Paul M. DeLuca

A consistent set of neutron kerma coefficients from thermal to 150 MeV for biologically important materials

Neutron cross sections for nonelastic and elastic reactions on a range of elements have been evaluated for incident energies up to 150 MeV. These cross sections agree well with experimental cross section data for charged-particle production as well as neutron and photon production. Therefore they can be used to determine kerma coefficients for calculations of energy deposition by neutrons in matter. Methods used to evaluate the neutron cross sections above 20 MeV, using nuclear model calculations and experimental data, are described. Below 20 MeV, the evaluated cross sections from the ENDF/B-VI library are adopted. Comparisons are shown between the evaluated charged-particle production cross sections and measured data. Kerma coefficients are derived from the neutron cross sections, for major isotopes of H, C, N, O, Al, Si, P, Ca, Fe, Cu, W, Pb, and for ICRU-muscle, A-150 tissue-equivalent plastic, and other compounds important for treatment planning and dosimetry. Numerous comparisons are made between our kerma coefficients and experimental kerma coefficient data, to validate our results, and agreement is found to be good. An important quantity in neutron dosimetry is the kerma coefficient ratio of ICRU-muscle to A-150 plastic. When this ratio is calculated from our kerma coefficient data, and averaged over the neutron energy spectra for higher-energy clinical therapy beams [three p (68) + Be beams, and a d (48.5) + Be beam], a value of 0.94 +/- 0.03 is obtained. Kerma ratios for water to A-150 plastic, and carbon to oxygen, are also compared with measurements where available.

Neutron production from beam-modifying devices in a modern double scattering proton therapy beam delivery system

In this work the neutron production in a passive beam delivery system was investigated. Secondary particles including neutrons are created as the proton beam interacts with beam shaping devices in the treatment head. Stray neutron exposure to the whole body may increase the risk that the patient develops a radiogenic cancer years or decades after radiotherapy. We simulated a passive proton beam delivery system with double scattering technology to determine the neutron production and energy distribution at 200 MeV proton energy. Specifically, we studied the neutron absorbed dose per therapeutic absorbed dose, the neutron absorbed dose per source particle and the neutron energy spectrum at various locations around the nozzle. We also investigated the neutron production along the nozzles central axis. The absorbed doses and neutron spectra were simulated with the MCNPX Monte Carlo code. The simulations revealed that the range modulation wheel (RMW) is the most intense neutron source of any of the beam spreading devices within the nozzle. This finding suggests that it may be helpful to refine the design of the RMW assembly, e.g., by adding local shielding, to suppress neutron-induced damage to components in the nozzle and to reduce the shielding thickness of the treatment vault. The simulations also revealed that the neutron dose to the patient is predominated by neutrons produced in the field defining collimator assembly, located just upstream of the patient.

V79 survival following simultaneous or sequential irradiation by 15-MeV neutrons and 60Co photons.

A unique tandem source irradiation facility, composed of an intense d-T neutron source and a 60Co teletherapy unit, was used to investigate biological responses for different neutron/photon configurations. V79 Chinese hamster cells, attached as monolayers in log-phase growth, were irradiated at 37 degrees C by either 14.8-MeV neutrons, 60Co, or a mixture of 40% neutrons and 60% photons in simultaneous or sequential application. Measurements of cell survival indicate an increased effectiveness in cell killing for simultaneously administered neutrons and photons compared to that measured or predicted for sequentially applied beam modalities. An understanding of the magnitude of these interactive effects is important both for calculating accurate effective doses for neutron radiotherapy of deep-seated tumors, for which the photon component is appreciable, and for determination of environmental hazards to people occupationally exposed to mixtures of photons and neutrons.

A prototype beam delivery system for the proton medical accelerator at Loma Linda.

A variable energy proton accelerator was commissioned at Fermi National Accelerator Laboratory for use in cancer treatment at the Loma Linda University Medical Center. The advantages of precise dose localization by proton therapy, while sparing nearby healthy tissue, are well documented [R. R. Wilson, Radiology 47, 487 (1946); M. Wagner, Med. Phys. 9, 749 (1982); M. Goitein and F. Chen, Med. Phys. 10, 831 (1983)]. One of the components of the proton therapy facility is a beam delivery system capable of delivering precise dose distributions to the target volume in the patient. To this end, a prototype beam delivery system was tested during the accelerators commissioning period. The beam delivery system consisted of a beam spreading device to produce a large, uniform field, a range modulator to generate a spread out Bragg peak (SOBP), and various beam detectors to measure intensity, beam centering, and dose distributions. The beam delivery system provided a uniform proton dose distribution in a cylindrical volume of 20-cm-diam area and 9-cm depth. The dose variations throughout the target volume were found to be less than +/- 5%. Modifications in the range modulator should reduce this considerably. The central axis dose rate in the region of the SOBP was found to be 0.4 cGy/spill with an incident beam intensity of 6.7 x 10(9) protons/spill. With an accelerator repetition rate of 30 spills/min and expected intensity of 2.5 x 10(10) protons/spill for patient treatment, this system can provide 50 cGy/min for a 20-cm-diam field and 9-cm range modulation.(ABSTRACT TRUNCATED AT 250 WORDS)

Maximum: A scanning photoelectron microscope at Aladdin

Abstract The recent successful installation of the 30-period undulator on Aladdin, the 1 GeV electron storage ring at the Synchrotron Radiation Center of the University of Wisconsin, opens new possibilities for photoelectron spectroscopy. In particular, the high brightness of the machine, together with innovative optics, make possible the application of photoelectron spectroscopy to high-resolution soft X-ray microscopy. We call this system MAXIMUM (for Multiple Application X-ray IMaging Undulator Microscope). The proposed optical system will have a lateral resolution of better than 1000 A and a resolving power of better than 200 at 100 eV. After monochromatization, the radiation will be focused on a pinhole that can range in diameter from 1 to 100 μm, and will be prepared by lithographic techniques on a thin nickel film. The image of the pinhole, suitably demagnified, will be relayed to the sample. The image resolution and magnification can be adjusted by changing the pinhole size and the scanning step. A Schwartzschild objective can produce a demagnified image of the pinhole which is diffraction limited even at a wavelength of 40 A. At 100 A and at a numerical aperture of 0.2, the objective can produce a 250 A diameter spot. High flux will be achieved with a Mo-Si multilayer coating, for which preliminary experiments have demonstrated reflectivities near 40% at normal incidence. Other focusing elements (Fresnel zone plates and Kirkpatrick-Baez objectives) will also be implemented.

