Boron Neutron Capture Therapy of Malignant Gliomas

Boron neutron capture therapy (BNCT) is a promising modality for biochemically targeted, highly selective radiation treatment of various cancers, including malignant gliomas. Currently available results demonstrate the beneficial effect of such therapy on survival of patients with both recurrent and newly diagnosed glioblastomas. The main drawback of BNCT in cases of previously irradiated neoplasms is high rates of symptomatic pseudoprogression and radiation necrosis. For prevention of these complications, concurrent administration of bevacizumab may be helpful. Further studies are needed to establish the optimal therapeutic protocols and to define the exact role of this management option in multimodality treatment strategies. Recent technological developments of accelerator-based neutron sources may simplify placement of the device for BNCT within clinical facilities and lead to wider application of this technique in cases of various cancers.

QA measurement of gamma-ray dose and neutron activation using TLD-400 for BNCT beam

TLD-400 (CaF₂:Mn) chips were applied for the gamma-ray dose measurement in a PMMA phantom exposed to a BNCT beam because of their very low neutron sensitivity. Since TLD-400 chips possess an adequate amount of Mn activator they have been employed in this work simultaneously for neuron activation measurement. The self-irradiation TL signals owing to the decay of the neutron induced ⁵⁶Mn activity have been applied for a calibration of the TLD-400 chip in situ, where the activities were measured by an HPGe detector system and the energy deposition per disintegration of ⁵⁶Mn was calculated by applying a Monte Carlo code. It was accidentally found that the irradiated TLD-400 chips were capable of emitting prominent scintillation lights owing to the induced ⁵⁶Mn activity, which can easily be recorded by the TLD reader without heating and after a calibration can be used to determine the ⁵⁶Mn activity.

Calibration of the borated ion chamber at NIST reactor thermal column

In boron neutron capture therapy and boron neutron capture enhanced fast neutron therapy, the absorbed dose of tissue due to the boron neutron capture reaction is difficult to measure directly. This dose can be computed from the measured thermal neutron fluence rate and the (10)B concentration at the site of interest. A borated tissue-equivalent (TE) ion chamber can be used to directly measure the boron dose in a phantom under irradiation by a neutron beam. Fermilab has two Exradin 0.5 cm(3) Spokas thimble TE ion chambers, one loaded with boron, available for such measurements. At the Fermilab Neutron Therapy Facility, these ion chambers are generally used with air as the filling gas. Since alpha particles and lithium ions from the (10)B(n,alpha)(7)Li reactions have very short ranges in air, the Bragg-Gray principle may not be satisfied for the borated TE ion chamber. A calibration method is described in this paper for the determination of boron capture dose using paired ion chambers. The two TE ion chambers were calibrated in the thermal column of the National Institute of Standards and Technology (NIST) research reactor. The borated TE ion chamber is loaded with 1,000 ppm of natural boron (184 ppm of (10)B). The NIST thermal column has a cadmium ratio of greater than 400 as determined by gold activation. The thermal neutron fluence rate during the calibration was determined using a NIST fission chamber to an accuracy of 5.1%. The chambers were calibrated at two different thermal neutron fluence rates: 5.11 x 10(6) and 4.46 x 10(7)n cm(-2) s(-1). The non-borated ion chamber reading was used to subtract collected charge not due to boron neutron capture reactions. An optically thick lithium slab was used to attenuate the thermal neutrons from the neutron beam port so the responses of the chambers could be corrected for fast neutrons and gamma rays in the beam. The calibration factor of the borated ion chamber was determined to be 1.83 x 10(9) +/- 5.5% (+/- 1sigma) n cm(-2) per nC at standard temperature and pressure condition.

Normalisation of prescribed dose in BNCT

Normalisation of prescribed dose in boron neutron capture therapy (BNCT) is needed to facilitate combining clinical data from different centres in the world to help expedite development of the modality. The approach being pursued within the BNCT community is based upon improving precision in the measurement and specification of absorbed dose. Beam characterisations using a common method are complete as are comparative dosimetry measurements between clinical centres in Europe and the USA. Results from treatment planning systems at these centres have been compared with measurements performed by MIT, and the scale factors determined are being confirmed with independent tests using measurements in an ellipsoidal water phantom. Dose normalisations have successfully been completed and applied to retrospectively analyse treatment plans from Brookhaven National Laboratory (1994-99) so that reported doses are consistently expressed with the trials performed during 1994-2003 at Harvard-MIT. Dose response relationships for adverse events and other endpoints can now be more accurately established.

