The alanine detector in BNCT dosimetry: dose response in thermal and epithermal neutron fields.

PURPOSE: The response of alanine solid state dosimeters to ionizing radiation strongly depends on particle type and energy. Due to nuclear interactions, neutron fields usually also consist of secondary particles such as photons and protons of diverse energies. Various experiments have been carried out in three different neutron beams to explore the alanine dose response behavior and to validate model predictions. Additionally, application in medical neutron fields for boron neutron capture therapy is discussed. METHODS: Alanine detectors have been irradiated in the thermal neutron field of the research reactor TRIGA Mainz, Germany, in five experimental conditions, generating different secondary particle spectra. Further irradiations have been made in the epithermal neutron beams at the research reactors FiR 1 in Helsinki, Finland, and Tsing Hua open pool reactor in HsinChu, Taiwan ROC. Readout has been performed with electron spin resonance spectrometry with reference to an absorbed dose standard in a (60)Co gamma ray beam. Absorbed doses and dose components have been calculated using the Monte Carlo codes fluka and mcnp. The relative effectiveness (RE), linking absorbed dose and detector response, has been calculated using the Hansen & Olsen alanine response model. RESULTS: The measured dose response of the alanine detector in the different experiments has been evaluated and compared to model predictions. Therefore, a relative effectiveness has been calculated for each dose component, accounting for its dependence on particle type and energy. Agreement within 5% between model and measurement has been achieved for most irradiated detectors. Significant differences have been observed in response behavior between thermal and epithermal neutron fields, especially regarding dose composition and depth dose curves. The calculated dose components could be verified with the experimental results in the different primary and secondary particle fields. CONCLUSIONS: The alanine detector can be used without difficulty in neutron fields. The response has been understood with the model used which includes the relative effectiveness. Results and the corresponding discussion lead to the conclusion that application in neutron fields for medical purpose is limited by its sensitivity but that it is a useful tool as supplement to other detectors and verification of neutron source descriptions.

The FiR 1 photon beam model adjustment according to in-air spectrum measurements with the Mg(Ar) ionization chamber.

The mixed neutron-photon beam of FiR 1 reactor is used for boron-neutron capture therapy (BNCT) in Finland. A beam model has been defined for patient treatment planning and dosimetric calculations. The neutron beam model has been validated with an activation foil measurements. The photon beam model has not been thoroughly validated against measurements, due to the fact that the beam photon dose rate is low, at most only 2% of the total weighted patient dose at FiR 1. However, improvement of the photon dose detection accuracy is worthwhile, since the beam photon dose is of concern in the beam dosimetry. In this study, we have performed ionization chamber measurements with multiple build-up caps of different thickness to adjust the calculated photon spectrum of a FiR 1 beam model.

Epithermal neutron beam interference with cardiac pacemakers.

In this paper, a phantom study was performed to evaluate the effect of an epithermal neutron beam irradiation on the cardiac pacemaker function. Severe malfunction occurred in the pacemakers after substantially lower dose from epithermal neutron irradiation than reported in the fast neutron or photon beams at the same dose rate level. In addition the pacemakers got activated, resulting in nuclides with half-lives from 25 min to 115 d. We suggest that BNCT should be administrated only after removal of the pacemaker from the vicinity of the tumor.

Comparative study of dose calculations with SERA and JCDS treatment planning systems.

Three treatment planning systems developed for clinical boron neutron capture therapy (BNCT) use are SERA developed by INL/Montana State University, NCTPlan developed by the Harvard-MIT and the CNEA group and JAEA computational dosimetry system (JCDS) developed by Japan Atomic Energy Agency (JAEA) in Japan. Previously, performance of the SERA and NCTPlan has been compared in various studies. In this preliminary study, the dose calculations performed with SERA and JCDS systems were compared in single brain cancer patient case with the FiR 1 epithermal neutron beam. A two-field brain cancer treatment plan was performed with the both codes. The dose components to normal brain, tumor and planning target volume (PTV) were calculated and compared in case of one radiation field and combined two fields. The depth dose distributions and the maximum doses in regions of interest were compared. Calculations with the treatment planning systems for the thermal neutron induced ((10)B and nitrogen) dose components and photon dose were in good agreement. Higher discrepancy in the fast neutron dose calculations was found. In case of combined two-field treatment plan, overall discrepancy of the maximum weighted dose was approximately 3% for normal brain and PTV and approximately 4% for tumor dose.

