Measurement of the dose equivalent of leakage radiation through an isocentric gantry used for neutron therapy

The leakage radiation through the shielding on an isocentric gantry of a neutron therapy machine was measured with a Rossi-type proportional counter. The dose equivalent of the leakage radiation was determined at two positions: (1) in the plane of the patient and (2) in the plane of the target. The dose equivalent of the leakage radiation is approximately the same as the leakage of a high-energy x-ray linac.     

Demonstration of three-dimensional deterministic radiation transport theory dose distribution analysis for boron neutron capture therapy

The Monte Carlo stochastic simulation technique has traditionally been the only well-recognized method for computing three-dimensional radiation dose distributions in connection with boron neutron capture therapy (BNCT) research. A deterministic approach to this problem would offer some advantages over the Monte Carlo method. This paper describes an application of a deterministic method to analytically simulate BNCT treatment of a canine head phantom using the epithermal neutron beam at the Brookhaven medical research reactor (BMRR). Calculations were performed with the TORT code from Oak Ridge National Laboratory (ORNL), an implementation of the discrete ordinates, or Sn method. Calculations were from first principles and used no empirical correction factors. The phantom surface was modeled by flat facets of approximately 1 cm2. The phantom interior was homogeneous. Energy-dependent neutron and photon scalar fluxes were calculated on a 32 x 16 x 22 mesh structure with 96 discrete directions in angular phase space. The calculation took 670 min on an Apollo DN10000 workstation. The results were subsequently integrated over energy to obtain full three-dimensional dose distributions. Isodose contours and depth-dose curves were plotted for several separate dose components of interest. Phantom measurements were made by measuring neutron activation (and therefore neutron flux) as a function of depth in copper-gold alloy wires that were inserted through catheters placed in holes drilled in the phantom. Measurements agreed with calculations to within about 15%. The calculations took about an order of magnitude longer than comparable Monte Carlo calculations but provided various conveniences, as well as a useful check.     

Macroscopic geometric heterogeneity effects in radiation dose distribution analysis for boron neutron capture therapy.

Calculations of radiation flux and dose distributions for boron neutron capture therapy (BNCT) of brain tumors are typically performed using sophisticated three-dimensional analytical models based on either a homogeneous approximation or a simplified few-region approximation to the actual highly heterogeneous geometry of the irradiation volume. Such models should be validated by comparison with calculations using detailed models in which all significant macroscopic tissue heterogeneities and geometric structures are explicitly represented as faithfully as possible. This paper describes such a validation exercise for BNCT of canine brain tumors. Geometric measurements of the canine anatomical structures of interest for this work were performed by dissecting and examining two essentially identical Labrador retriever heads. Chemical analyses of various tissue samples taken during the dissections were conducted to obtain measurements of elemental compositions for the tissues of interest. The resulting geometry and tissue composition data were then used to construct a detailed heterogeneous calculational model of the Labrador head. Calculations of three-dimensional radiation flux distributions pertinent to BNCT were performed for this model using the TORT discrete-ordinates radiation transport code. The calculations were repeated for a corresponding volume-weighted homogeneous-tissue model. Comparison of the results showed that peak neutron and photon flux magnitudes were quite similar for the two models (within 5%), but that the spatial flux profiles were shifted in the heterogeneous model such that the fluxes in some locations away from the peak differed from the corresponding fluxes in the homogeneous model by as much as 10%-20%. Differences of this magnitude can be therapeutically significant, emphasizing the need for proper validation of simplified treatment planning models.     

Enhancement of glutamic-pyruvic transaminase activity in leptospirae by addition of rabbit serum

Glutamic-pyruvic transaminase (GPT) in leptospirae was shown to have an optimal temperature of 30 C and pH 8.0. It was activated by cofactor pyridoxal phosphate, similar to GPT from other sources. However, GPT in all leptospirae, such as Leptospira canicola, L. icterohaemorrhagiae, L. autumnalis, and L. biflexa, evidently had less activity even when pyridoxal phosphate was present, in comparison with the cultivable spirochete Reiter treponeme and Streptococcus faecalis. The low level of activity of leptospiral GPT was enhanced more than 2.5-fold by the addition of about 10% rabbit serum, whereas GPT of S. faecalis did not show such enhancement. Enhancement of leptospiral GPT was greater with rabbit serum than with bovine or horse serum.     