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.

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.

Advantage and limitations of weighting factors and weighted dose quantities and their units in boron neutron capture therapy

Defining the parameters influencing the biological reaction due to absorbed dose is a continuous topic of research. The main goal of radiobiological research is to translate the measurable dose of ionizing radiation to a quantitative expression of biological effect. Mathematical models based on different biological approaches (e.g., skin reaction, cell culture) provide some estimations that are often misleading and, to some extent, dangerous. Conventional radiotherapy is the simplest case because the primary radiation and secondary radiation are both low linear energy transfer (LET) radiation and have about the same relative biological effectiveness (RBE). Nevertheless, for this one-dose-component case, the dose-effect curves are not linear. In fact, the total absorbed dose and the absorbed dose per fraction as well as the time schedule of the fractionation scheme influence the biological effects. Mathematical models such as the linear-quadratic model can only approximate biological effects. With regard to biological effects, fast neutron therapy is more complex than conventional radiotherapy. Fast neutron beams are always contaminated by gamma rays. As a consequence, biological effects are due to two components, a high-LET component (neutrons) and a low-LET component (photons). A straight transfer of knowledge from conventional radiotherapy to fast neutron therapy is, therefore, not possible: RBE depends on the delivered dose and several other parameters. For dose reporting, the European protocol for fast neutron dosimetry recommends that the total absorbed dose with gamma-ray absorbed dose in brackets is stated. However, boron neutron capture therapy (BNCT) is an even more complex case, because the total absorbed dose is due to four dose components with different LET and RBE. In addition, the terminology and units used by the different BNCT groups is confusing: absorbed dose and weighted dose are both to be stated in grays and are never "photon equivalent." The ICRU/IAEA made proposals, which should be followed by all BNCT groups, to report always the four absorbed dose components, boron dose DB, proton dose Dp, gamma-ray dose Dgamma, and neutron dose Dn, as well as the sum DT of all components, as total absorbed dose, together with the total weighted dose Dw (to be used only for internal purposes, indicating the used weighting factors) at all points of interest and the treatment conditions.

Monte Carlo model of the Studsvik BNCT clinical beam: Description and validation

The neutron beam at the Studsvik facility for boron neutron capture therapy (BNCT) and the validation of the related computational model developed for the MCNP-4B Monte Carlo code are presented. Several measurements performed at the epithermal neutron port used for clinical trials have been made in order to validate the Monte Carlo computational model. The good general agreement between the MCNP calculations and the experimental results has provided an adequate check of the calculation procedure. In particular, at the nominal reactor power of 1 MW, the calculated in-air epithermal neutron flux in the energy interval between 0.4 eV-10 keV is 3.24 x 10(9) n cm(-2) s(-1) (+/- 1.2% 1 std. dev.) while the measured value is 3.30 x 10(9) n cm(-20 s(-1) (+/- 5.0% 1 std. dev.). Furthermore, the calculated in-phantom thermal neutron flux, equal to 6.43 x 10(9) n cm(-2) s(-1) (+/- 1.0% 1 std. dev.), and the corresponding measured value of 6.33 X 10(9) n cm(-2) s(-1) (+/- 5.3% 1 std. dev.) agree within their respective uncertainties. The only statistically significant disagreement is a discrepancy of 39% between the MCNP calculations of the in-air photon kerma and the corresponding experimental value. Despite this, a quite acceptable overall in-phantom beam performance was obtained, with a maximum value of the therapeutic ratio (the ratio between the local tumor dose and the maximum healthy tissue dose) equal to 6.7. The described MCNP model of the Studsvik facility has been deemed adequate to evaluate further improvements in the beam design as well as to plan experimental work.

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.

