New irradiation facility for biomedical applications at the RA-3 reactor thermal column.

A new irradiation facility has been developed in the RA-3 reactor in order to perform trials for the treatment of liver metastases using boron neutron capture therapy (BNCT). RA-3 is a production research reactor that works continuously five days a week. It had a thermal column with a small cross section access tunnel that was not accessible during operation. The objective of the work was to perform the necessary modifications to obtain a facility for irradiating a portion of the human liver. This irradiation facility must be operated without disrupting the normal reactor schedule and requires a highly thermalized neutron spectrum, a thermal flux of around 10(10) n cm(-2)s(-1) that is as isotropic and uniform as possible, as well as on-line instrumentation. The main modifications consist of enlarging the access tunnel inside the thermal column to the suitable dimensions, reducing the gamma dose rate at the irradiation position, and constructing properly shielded entrance gates enabled by logical control to safely irradiate and withdraw samples with the reactor at full power. Activation foils and a neutron shielded graphite ionization chamber were used for a preliminary in-air characterization of the irradiation site. The constructed facility is very practical and easy to use. Operational authorization was obtained from radioprotection personnel after confirming radiation levels did not significantly increase after the modification. A highly thermalized and homogenous irradiation field was obtained. Measurements in the empty cavity showed a thermal flux near 10(10) n cm(-2)s(-1), a cadmium ratio of 4100 for gold foils and a gamma dose rate of approximately 5 Gy h(-1).

Preliminary liver dose estimation in the new facility for biomedical applications at the RA-3 reactor.

As a part of the project concerning the irradiation of a section of the human liver left lobe, a preliminary estimation of the expected dose was performed. To obtain proper input values for the calculation, neutron flux and gamma dose rate characterization were carried out using adequate portions of cow or pig liver covered with demineralized water simulating the preservation solution. Irradiations were done inside a container specially designed to fulfill temperature preservation of the organ and a reproducible irradiation position (which will be of importance for future planification purposes). Implantable rhodium based self-powered neutron detectors were developed to obtain neutron flux profiles both external and internal. Implantation of SPND was done along the central longitudinal axis of the samples, where lowest flux is expected. Gamma dose rate was obtained using a neutron shielded graphite ionization chamber moved along external surfaces of the samples. The internal neutron profile resulted uniform enough to allow for a single and static irradiation of the liver. For dose estimation, irradiation condition was set in order to obtain a maximum of 15 Gy-eq in healthy tissue. Additionally, literature reported boron concentrations of 47 ppm in tumor and 8 ppm in healthy tissue and a more conservative relationship (30/10 ppm) were used. To make a conservative estimation of the dose the following considerations were done: i). Minimum measured neutron flux inside the sample (approximately 5 x 10(9) n cm-2 s-1) was considered to calculate dose in tumor. (ii). Maximum measured neutron flux (considering both internal as external profiles) was used to calculate dose in healthy tissue (approximately 8.7 x 10(9) n cm-2 s-1). (iii). Maximum measured gamma dose rate (approximately 13.5 Gy h-1) was considered for both tumor and healthy tissue. Tumor tissue dose was approximately 69 Gy-eq for 47 ppm of (10)B and approximately 42 Gy-eq for 30 ppm, for a maximum dose of 15 Gy-eq in healthy tissue. As can be seen from these results, even for the most conservative case, minimum tumor dose will be acceptable from the treatment point of view, which shows that the irradiation conditions at this facility have quite good characteristics for the proposed irradiation.

Monte Carlo calculation for the development of a BNCT neutron source (1eV – 10 KeV) using MCNP code

Different materials have been studies in order to produce the epithermal neutron beam between 1eV - 10 KeV, which extensively used to irradiate patients with brain tumors such as (GBM). For this purpose we have studied three different neutrons moderators (H2O, D2O and BeO) and their combinations, four reflectors (Al2O3, C, Bi, and Pb) and two filters (Cd and Bi). Results of calculation show, That the best obtained assembly configuration correspond to the combination of the three moderators H2O, BeO and D2O jointly to Al2O3 reflector and two filter Cd +Bi optimize the spectrum of the epithermal neutron at 72 percent, and minimize the thermal neutron to 4 percent and thus it can be used to treat the deep brain tumor. Our calculations have been performed by means of the Monte Carlo N- particle code MCNP 5C.

A study of neutron spectra from medical linear accelerators.

Medical accelerators with photon energies over 10 MeV generate an undesired fast neutron contamination in the therapeutic beam. In this work, the Monte Carlo code MCNP was used to simulate the transport of these photoneutrons across the head of various medical accelerators of high energy. The average and most probable neutron energies were obtained from these spectra, before and after crossing the accelerator shielding. The degradation of these spectra, when they cross concrete barriers with thickness which vary between 25 and 100 cm, was also studied.

Comparison of the performance of two NCT treatment planning systems using the therapeutic beam of the RA-6 reactor.

