioning experience, criteria for optimizing the scan patterns were derived. A numerical integration model was used to perform initial optimization of the resulting dose distribution. The selected scan patterns were then experimentally validated via test irradiation of Gafchromic films. Generic spiral and linear scan forms are proposed capable of delivering uniform circular and rectangular fields in continuous scanning mode. The test irradiations performed indicate that dose uniformity is within +/- 3% for both scan forms and that penumbra of the uncollimated field can approach the radius of the pristine beam spot. A well designed uniform scanning system can have a large library of uniform circular and rectangular fields of different sizes, which would increase beam utilization and minimize out-of-field dose to the patient.

Energy spectrum control for modulated proton beams.

In proton therapy delivered with range modulated beams, the energy spectrum of protons entering the delivery nozzle can affect the dose uniformity within the target region and the dose gradient around its periphery. For a cyclotron with a fixed extraction energy, a rangeshifter is used to change the energy but this produces increasing energy spreads for decreasing energies. This study investigated the magnitude of the effects of different energy spreads on dose uniformity and distal edge dose gradient and determined the limits for controlling the incident spectrum. A multilayer Faraday cup (MLFC) was calibrated against depth dose curves measured in water for nonmodulated beams with various incident spectra. Depth dose curves were measured in a water phantom and in a multilayer ionization chamber detector for modulated beams using different incident energy spreads. Some nozzle entrance energy spectra can produce unacceptable dose nonuniformities of up to +/-21% over the modulated region. For modulated beams and small beam ranges, the width of the distal penumbra can vary by a factor of 2.5. When the energy spread was controlled within the defined limits, the dose nonuniformity was less than +/-3%. To facilitate understanding of the results, the data were compared to the measured and Monte Carlo calculated data from a variable extraction energy synchrotron which has a narrow spectrum for all energies. Dose uniformity is only maintained within prescription limits when the energy spread is controlled. At low energies, a large spread can be beneficial for extending the energy range at which a single range modulator device can be used. An MLFC can be used as part of a feedback to provide specified energy spreads for different energies.

Improved Halbach sensor for NMR scanning of drill cores.

A lightweight Halbach magnet system for use in nuclear magnetic resonance (NMR) studies on drill cores was designed and built. It features an improved homogeneous magnetic field with a strength of 0.22 T and a maximum accessible sensitive volume. Additionally, it is furnished with a sliding table for automatic scans of cylindrical samples. This device is optimized for nondestructive online measurements of porosity and pore size distributions of water-saturated full cylindrical and split semicylindrical drill cores of different diameters. The porosity of core plugs with diameters from 20 to 80 mm can be measured routinely using exchangeable radiofrequency coils. Advanced NMR techniques that provide 2D T(1)-T(2) correlations with an average measurement time of 30 min and permeability estimates can be performed with a special insert suitable for small core plugs with diameter and length of 20 mm.

Advances of unilateral mobile NMR in nondestructive materials testing.

Unilateral mobile NMR employs portable instrumentation with sensors, which are applied to the object from one side. Based on the principles of well-logging NMR, a hand-held sensor, the NMR-MOUSE (MObile Universal Surface Explorer) has been developed for nondestructive materials testing. In the following, a number of new applications of unilateral NMR in materials science are reviewed. They are the state assessment of polyethylene pipes, the characterization of wood, the in situ evaluation of stone conservation treatment, high-resolution profiling of rubber tubes and 2-D imaging for defect analysis in rubber products.

Combined X-Y scanning magnet for conformal proton radiation therapy.

Light-ion beams have several features that make them very effective in radiation therapy applications. These include favorable depth dose distribution, finite penetration range, and high radiobiological efficiency. Moreover, magnetic scanning methods allow one to spread an ion beam to an exact image of a complex tumor shape. The ion scanning system usually consists of two magnets, each scanning horizontal and vertical directions independently. This paper discusses the design for a novel combined X-Y beam scanning magnet which is under development for the Midwest Proton Radiotherapy Institute.