James B. Smathers

Composition of A‐150 tissue‐equivalent plastic

In recent years, the use of tissue-equivalent materials has become quite common in fast-neutron dosimetry, with the A-150 plastic developed by Shonka et al. probably the most popular. Information on this specific plastic is scantily reported in the literature and as a consequence a preponderance of authors unknowingly reference an article by Shonka describing an early version of a tissue substitute plastic but having a different elemental composition than the present A-150 formulation. We have reviewed the results of 21 chemical analyses which have occurred over a time span of four years on a total of 14 samples of A-150 plastic and based on these data and the formulation of the plastic, have arrived at a suggested composition for A-150 tissue-equivalent plastic. The ambiguities of water absorption by nylon, one of the components of the plastic, and the uncertainty this reflects in the composition of the plastic were evaluated.

Dosimetry intercomparisons between fast-neutron radiotherapy facilities.

Neutron dosimetry intercomparison visits have been made by physicists from the M. D. Anderson Hospital-Texas A&M University Project to the Naval Research Laboratory, the University of Washington, and the MRC Cyclotron at Hammersmith Hospital. The Naval Research Laboratory and University of Washington physicists have made dosimetry intercomparisons at the Texas A&M Variable-Energy Cyclotron (TAMVEC). The parameters that are usually measured during these visits are tissue kerma in air, tissue dose at depth of dose maximum, relative central-axis depth dose, neutron/gamma ratios in air and in phantom, and photon calibrations of ionization chambers. In addition, beam profiles and dose buildup curves are sometimes measured. Other parameters that are compared are values of W, stopping power ratios, kerma corrections, and calculations that lead to the statement of tumor doses for patients. This paper presents some of the results of the intercomparisons and discusses the implications of the findings.

Neutron energy spectra of d (49)‐Be and p (41)‐Be neutron radiotherapy sources

Zero-degree neutron energy spectra for the p(41)-Be and d(49)-Be reactions were measured by time-of-flight for neutrons with energies above 1.9 and 1.4 MeV, respectively. Spectral changes resulting from the addition of copper, aluminum, and polyethylene filters to unfiltered beams were determined. Integral yields, average energies, filter material attenuation coefficients, and kerma fractions were computed for these spectra. Calculated spectra for neutron beams filtered by various thicknesses of polyethylene compared favorably with experimental results

Dosimetric properties of the fast neutron therapy beams at TAMVEC

Neutrons produced by bombarding beryllium with 16-, 30-, and 50-MeV deuterons are compared in terms of dose rate, skin sparing, depth dose, and field flatness. In general, the dosimetric properties improved as the energy increased, but at 50 MeV flattening filters were necessary. Isodose curves for treatment planning were generated, using the decrement-line method, and were compared to curves measured by a computer-controlled isodose plotter. This system was also employed to measure the isodose curves for wedge fields. Dosimetry checks on various patients were made, using silicon diodes as in vivo fast neutron dosimeters.

Thallium-201 production with the idle beam from neutron therapy.

Deuteron reactions have been studied for the possibility of producing 201Tl so that this radioisotope could be made as an adjunct to neutron therapy. The cross‐sections for the (d,x n) reactions for 203Tl and 205Tl target nuclei have been measured for x=3, 4, and 5. From these data the yields of 201Tl for a typical treatment day have been calculated.

Physics intercomparisons for neutron radiation therapy

Abstract For several years, a concerted effort has been going on to ensure that all the groups involved in neutron dosimetry have intercompared their absorbed dose standards. These intercomparisons have relevance to both neutron radiotherapy and neutron radiobiology. An International Neutron Dosimetry Intercomparison (INDI) sponsored by the International Commission on Radiation Units and Measurements, has been completed at the Radiological Accelerator Facility at Brookhaven National Laboratory with fourteen groups from six countries, participating. In Europe, the European Neutron Dosimetry Intercomparison Project (ENDIP) has also been conducting neutron dose intercomparison along similar lines. In the United States of America, an informal group of medical physicists associated with the ongoing therapy programs have also carried out extensive intercomparison between their institutions. They have also made measurements at several of the fast neutron therapy projects in Europe as well as participating in the INDI and ENDIP projects. Through the U.S.-Japan Co-operation Cancer Research Program intercomparison measurements have been made between the U.S. center and the Japanese centers. The Japanese were also involved in the INDI measurements. It is, therefore, apparent that extensive world-wide intercomparisons have been made and a summary of these measurements will be given. Several conclusions relating to types of dosimeters and precautions that need to be made in neutron dosimetry can be drawn from these measurements.

Chelate enhancement of the sensitivity for magnesium in neutron activation analysis

Abstract A chelation technique is described in which the high sensitivity of neutron activation analysis for bromine is used to improve, the activation sensitivity for magnesium. Magnesium is extracted with 5,7-dibromo-8-hydroxyquinoline in chloroform in the presence of 2,4,6-trimethylpyridine, the chelate is isolated by paper chromatography, and the amount of magnesium present is calculated from the bromine activity after activation. Experimental variables are discussed. The sensitivity limit for magnesium is ca. 0.1 μg.

Modification of the 50% maximum dose depth for 41-MeV (p+,Be) neutrons by use of filtration and/or transmission targets.

Several target configurations for the 41-MeV (p+,Be) reaction have been evaluated for the characteristics of the radiation field produced; depth dose, dose rate per microA, From analysis, it is concluded that to achieve the desired 13.2-cm depth for 50% of maximum dose and acceptable dose rate at a target-to-skin distance (TSD) of 125-150 cm, the neutron spectra must be filtered to preferentially absorb the lower-energy neutrons. Further increases in depth of 50% of maximum dose and a significant reduction in beryllium heating problems result if a partial transmission target is used with the terminal 30% of proton energy being deposited in a copper target backing.

A Medically Dedicated Cyclotron Facility

The use of cyclotrons for clinical work is not new. In fact, one of the first applications for the early cyclotrons built by Lawrence at Berkeley was the production of radioactive isotopes for medical purposes and the production of fast neutrons for treatment of cancer patients (neutron radiotherapy) and although several compact cyclotrons have been installed in medical institutions in the U.S. for isotope production no installation has taken place for both radiotherapy purposes and isotope production. In fact, less than half a dozen for such combined purposes have been installed world wide.

Fast‐neutron dose rate vs energy for the d+Be reaction—a reanalysis

The differences in the published information concerning tissue kerma in air vs deuteron energy for the d+Be reaction are analyzed in light of some recent measurements. The reason for the discrepancy is determined to be a lack of electron suppression on the Be target in some earlier measurements, and the relation ln(tissue kerma)=ln(1.356 X 10(-4)+2.97lnE is found to fit the measured data over the deuteron energy range 11-50 MeV.