This work evaluates the performance of two NCT treatment planning systems: NCTPlan, developed by the CNEA and the Harvard-MIT group, and SERA, developed by the INEEL/Montana State University group. The study was performed in some simple geometries with the therapeutical hyperthermal beam of the RA-6 facility at Bariloche, Argentina. The first geometry was a rectangular phantom and calculations and measurements were made along the central beam axis and along a parellel axis, 4 cm apart from the central beam axis. Measurements and calculations were also performed in a cylindrical phantom, to explore the behavior of the treatment planning systems in a geometry simulating an extremity, in accordance with the CNEA clinical protocol. Comments on differences in source definitions and cross sections libraries are also included in the text. It can be seen that both codes give acceptable results on the central beam axis and on a lateral axis, showing good agreement with experimental results.

Implantable self-powered detector for on-line determination of neutron flux in patients during NCT treatment.

A novel system to determine thermal neutron flux in real time during NCT treatments was developed in the National Atomic Energy Commission of Argentina. The system is based on a special self-powered detector that can be implanted in patients owing to its small size and biocompatibility. High voltage is not required to operate this kind of detectors, which is a considerable advantage in terms of medical uses. By choosing the appropriate materials, it was possible to obtain a prototype with thermal neutron sensitivity providing for an adequate signal level in typical NCT thermal fluxes. It was also possible to minimize gamma response in order to neglect its contribution.

Boron neutron capture therapy of skin melanomas at the RA-6 reactor: a procedural approach to beam set up and performance evaluation for upcoming clinical trials.

This article reports on the progress of the modeling and experimental characterization of the RA-6 reactor neutron beam, designed for the upcoming BNCT clinical trials of skin melanoma, and presents the first theoretical analysis of such beam performance. The aspects relating to surface source modeling and assessment, beam dosimetry, treatment planning system calibration, and treatment planning optimization are presented herein. Several methods and criteria were established in order to provide guidance for future clinical studies conducted in this facility. Following a realistic model, the theoretical analysis was based on a clinical case of malignant melanoma in extremities. Owing to the complex geometry of the tumor, this particular clinical case represents one of the most difficult lesions to be treated. This article discusses the thorough evaluation stage that has led to the optimization of the treatment planning procedure. Two candidate plans were proposed, and dose-volume distributions in the target volume were evaluated on the basis of the application of a series of criteria that define the critical normal structures which limit the dose delivered. In spite of the complexity of the clinical case under review, results showed that only 4% of the tumor volume is underdosed in cases of mean blood 10B concentration values, even in the most unfavorable analysis. The overall results suggest that this BNCT facility is prepared to rigorously explore the clinical efficacy of the RA-6 beam and the BNCT treatment modality for peripheral melanomas.

Real-time measurement of low-energy-range neutron spectra on board the space shuttle STS-89 (S/MM-8).

We have developed a real-time, Bonner Ball-type (neutron energy range is from thermal to 15 MeV) neutron spectral measurement system (Bonner Ball Neutron Detector (BBND)) for use on board the International Space Station (ISS). From measurements taken inside STS-89 (S/MM-8), we successfully distinguished neutrons from protons and other particles in a mixed radiation field; a task hitherto considered difficult. Although the experimental period was short, only 3.5 days (January 24-27, 1998), we were able to obtain energy spectral data and the Earth's neutron dose-equivalent map for the ISS orbital conditions (altitude 400 km, orbit inclination angle 51.6 degrees). A method for calculating the neutron energy spectrum and compensating for the particle interaction with the sensors is also described in detail.

Genetic effects induced by neutrons in Drosophila melanogaster I. Determination of absorbed dose.

A method to obtain the absorbed dose in Drosophila melanogaster irradiated in the thermal column facility of the Triga Mark III Reactor has been developed. The method is based on the measurements of neutron activation of gold foils produced by neutron capture to obtain the neutron fluxes. These fluxes, combined with the calculations of kinetic energy released per unit mass, enables one to obtain the absorbed doses in Drosophila melanogaster.

Procedimentos para pacientes portadores de marcapasso e submetidos à radioterapia / Procedures for radiation therapy in patients using cardiac pacemarkers

Os modernos marcapassos cardíacos podem falhar quando submetidos a radiaçäo ionizante da ordem de 10 Gy e exibir alteraçös funcionais com doses täo baixas quanto 2 Gy. A interferência eletromagnética também pode afetar seu desempenho, mas este efeito näo é problema sério quando consideramos os equipamentos de radioterapia atuais. Devido ao aumento significativo de aceleradores lineares no Brasil e, com isto, um aumento da probabilidade de tratamento de pacientes portadores de marcapassos cardíacos permanentes com esses equipamentos, apresentamos uma revisäo e discussäo dos problemas em potencial associados e sugerimos alguns cuidados que adotamos e praticamos no Serviço de Radioterapia do Hospital Israelita Albert Einstein