The EORTC Boron Neutron Capture Therapy (BNCT) Group: achievements and future projects

Boron Neutron Capture Therapy (BNCT) is an experimental treatment modality that takes place in a nuclear research reactor. To progress from preclinical studies to patient treatment is a challenge requiring strict quality management and special solutions to licensing, liability, insurance, responsibility and logistics. The European Organisation for the Research and Treatment of Cancer (EORTC) BNCT group has started the first European clinical trial of BNCT for glioblastoma patients at the European High Flux Reactor (HFR) in Petten, The Netherlands, conducted by the Department of Radiotherapy of the University of Essen, Germany. A very strict quality management had to be installed following the European rules on safety and quality assurance for nuclear research reactors, for radioprotection, for radiotherapy and for clinical trials. The EORTC BNCT Group has created a virtual European-wide hospital to handle the complex management of patients treated with BNCT. New clinical trials are currently under development.

Comparison of quality assurance for performance and safety characteristics of the facility for Boron Neutron Capture therapy in Petten/NL with medical electron accelerators

BACKGROUND AND PURPOSE

The European Council Directive on health protection 97/43/EURATOM requires radiotherapy quality assurance programmes for performance and safety characteristics including acceptance and repeated tests. For Boron Neutron Capture therapy (BNCT) at the High Flux Reactor (HFR) in Petten/NL such a programme has been developed on the basis of IEC publications for medical electron accelerators.

RESULTS

The fundamental differences of clinical dosimetry for medical electron accelerators and BNCT are presented and the order of magnitude of dose components and their stability and that of the main other influencing parameter 10B concentration for BNCT patient treatments. A comparison is given for requirements for accelerators and BNCT units indicating items which are not transferable, equal or additional. Preliminary results of in vivo measurements done with a set of 55Mn, 63Cu and 197Au activation foils for all single fields for the four fractions at all 15 treated patients show with < +/- 4% up to now a worse reproducibility than the used dose monitoring systems (+/- 1.5%) caused by influence of hair position on the foil-skull distance.

CONCLUSIONS

Despite the more complex clinical dosimetry (because of four relevant dose components, partly of different linear energy transfer (LET)) BNCT can be regulated following the principles of quality assurance procedures for therapy with medical electron accelerators. The reproducibility of applied neutron fluence (proportional to absorbed doses) and the main safety aspects are equal for all teletherapy methods including BNCT.

Organisation and management of the first clinical trial of BNCT in Europe (EORTC protocol 11961).EORTC BNCT study group

Boron Neutron Capture Therapy is based on the ability of the isotope 10B to capture thermal neutrons and to disintegrate instantaneously producing high LET particles. The only neutron beam available in Europe for such a treatment is based at the European High Flux Reactor HFR at Petten (The Netherlands). The European Commission, owners of the reactor, decided that the potential benefit of the facility should be opened to all European citizens and therefore insisted on a multinational approach to perform the first clinical trial in Europe on BNCT. This precondition had to be respected as well as the national laws and regulations. Together with the Dutch authorities actions were undertaken to overcome the obvious legal problems. Furthermore, the clinical trial at Petten takes place in a nuclear research reactor, which apart from being conducted in a non-hospital environment, is per se known to be dangerous. It was therefore of the utmost importance that special attention is given to safety, beyond normal rules, and to the training of staff. In itself, the trial is an unusual Phase I study, introducing a new drug with a new irradiation modality, with really an unknown dose-effect relationship. This trial must follow optimal procedures, which underscore the quality and qualified manner of performance.

Beam collimation and bolusing material optimizations for 10boron neutron capture enhancement of fast neutron (BNCEFN): definition of the optimum irradiation technique

PURPOSE

In boron-10 neutron capture enhancement of fast neutron irradiation (BNCEFN), the dose enhancement is correlated to the 10B concentration and thermal neutron flux. A new irradiation technique is presented to optimize the thermal neutron flux.

METHODS AND MATERIALS

The coupled FLUKA and MCNP-4A Monte Carlo codes were used to simulate the neutron production and transport for the Nice and Orleans facilities.