Predictions of a stochastic model of bone marrow cell survival in high dose rate radiation fields with arbitrary neutron to gamma-ray absorbed dose rate ratios

In this paper, a stochastic model of cell survival, which was developed by Cotlet and Blue, based on the work of Jones, is extended to describe bone marrow cell survival in high dose rate radiation fields with arbitrary neutron to gamma-ray absorbed dose rate ratios. Mathematical formulas are obtained that describe the interaction of the neutron and gamma-ray components of the absorbed dose, for radiation fields with arbitrary neutron to gamma-ray dose rate ratios, for exposures of cells to various absorbed doses, at various high dose rates.     

Enhancement of miscibility between hemicellulose and lignin by addition of their copolymer,the lignin-carbohydrate complex

The miscibility between hemicellulose and lignin from beech wood was investigated from observation of their glass transition temperature, along with the effect of addition of their copolymer, the lignin-carbohydrate complex (LCC). The binary and ternary blend systems of hemicellulose, lignin and LCC change from immiscible to miscible by increasing the temperature. This behavior shows that this blend system has an upper critical solution temperature (UCST). The binodal temperature decreases with increasing amount of LCC added. This behavior was analyzed by the temperature and composition dependence of the Flory polymer-polymer interaction parameter (χ₁₂). The value of χ₁₂ estimated from the phase diagrams indicates that χ₁₂ decreases with an increase in both the lignin content and the temperature.     

Improvement of dose distribution by central beam shielding in boron neutron capture therapy

Since boron neutron capture therapy (BNCT) with epithermal neutron beams started at the Kyoto University Reactor (KUR) in June 2002, nearly 200 BNCT treatments have been carried out. The epithermal neutron irradiation significantly improves the dose distribution, compared with the previous irradiation mainly using thermal neutrons. However, the treatable depth limit still remains. One effective technique to improve the limit is the central shield method. Simulations were performed for the incident neutron energies and the annular components of the neutron source. It was clear that thermal neutron flux distribution could be improved by decreasing the lower energy neutron component and the inner annular component of the incident beam. It was found that a central shield of 4-6 cm diameter and 10 mm thickness is effective for the 12 cm diameter irradiation field. In BNCT at KUR, the depth dose distribution can be much improved by the central shield method, resulting in a relative increase of the dose at 8 cm depth by about 30%. In addition to the depth dose distribution, the depth dose profile is also improved. As the dose rate in the central area is reduced by the additional shielding, the necessary irradiation time, however, increases by about 30% compared to normal treatment.     

Evaluation of neutron dose in the maze of medical electron accelerators

MCNP code was used to simulate neutron and prompt gamma ray transport for a range of maze geometrical parameters, wall composition, and wall surface lining. Verification measurements were performed at two medical electron accelerator facilities. A very good agreement was observed between the results of the measurements and the MCNP simulation. MCNP code results were compared with the results of analytical equations used for the calculation of maze effectiveness, derived by Kersey and McCall. A good agreement exists between the simulation results and the results of the analytical methods for maze lengths longer than 8.5 m. However, the results of the present study showed that for shorter maze lengths, Kersey's method tended to overestimate neutron dose at the door entrance, whereas McCall's method with the neutron room scattered correction applied, showed an underestimation of neutron dose. Furthermore, according to MCNP simulation results, the use of barytes concrete instead of standard concrete as room shielding material, reduced neutron dose at the door entrance by about 20%. Finally, it was shown that lining with layers of wood and borated polyethylene significantly reduced the neutron dose at the door entrance by 45% and 65%, respectively.     

