A reappraisal of the reported dose equivalents at the boundary of the University of California Radiation Laboratory during the early days of Bevatron operation.

The Bevatron of the Lawrence Berkeley National Laboratory operated with no permanent shielding-roof from 1954 to 1962. Neutron fluences measured at the laboratory perimeter reached a maximum in 1959, and were reported as an annual dose equivalent of 8.1 mSv (54% of the then operative radiation limit). The addition of temporary local shielding and improved operational techniques subsequently led to a steady decline in dose equivalent at the laboratory perimeter. A permanent concrete shielding-roof was constructed in 1962. In those early years of operation the reported dose equivalent, H, was derived from a measured total neutron fluence, phi, and an estimated spectrum-weighted fluence to dose equivalent conversion coefficient, (g), where H= (g) phi. The uncertainty in H was almost entirely due to the uncertainty in (g). While the measurements of phi were accurate the estimates of (g) were quite crude and depended upon measurements of average neutron energy, on assumptions about the shape of the neutron energy spectrum, and primitive values of fluence to dose equivalent conversion coefficients for monoenergetic neutrons. These early reported dose equivalents were known to be overestimated. This paper has reappraised the dose equivalents in the light of better information now available. Environmental neutron spectra have been calculated which more accurately correspond to the operational conditions of the Bevatron in the 1950s and early 1960s. than did those spectra available at that time. A new fluence to dose equivalent conversion function based on the latest data and for isotropic irradiation geometry was developed. From these two parameters better estimates of the coefficient (g) were determined and compared with the earlier values. From this reappraisal it is shown that the early reported dose equivalents were conservative by a factor of at least five.

The relative biological effectiveness of mixed fission-neutron-gamma radiation on the hematopoietic syndrome in the canine: effect of therapy on survival.

Acute lethality syndromes produced by the accidental exposure of humans to mixed neutron and gamma radiation from external sources can be related to acute lethality from photon irradiation using the relative biological effectiveness (RBE) for common end points. We used the canine as a model to study injury following exposure to mixed neutron and gamma radiation from the AFRRI TRIGA reactor. Exposures from the reactor were steady-state mode (40 cGy/min, bilateral) with an average neutron energy of 0.85 MeV; tissue-air ratio = 0.59 at midline abdominal. Healthy male and female canines were irradiated free-in-air behind a 6-in. lead wall; the neutron-gamma ratio was 5.4:1 at the entrance skin surface; exposures are reported as midline tissue doses. Bilateral exposure resulted in an LD50/30 of 153 cGy without therapeutic clinical support. Addition of clinical support consisting of fluids, antibiotics, and fresh irradiated platelets/whole blood increased the bilateral LD50/30 to 185 cGy, a dose modifying factor (DMF) of 1.21. This corresponds to respective LD50/30 values for bilateral 60Co gamma exposures of 260 and 338 cGy for nonsupported and clinically supported animals, and a DMF of 1.30. The RBE based on the values determined at midline tissue is approximately 1.69. Clinical support after bilateral irradiation produced a similar DMF to those of mixed fission neutrons and gamma rays and 60Co gamma rays alone. The RBE of 1.69 for midline tissue bilateral exposures is higher than 1, an RBE often cited for large animals. Therapeutic support administered to lethally irradiated canines significantly improved survival and increased the LD50/30 independent of radiation quality.

Time- and dose-dependent changes in neuronal activity produced by X radiation in brain slices.

A new method of exposing tissues to X rays in a lead Faraday cage has made it possible to examine directly radiation damage to isolated neuronal tissue. Thin slices of hippocampus from brains of euthanized guinea pigs were exposed to 17.4 ke V X radiation. Electrophysiological recordings were made before, during, and after exposure to doses between 5 and 65 Gy at a dose rate of 1.54 Gy/min. Following exposure to doses of 40 Gy and greater, the synaptic potential was enhanced, reaching a steady level soon after exposure. The ability of the synaptic potential to generate a spike was reduced and damage progressed after termination of the radiation exposure. Recovery was not observed following termination of exposure. These results demonstrate that an isolated neuronal network can show complex changes in electrophysiological properties following moderate doses of ionizing radiation. An investigation of radiation damage directly to neurons in vitro will contribute to the understanding of the underlying mechanisms of radiation-induced nervous system dysfunction.

Nonuniform irradiation of the canine intestine. II. Dosimetry.

An experimental model has been developed for quantitative studies of radiobiological damage to the canine small intestine following partial-body nonuniform irradiation. Animals were irradiated with 60Co gamma rays to simulate the nonuniform irradiation which do occur in victims of radiation accidents. The model used a short source-to-surface distance for unilateral irradiations to produce a dose gradient of a factor of two laterally across the canine intestinal region. The remainder of the animal's body was shielded to prevent lethal damage to the bone marrow. In situ dosimetry measurements were made using thermoluminescent dosimeters to determine the radiation dose delivered as a function of position along a segment of the small intestine. This system made it possible to correlate the radiation dose delivered at a specific point along the small intestine with the macroscopic and microscopic appearance of the intestinal mucosa at that point, as determined by direct observation and biopsy using a fiberoptic endoscope. A key feature of this model is that dosimetry data for multiple sites, which receive a graded range of radiation doses, can be correlated with biological measurements to obtain a dose-response curve. This model is being used to evaluate the efficacy of new therapeutic procedures to improve survival following nonuniform irradiation.