RESULTS

The new irradiation technique consists of a 20-cm lead blocks additional collimator, placed close to the patient's head, which is embedded in a pure graphite cube. A 24-fold thermal neutron flux increase is calculated between a 5 x 5 cm2 primary collimated field, with the patient's head in the air, and the same field size irradiated with the optimum irradiation technique. This increase is more important for the p(60)+Be Nice beam than for the p(34)+Be Orleans one. The thermal neutron flux is 2.1 x 10(10) n(th)/Gy for each facility. Assuming a 100 microg/g 10B concentration, a physical dose enhancement of 22% is calculated. Moreover, the thermal neutron flux becomes independent of the field size and the phantom head size.

CONCLUSION

This technique allows conformal irradiation of the tumor bed, while the thermal neutron flux is enhanced, and spreads far around the tumor.

Measurements of the neutron yields from 7Li(p,n)7Be reaction (thick target) with incident energies from 1.885 to 2.0 MeV

Accelerator-based neutron source have been considered to be practical for boron neutron capture therapy (BNCT). Based on experience with a parameters of the Brookhaven National Laboratory BMRR reactor neutron source, which has been used in treatment experiments, the future accelerator-based neutron source for BNCT should have the properties of low energy distribution (< 100 keV) and high flux (about 10(9) neutrons per second per square centimeter) in the patient zone. Using protons to bombard thick 7Li targets, generating neutrons via the 7Li(p,n)7Be reaction, is one of the optimal choices for this kind of neutron source. Neutron yield data versus incident energy are necessary in order to select the proper incident energy and for estimating how high the incident proton current should be. The required proton beam current intensity is one of the key parameters for an accelerator useful for BNCT. In the present work, neutron yields of the 7Li(p,n)7Be reaction with a thick lithium target and incident energies of 1.885 and 1.9 MeV were measured at 0 degree with respect to the incident beam direction. The results are (3.08 +/- 0.17) x 10(12) and (5.71 +/- 0.32) x 10(12) neutrons/C sr, respectively. Neutron yield angular distribution measurements at 2 MeV incident energy were also performed. The proton beams were generated by the Peking University 4.5 MV electrostatic accelerator. The emitted neutrons from these reactions have the advantages of low energy distribution and forward angular distribution, which are requirements for a BNCT neutron source. The data obtained in this work can be used as a reference to study the accelerator-based neutron sources for BNCT.

Disposition and tissue distribution of boron after infusion of borocaptate sodium in patients with malignant brain tumors

PURPOSE

In the frame of the Czech boron neutron capture therapy (BNCT) project, a clinical Phase I study of borocaptate sodium [Na2B12H11SH (BSH)] as the boron-10 delivery agent was performed to obtain data on disposition and tissue distribution of boron after an infusion of this compound, as well as to establish an optimal protocol for BNCT of malignant cerebral tumors.

METHODS AND MATERIALS

The kinetics of boron disposition after an infusion of borocaptate sodium (25 mg/kg body wt over the period of 1 h) was studied in a group of 10 patients with astrocytoma or glioblastoma of cerebral hemispheres using a modification of the Soloway-Messer colorimetric method. The boron content of tissues (tumor, healthy brain, dura mater, muscle, skin, and cranial bone) removed during the operation performed with latencies varying between 3 and 18 h was investigated by atomic emission spectrometry.

RESULTS

Compartmental analysis of boron blood concentrations has shown that in the majority of patients (four males and three females), the concentration decline can be adequately described by a two-compartment pharmacokinetic model (i.e., by a biexponential relationship). The calculated half-lives of the initial (fast) phase of the concentration decline varied between 0.85 and 3.65 h, whereas the half-life values for the terminal (slow) phase ranged between 22.2 and 111.8 h. However, in the remaining three patients (all females), the goodness of fit of the boron concentration data was significantly better when a pharmacokinetic model with three compartments was assumed. In these patients, therefore, an additional ultrafast phase with a half-life varying between 17 and 37 min was detected in the beginning of the boron blood concentration decline. On the other hand, in one of these patients, the half-life of the terminal phase was found to be 415 h (i.e., more than 17 days). Such a long persistence in the body is explained by the very high value of the total distribution volume, indicating extensive binding of BSH in peripheral tissues. Another reason may be enterohepatic recycling of BSH.

CONCLUSION

Tumor-to-blood ratios higher than 1.5, which are necessary for an effective outcome of BNCT, can be obtained only if the time interval elapsing between the onset of surgery and termination of BSH infusion is at least 12 h.