Enhancement of hybridoma formation by addition of insulin to HAT medium (HIAT)

The effects of insulin on the formation of hybridoma clones following fusion experiments with SRBC immunized BALB/c mouse spleen cells and P3U1 mouse plasmacytoma cells were evaluated. The addition of insulin to HAT medium (HIAT) resulted in significant increases in the number and size of hybridoma colonies generated. The total number of anti-SRBC antibody-secreting clones also increased as much as sevenfold using insulin-supplemented medium compared to HAT alone. In view of the increasing interest in hybridoma technology for monoclonal antibody production, the use of insulin-supplemented medium (HIAT) may significantly expedite ongoing work by providing a more efficient method for the establishment of stable clones.     

Tracking the dose distribution in radiation therapy by accounting for variable anatomy

The goal of this research is to calculate the daily and cumulative dose distribution received by the radiotherapy patient while accounting for variable anatomy, by tracking the dose distribution delivered to tissue elements (voxels) that move within the patient. Non-linear image registration techniques (i.e., thin-plate splines) are used along with a conventional treatment planning system to combine the dose distributions computed for each 3D computed tomography (CT) study taken during treatment. For a clinical prostate case, we demonstrate that there are significant localized dose differences due to systematic voxel motion in a single fraction as well as in 15 cumulative fractions. The largest positive dose differences in rectum, bladder and seminal vesicles were 29%, 2% and 24%, respectively, after the first fraction of radiation treatment compared to the planned dose. After 15 cumulative fractions, the largest positive dose differences in rectum, bladder and seminal vesicles were 23%, 32% and 18%, respectively, compared to the planned dose. A sensitivity analysis of control point placement is also presented. This method provides an important understanding of actual delivered doses and has the potential to provide quantitative information to use as a guide for adaptive radiation treatments.     

Measurements of the equivalent whole-body dose during radiation therapy by cytogenetic methods.

Estimates of equivalent whole-body dose following partial body exposure can be performed using different biophysical models. Calculations should be compared with biodosimetry data, but measurements are complicated by mitotic selection induced in target cells after localized irradiation. In this paper we measured chromosomal aberrations in peripheral blood lymphocytes during radiotherapy, and estimated the equivalent whole-body dose absorbed, by using the novel technique of interphase chromosome painting. Premature chromosome condensation was induced in stimulated lymphocytes by incubation in calyculin A, and slides were hybridized in situ with whole-chromosome DNA probes specific for human chromosomes 2 and 4. Reciprocal exchanges were used to estimate the equivalent whole-body dose, based on individual pre-treatment in vitro calibration curves. Equivalent whole-body dose increased as a function of the number of fractions, and reached a plateau at high fraction numbers. Chromosomal aberration yields were dependent on field size, tumour position and concurrent chemotherapy. Results suggest that interphase chromosome painting is a simple technique able to give a reliable estimate of the equivalent whole-body dose absorbed during therapeutic partial-body irradiation.     

Analytic estimates of secondary neutron dose in proton therapy

Proton beam losses in various components of a treatment nozzle generate secondary neutrons, which bring unwanted out of field dose during treatments. The purpose of this study was to develop an analytic method for estimating neutron dose to a distant organ at risk during proton therapy. Based on radiation shielding calculation methods proposed by Sullivan, we developed an analytical model for converting the proton beam losses in the nozzle components and in the treatment volume into the secondary neutron dose at a point of interest. Using the MCNPx Monte Carlo code, we benchmarked the neutron dose rates generated by the proton beam stopped at various media. The Monte Carlo calculations confirmed the validity of the analytical model for simple beam stop geometry. The analytical model was then applied to neutron dose equivalent measurements performed on double scattering and uniform scanning nozzles at the Midwest Proton Radiotherapy Institute (MPRI). Good agreement was obtained between the model predictions and the data measured at MPRI. This work provides a method for estimating analytically the neutron dose equivalent to a distant organ at risk. This method can be used as a tool for optimizing dose delivery techniques in proton therapy.     