Longitudinal relaxation times of 129Xe in rat tissue homogenates at 9.4 T

Longitudinal relaxation times of 129Xe were measured in homogenates of rat brain, kidney, liver, and lung at varying oxygenation levels as a means to assess the feasibility of magnetic resonance (MR) imaging of tissue using laser‐polarized (LP) 129Xe as the signal source. The measured relaxation times ranged from 4.4 ± 0.4 sec in deoxygenated lung homogenate to 22 ± 2 sec in deoxygenated brain homogenate. When the LP gas is introduced to the subject via inhalation, these relaxation times are long enough to allow accumulation and subsequent MR imaging of LP 129Xe in tissues. Imaging of dissolved LP 129Xe will yield an intrinsic signal‐to‐noise ratio (SNR) that is approximately 3% of the proton intrinsic SNR. This relatively low intrinsic SNR is expected to be adequate for some tracer applications. T1 of 129Xe was found to depend on the oxygenation level of the tissue, and the effect of oxygenation is likely dependent on the amount of hemoglobin in the tissue homogenate. Magn Reson Med 41:933–938, 1999.

SURVIVAL OF PARENCHYMAL HEPATOCYTES IRRADIATED WITH 14.3 MeV NEUTRONS

The purpose of these experiments was to estimate the RBE of neutrons for parenchymal hepatocytes as a function of neutron dose and to determine the ability of liver cells to repair potentially lethal damage (PLD) after neutron exposure. Hepatocyte reproductive survival was used as the biological end point in these studies and hepatocyte survival was determined with an in vivo transplantation clonogenic assay system. The 14.3 MeV neutrons were generated by a D-T reaction at the University of Wisconsins gas target neutron source. The average neutron dose rate was 20 cGy/min. The estimated survival data for neutron exposed hepatocytes were best described by a single hit-single target model (i.e., n = 1.0) with a D0 = 170 cGy. In contrast to the results obtained with 60Co, hepatocytes exposed to neutrons are unable to repair PLD. The RBE value, when the reproductive survival was estimated 30 min after radiation exposure, is independent of neutron dose and equal to 1.6 +/- 0.1. In contrast, when the reproductive survival was estimated 24 hrs after radiation exposure, the RBE was found to increase with decreasing neutron dose and equal 4.2 +/- 0.5 at 50 cGy.

Synchrotron-Produced Ultrasoft X Rays: Equivalent Cell Survival at the Isoattenuating Energies 273 eV and 860 eV

In this paper we report on survival of Chinese hamster V79 and mouse C3H 10T1/2 cells after irradiation with synchrotron-produced 273 eV and 860 eV ultrasoft X rays. These two energies, which are available by multilayer monochromatization of the synchrotron output spectrum, exhibit equal attenuation within living cells. Such an isoattenuating energy pair allows the direct examination of how biological effectiveness varies with the energy of the ultrasoft X rays. In comparing survival results, we find similar biological effectiveness of these two energies for both the C3H 10T1/2 and the V79 cells. These results are not consistent with previous findings of increasing RBE with decreasing ultrasoft X-ray energies. In addition, after correcting for mean nuclear dose based on measurements of cell thickness obtained with confocal microscopy, we find no significant differences in survival between the two ultrasoft X-ray energies and 250 kVp X rays. These results suggest that RBE does not increase with decreasing energy of ultrasoft X rays between 860 eV and 273 eV. The possible impact of our results on past results for ultrasoft X rays is discussed.

Shielding measurements for 230-Mev protons

Energetic neutrons, produced as protons interact with matter, dominate the radiation shielding environment for proton accelerators. Because of the scarcity of data describing the shielding required to protect personnel from these neutrons, absorbed dose and dose-equivalent values are measured as a function of depth in a thick concrete shield at neutron emission angles of 0, 22, 45, and 90 deg for 230-MeV protons incident upon stopping-length aluminum, iron, and lead targets. Neutron attenuation lengths vary sharply with angle but are independent of the target material. Comparing results with prior shielding calculations, the High-Energy Transport Code overestimates neutron production and attenuation lengths in the forward direction. Analytical methods compare favorably in the forward direction but overestimate the production and attenuation lengths at large angles. The results presented are useful for determining the shielding requirements for proton radiotherapy facilities and as a benchmark for future calculations.

Kerma factor of carbon for 14.1-MeV neutrons.

Using microdosimetric techniques, a direct measurement was made of the kerma factor of carbon for 14.1-MeV neutrons. Kerma was inferred from charged particle energy depositions measured with a small graphite-walled proportional counter. Measurements with an ionization chamber and a proportional counter, both constructed with A150 plastic walls, as well as induced 24Na activity from the 27Al(n, alpha) reaction, determined the neutron fluence. The resulting carbon kerma factor was 0.178 +/- 0.11 X 10(-8) cGy X cm2 which is lower than published tabulations but in agreement with recent microscopic cross-section measurements.