J. Eenmaa

Uniformity in dosimetry protocols for therapeutic applications of fast neutron beams.

In this supplement to both the American and the European protocols for clinical neutron dosimetry, new recommendations are given with respect to the basic physical parameters and experimental techniques employed. For neutron dosimetry, the air kerma or exposure calibration in a photon beam is the most suitable method for the calibration of tissue-equivalent ionization chambers until calibration in a standard neutron field becomes available. More recent data are recommended for the physical parameters required for the photon calibration as well as for the measurements in the neutron beam. Water is recommended as the reference phantom material due to its similarity in absorption and scattering properties to muscle. The resulting overall change in absorbed dose calculated according to this supplement, compared with the original protocols, will be smaller than about +/- 2% due to differences in the basic physical parameters. An additional change of several percent occurs at depth in a phantom as a result of the difference between water and the muscle-equivalent liquid formerly recommended as the reference phantom material.

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.

Stereotactic Neutron Radiosurgery for Arteriovenous Malformations of the Brain

A technique employing single fraction neutron radiosurgery for treatment of intracranial vascular malformations has been developed at the University of Washington and is described in this report. The natural history of arteriovenous malformations of the brain is briefly reviewed, along with currently available therapeutic methods for treatment of these lesions. The characteristics of the neutron beam used for radiosurgery are described, along with methods for patient immobilization, radiation treatment planning, dosimetry, and delivery of treatment using this technique.

Measurement of photon dose fraction in a neutron radiotherapy beam.

Photon dose fractions (PDFs) have been measured in and around a neutron radiotherapy beam with a tissue-equivalent proportional counter (TEPC) and with paired ion chambers. The PDFs were found to increase linearly with increasing field size and width depth in phantom. PDFs were shown to decrease with decreasing phantom size and to be larger in the shielded region of the phantom than in the direct beam. Uncertainties in the PDF values were estimated to be 10%-15% for the TEPC measurements but about 50% for the measurement made with ion chambers.

Dosimetric properties of neutrons from 21-MeV deuteron bombardment of a deuterium gas target.

Spectra, yields, average energies, and kerma rates in tissue of neutrons from 21-MeV deuteron bombardment of deuterium gas targets have been calculated for target thicknesses of 1, 3.5, and 5 MeV. A high pressure gas cell was constructed and was filled with 33 atm of D2 gas (equivalent to an energy loss of 3.5 MeV for 21-MeV deuterons); dose rate, dose buildup, and depth-dose properties of neutrons produced by the D(d,n) reaction were measured. Dosimetric properties of these neutrons are superior to those of neutrons from a thick Be target bombarded by a deuteron beam of the same energy.

Correction factors for neutron dose changes caused by inhomogeneities

A technique is presented for calculating correction factors for neutron dose changes caused by inhomogeneities. The technique is particularly applicable to geometries in which the inhomogeneity lies adjacent to, but not on, a direct path from the radiation source to the point of dosimetric interest; it yields results accurate to a few percent. Best results are obtained for inhomogeneities located at shallow depths.

Dosimetry intercomparisons and protocols for therapeutic applications of fast neutron beams

For an adequate evaluation and comparison of clinical results obtained at different centers, the neutron dosimetry procedures should be consistent. A considerable number of neutron dosimetry intercomparisons has been performed during the past 5 years; the general conclusions resulting from these intercomparisons will be summarized. Protocols for neutron dosimetry for radiotherapy have been drafted for the European and American groups involved in fast neutron radiotherapy. The highlights of the protocols will be discussed and the differences between them will be reported. Although adoption of uniform basic parameters is desirable, it seems equally important to standardize the experimental techniques employed for the determination of absorbed dose. The introduction of a secondary standard or a transfer dosimeter would be of great importance for the consistency of neutron dosimetry for biological and medical applications.

Treatment planning for neutron radiation therapy

Abstract Treatment-planning techniques developed for photon therapy have been freely borrowed for fast-neutron beam therapy, including methods of storing the measured data and subsequent manipulation to correct for patient contour, inhomogeneities, oblique incidence of the beam, and the presence of beam-modifying devices. These corrections account for discontinuities in density in the path of the ray from source to volume of interest, but not for their location along that ray, discontinuities in adjacent rays, or the finite size of the patients. Given appropriate choice of effective density and attenuation for the discontinuity, resulting errors are minimal for the photon energies in common use and in most clinically realistic geometries. In fast-neutron beam therapy, however, experiment shows that for accurate prediction of dose in the irradiated volume, corrections must be made for the finite size and shape of the irradiated volume and the location of discontinuities along the ray from the source to the volume of interest. Corrections must also be made for discontinuities present along adjacent rays, though absent along the ray through the volume of interest. Corrections are influenced by both density and hydrogen content, being greatest for lung, then bone, and least for fat. Methods will be discussed. The problem of incorporating the biological data to provide biologically effective dose will be considered.

A physics cyclotron adapted for fast neutron beam therapy

The adaptation of the University of Washington physics cyclotron to fast neutron beam radiation therapy was described. The approaches to the problem were described, including acquisition of basic machine data. The target, main barrier assemblies, collimators, dose control and safety system design and installation were outlined. The performance, including dose distributions, photon contamination, collimation achieved, etc. were described.

Fast neutron beam dosimetry: comparison of the ion chamber and proportional counter approaches

Data were presented comparing the absorbed dose in tissue exposed to beams of fast neutrons as measured by ion chambers and tissue-equivalent proportional counters. The fast neutron beams studied include 14 MeV monoenergetic beams and those generated by high energy deuterons incident on beryllium. The ion chamber approaches include the tissue-equivalent plastic, tissue-equivalent gas and polyethylene wall-ethylene gas systems. The proportional counters were of the tissue-equivalent wall-tissue-equivalent gas type. Sources of uncertainty were discussed, including values of stopping power ratios, W for the gases, differences in atomic composition between the wall and the gases, and separation of the photon component of the total dose.