An international dosimetry exchange for boron neutron capture therapy. Part I: Absorbed dose measurements.

An international collaboration was organized to undertake a dosimetry exchange to enable the future combination of clinical data from different centers conducting neutron capture therapy trials. As a first step (Part I) the dosimetry group from the Americas, represented by MIT, visited the clinical centers at Studsvik (Sweden), VTT Espoo (Finland), and the Nuclear Research Institute (NRI) at Rez (Czech Republic). A combined VTT/NRI group reciprocated with a visit to MIT. Each participant performed a series of dosimetry measurements under equivalent irradiation conditions using methods appropriate to their clinical protocols. This entailed in-air measurements and dose versus depth measurements in a large water phantom. Thermal neutron flux as well as fast neutron and photon absorbed dose rates were measured. Satisfactory agreement in determining absorbed dose within the experimental uncertainties was obtained between the different groups although the measurement uncertainties are large, ranging between 3% and 30% depending upon the dose component and the depth of measurement. To improve the precision in the specification of absorbed dose amongst the participants, the individually measured dose components were normalized to the results from a single method. Assuming a boron concentration of 15 microg g(-1) that is typical of concentrations realized clinically with the boron delivery compound boronophenylalanine-fructose, systematic discrepancies in the specification of the total biologically weighted dose of up to 10% were apparent between the different groups. The results from these measurements will be used in future to normalize treatment plan calculations between the different clinical dosimetry protocols as Part II of this study.

MCNP study for epithermal neutron irradiation of an isolated liver at the Finnish BNCT facility.

A successful boron neutron capture treatment (BNCT) of a patient with multiple liver metastases has been first given in Italy, by placing the removed organ into the thermal neutron column of the Triga research reactor of the University of Pavia. In Finland, FiR 1 Triga reactor with an epithermal neutron beam well suited for BNCT has been extensively used to irradiate patients with brain tumors such as glioblastoma and recently also head and neck tumors. In this work we have studied by MCNP Monte Carlo simulations, whether it would be beneficial to treat an isolated liver with epithermal neutrons instead of thermal ones. The results show, that the epithermal field penetrates deeper into the liver and creates a build-up distribution of the boron dose. Our results strongly encourage further studying of irradiation arrangement of an isolated liver with epithermal neutron fields.

The TL analysis methods used to determine absorbed gamma doses in vivo for the BNCT patients treated at FiR 1.

The gamma dose determination using thermoluminescent (TL) dosimeters in mixed neutron-gamma fields, such as in boron neutron capture therapy (BNCT), is difficult due to the thermal neutron sensitivity of the detectors; especially when equipment capable of glow curve analysis is not available. The two TL analysis methods used previously in Finnish BNCT to correct the measured TL signal to obtain absorbed gamma dose in vivo were studied and compared, and an enhanced method was introduced. The three TL methods were found surprisingly consistent despite, e.g. the rough estimate made in the first method.

Design and construction of shoulder recesses into the beam aperture shields for improved patient positioning at the FiR 1 BNCT facility.

Improvements have been made at the FiR 1 BNCT facility to ease the positioning of the patient with a tumor in the head and neck region into a lateral neutron beam. Shoulder recesses were constructed horizontally on both sides of the beam aperture. When shoulder recesses are not needed, they are filled with neutron attenuating filling blocks. MCNP simulations using an anthropomorphic human model BOMAB phantom showed that the main contribution to the increase in the effective dose to the patient's body due to the shoulder recesses was from the neutron dose of the arm. In a position when one arm is inside the shoulder recess, the maximal effective dose of the patient was estimated to be 0.7Sv/h. Dose measurements using the twin ionization chamber technique showed that the neutron dose increased on the sides as predicted by the MCNP model but there was no noticeable change in the gamma doses. When making the recesses into the lithium containing neutron shield material tritium contamination was confined using an underpressurized glove box and machine tools with local exhaust. The shoulder recesses give space for more flexible patient positioning and can be considered as a significant improvement of the Finnish BNCT facility.

Quality assurance procedures for the neutron beam monitors at the FiR 1 BNCT facility.

In order to assure the stability of the beam, the reliability of the beam monitoring system and the quality of the patient dose delivered, several procedures are followed at the FiR 1 epithermal beam in Finland. Routine procedures include in-phantom activation measurements before each patient treatment and a long-term follow-up of the results. The sensitivity of the beam monitors to external objects in the beam and to variations in the control rod positions in the reactor has been checked and found insignificant. The linearity of the beam monitor channels has been checked with activation measurements. It was found that due to saturation effects a correction of 11% has to be applied when extrapolating results from experiments at low power to full power using the reference monitor channel. The correction is even larger for other channels with higher count rates.