Radiation quality and rat motor performance.

The effects of bremsstrahlung, electron, gamma, and neutron radiations were investigated on the motor performance of male Sprague-Dawley rats. Rats were irradiated at a midline tissue dose rate of 20 Gy/min +/- 1 with one of the following: 18.6-MeV electrons (N = 40) or 18.1-MVp bremsstrahlung (N = 57) from a linear accelerator, 60Co 1.25-MeV gamma-ray photons (N = 48), or reactor neutrons at 1.67 MeV tissue-kerma weighted-mean energy (N = 43). Radiation effects were determined by establishing median effective doses (ED50) for rats trained on an accelerod, a shock-avoidance motor performance test. ED50's were based on 10-min postexposure performance. The ED50's were 61 Gy for electrons, 81 Gy for bremsstrahlung, 89 Gy for gamma-ray photons, and 98 Gy for neutrons. In terms of relative biological effectiveness to produce early performance decrement (10 min from the start of irradiation), significant differences existed between the electrons and the other three fields and between the bremsstrahlung and neutron fields. These differences could not be explained by macroscopic dose distribution patterns in the irradiated animals. The data imply that different radiation qualities are not equally effective at disrupting performance, with high-energy electrons being the most effective and neutrons the least.

A low-energy x-ray irradiator for electrophysiological studies.

A 50 kVp molybdenum target/filter x-ray tube has been installed inside a lead-shielded Faraday cage. High-dose rates of up to 1.54 Gy min-1 (17.4 keV weighted average photons) have been used to conduct local in vitro irradiations of the hippocampal region of guinea pig brains. Electrophysiological recordings of subtle changes in neuronal activity indicate this system is suitable for this application.

Lack of inverse dose-rate effect on fission neutron induced transformation of C3H/10T1/2 cells.

Exponential and density-inhibited cultures of C3H/10T1/2 cells were exposed to a single dose of 0.3 Gy of fission neutrons delivered at rates ranging from 0.005 to 0.1 Gy/min. No discernible effect upon cell survival or transformation was observed by a lowering of the fission neutron dose rate in either exponential or plateau cultures. At the level of 2.3 x 10(-4) transformants per surviving cell, the RBE for neoplastic transformation was three at acute dose rates and ten at the lowest dose rate studied (0.005 Gy/min for neutrons and 0.01 Gy/min for X-rays).

Xeromammographic automatic exposure termination.

A method of automatic exposure termination (AET) for xeromammography has been devised, significantly reducing the rate of repeat exposures due to poor choice of manual exposure factors. AET images are of good quality and are reliably produced. The concept of AET is based on the existence of an optimal transmitted exposure to the selenium plate, which is easily determined experimentally. In routine clinical xeromammography, a repeat rate of 20% was eliminated by the use of AET.

Negative-mode soft-tissue xeroradiography.

Positive-mode xeroradiography is an excellent method of visualizing soft-tissue structures in mammography and other procedures; however, toner deletions over a large area may preclude adequate delineation of the soft tissues near high-density structures such as bone. Use of the negative mode in the development cycle provides excellent demonstration of the soft tissues without toner deletions and requires only about 70% of the exposure needed in the positive mode. Negative-mode images also exhibit greater changes in radiographic contrast than those observed with the positive mode. The extent of achilles tendon lacerations was shown particularly well by this technique.

Evaluation of xeroradiographic image quality.

A theory of edge enhancement has been developed to describe xeroradiographic images of linear step objects. The theory is shown to give an accurate description of the relationship between edge enhancement, subject contrast, radiation exposure, selenium charging potential and developer bias potential. An optimal transmitted x-ray exposure exists which gives the maximum edge enhancement. For low contrast structures, this same optimal exposure was found to yield the most acceptable images as judged subjectively. This finding has led to the successful application of automatic exposure termination in mammographic examinations using xeroradiography.

A circular test pattern for evaluating x-ray tube focal spots.

The design and use of a circular x-ray resolution test pattern is presented. This pattern is shown to yield equivalent focal spot dimensions equal to those obtained with the more commonly used star pattern. The advantage of using the circular pattern is that it obviates the necessity of positioning the pattern strictly along the x-ray beam central axis. A major source of experimental error in making focal spot measurements is therefore circumvented, especially in the case of x-ray tubes with a small target angle.

Collimator misalignment in high energy therapy machines.

Measurements of the intensity distribution of a 60-Co beam revealed a significant field distortion caused by minor misalignment of the collimator assembly which was not detected by routine methods. Accurate monitoring of radiation intensity distribution and periodic visual inspection of collimators are suggested for early detection of this problem.

Coexistence of several putative neurotransmitters in single identified neurons of Aplysia.

By sensitive enzymatic micromethods several putative neurotransmitters were measured in four identifiable neurons of Aplysia californica (R-2, R-14, L-11, and C-1). Serotonin was found in all of these neurons, and octopamine in all but C-1. Acetylcholine has been previously reported to be present in R-2 and L-11. The catecholamines, norepinephrine and dopamine, were not detected in the four cells examined. The possible biological consequence of the presence of several putative transmitters in single identifiable neurons is discussed.