Improvement of ESR dosimetry for thermal neutron beams through the addition of gadolinium

In this paper, the addition of gadolinium is proposed as a useful tool to enhance the electron spin resonance (ESR) sensitivity of organic compounds to thermal neutrons. The target of this work is the detection, through the ESR technique, of the thermal neutron fluence in a mixed field of photons and neutrons. Gadolinium was chosen because it has a very high capture cross section to thermal neutrons; its nuclear reaction with thermal neutrons induces complex inner shell transitions that generate, besides other particles, Auger electrons, which in turn release their energy in the neighborhood (only several nanometers) of the place of reaction. Gadolinium was added to two organic molecules: alanine and ammonium tartrate. The main result obtained was a greater neutron sensitivity for dosimeters with gadolinium than for those without gadolinium for both organic molecules used. Since a dosimeter pair is required to discriminate between the two components of a mixed field, we studied the response of each dosimeter pair irradiated in a mixed field. Through a blind test we verified the usefulness of this dosimetric system and we obtained an estimate of the fluence in the mixed field with a relative uncertainty of 3%, when the pair composed of an alanine dosimeter and a dosimeter with alanine and gadolinium is used.     

An analytic model of neutron ambient dose equivalent and equivalent dose for proton radiotherapy

Stray neutrons generated in passively scattered proton therapy are of concern because they increase the risk that a patient will develop a second cancer. Several investigations characterized stray neutrons in proton therapy using experimental measurements and Monte Carlo simulations, but capabilities of analytical methods to predict neutron exposures are less well developed. The goal of this study was to develop a new analytical model to calculate neutron ambient dose equivalent in air and equivalent dose in phantom based on Monte Carlo modeling of a passively scattered proton therapy unit. The accuracy of the new analytical model is superior to a previous analytical model and comparable to the accuracy of typical Monte Carlo simulations and measurements. Predictions from the new analytical model agreed reasonably well with corresponding values predicted by a Monte Carlo code using an anthropomorphic phantom.     

Sensitivity of different dose scoring methods on organ-specific neutron dose calculations in proton therapy

Scattered doses, e.g. neutron doses in proton therapy, are of concern in radiation therapy. Although measured data are the gold standard, Monte Carlo simulations allow a more realistic consideration of patient anatomy via whole-body phantoms. When calculating neutron doses with Monte Carlo techniques, the dose can be scored in different ways because neutrons deposit dose indirectly. The purpose of this study was to assess the differences in neutron dose predictions when using different dose scoring methods. Two methods were tested. In the first method, the organ dose was calculated by accumulating dose from each individual dose deposition event with a particle-specific radiation weighting factor applied. Alternatively, we applied a method where the calculation was done by averaging the dose over the total number of events irrespective of particle type and applying average neutron radiation weighting factors. In addition, we assessed the sensitivity of different neutron quality factor assignments based on two recommendations by the International Commission on Radiological Protection (ICRP). We found that the scoring procedure can lead to differences in the organ equivalent dose of about 25%. As to the ICRP definition of neutron quality factors, the most recent recommendation results in about 10% higher organ doses.     

Enhancement of Neisseria meningitidis infection in mice by addition of iron bound to transferrin.

Weanling mice were inoculated intracerebrally with selected vesicular stomatitis virus (VSV) complementation group II and III temperature-sensitive (ts) mutants. Of the VSV ts mutants studied, only ts G32, a group III complementation mutant, appeared neurovirulent. Interestingly, neither the capacity to replicate in central nervous system tissue nor the ability to replicate in certain neurally derived continuous cell lines at semipermissive or nonpermissive temperatures appeared different among the VSV ts mutants employed. Finally, the pathological alterations in central nervous system tissue produced by VSV ts G32 were entirely different than those produced by G31 VSV ts in the group III mutant. These studies support the hypothesis that both the virological and neuropathological features produced by different VSV ts mutants are dependent upon the unique characteristics of each mutant, rather than upon a common biochemical defect shared by all members of a complementation group.