Computational study of the required dimensions for standard sized phantoms in boron neutron capture therapy dosimetry.

The minimum size of a water phantom used for calibration of an epithermal neutron beam of the boron neutron capture therapy (BNCT) facility at the VTT FiR 1 research reactor is studied by Monte Carlo simulations. The criteria for the size of the phantom were established relative to the neutron and photon radiation fields present at the thermal neutron fluence maximum in the central beam axis (considered as the reference point). At the reference point, for the most commonly used beam aperture size at FiR 1 (14 cm diameter), less than 1% disturbance of the neutron and gamma radiation fields in a phantom were achieved with a minimum a 30 cm x 30 cm cross section of the phantom. For the largest 20 cm diameter beam aperture size, a minimum 40 cm x 40 cm cross-section of the phantom and depth of 20 cm was required to achieve undisturbed radiation field. This size can be considered as the minimum requirement for a reference phantom for dosimetry at FiR 1. The secondary objective was to determine the phantom dimensions for full characterization of the FiR 1 beam in a rectangular water phantom. In the water scanning phantom, isodoses down to the 5% level are measured for the verifications of the beam model in the dosimetric and treatment planning calculations. The dose distribution results without effects caused by the limited phantom size were achieved for the maximum aperture diameter (20 cm) with a 56 cm x 56 cm x 28 cm rectangular phantom. A similar approach to study the required minimum dimensions of the reference and water scanning phantoms can be used for epithermal neutron beams at the other BNCT facilities.

Study of the relative dose-response of BANG-3 polymer gel dosimeters in epithermal neutron irradiation.

Polymer gels have been reported as a new, potential tool for dosimetry in mixed neutron-gamma radiation fields. In this work, BANG-3 (MGS Research Inc.) gel vials from three production batches were irradiated with 6 MV photons of a Varian Clinac 2100 C linear accelerator and with the epithermal neutron beam of the Finnish boron neutron capture therapy (BNCT) facility at the FiR 1 nuclear reactor. The gel is tissue equivalent in main elemental composition and density and its T2 relaxation time is dependent on the absorbed dose. The T2 relaxation time map of the irradiated gel vials was measured with a 1.5 T magnetic resonance (MR) scanner using spin echo sequence. The absorbed doses of neutron irradiation were calculated using DORT computer code, and the accuracy of the calculational model was verified by measuring gamma ray dose rate with thermoluminescent dosimeters and 55Mn(n,gamma) activation reaction rate with activation detectors. The response of the BANG-3 gel dosimeter for total absorbed dose in the neutron irradiation was linear, and the magnitude of the response relative to the response in the photon irradiation was observed to vary between different gel batches. The results support the potential of polymer gels in BNCT dosimetry, especially for the verification of two- or three-dimensional dose distributions.

Application of the new MultiTrans SP3 radiation transport code in BNCT dose planning.

Dose planning in boron neutron capture therapy (BNCT) is a complex problem and requires sophisticated numerical methods. In the framework of the Finnish BNCT project, new deterministic three-dimensional radiation transport code MultiTrans SP3 has been developed at VTT Chemical Technology, based on a novel application of the tree multigrid technique. To test the applicability of this new code in a realistic BNCT dose planning problem, cylindrical PMMA (polymethyl-methacrylate) phantom was chosen as a benchmark case. It is a convenient benchmark, as it has been modeled by several different codes, including well-known DORT and MCNP. Extensive measured data also exist. In this paper, a comparison of the new MultiTrans SP3 code with other methods is presented for the PMMA phantom case. Results show that the total neutron dose rate to ICRU adult brain calculated by the MultiTrans SP3 code differs less than 4% in 2 cm depth in phantom (in thermal maximum) from the DORT calculation. Results also show that the calculated 197Au(n,gamma) and 55Mn(n,gamma) reaction rates in 2 cm depth in phantom differ less than 4% and 1% from the measured values, respectively. However, the photon dose calculated by the MultiTrans SP3 code seems to be incorrect in this PMMA phantom case, which requires further studying. As expected, the deterministic MultiTrans SP3 code is over an order of magnitude faster than stochastic Monte Carlo codes (with similar resolution), thus providing a very efficient tool for BNCT dose planning.