Microdosimetry for boron neutron capture therapy: a review

Radionuclide carriers for targeting of cancer

This review describes strategies for the delivery of therapeutic radionuclides to tumor sites. Therapeutic approaches are summarized in terms of tumor location in the body, and tumor morphology. These determine the radionuclides of choice for suggested targeting ligands, and the type of delivery carriers. This review is not exhaustive in examples of radionuclide carriers for targeted cancer therapy. Our purpose is two-fold: to give an integrated picture of the general strategies and molecular constructs currently explored for the delivery of therapeutic radionuclides, and to identify challenges that need to be addressed. Internal radiotherapies for targeting of cancer are at a very exciting and creative stage. It is expected that the current emphasis on multidisciplinary approaches for exploring such therapeutic directions should enable internal radiotherapy to reach its full potential.

Surface-active liposomes for targeted cancer therapy

An overview of liposome-based drug-delivery carriers to cancer cells is presented. Properties related to interfacial interactions between liposomes and the biological milieu that determine the fate of liposomes in vivo are discussed. Original approaches to improve specificity for the target and to control the structural responsiveness of liposomes, depending on their immediate environment, with the aim of enhancing the delivered therapeutic doses, are summarized. This review is not exhaustive on research examples of liposomes as carriers for cancer therapy but, rather, aims to describe major directions of designs and strategies over recent years. The current therapeutic trends that exhibit increasingly higher complexity in structures and responses are also discussed.

Boron distribution in the normal rat brain after intravenous injection of boronophenylalanine-fructose

Boron neutron capture therapy (BNCT) is an experimental form of radiation therapy for malignant brain tumors and peripheral melanoma. The micro-distribution of the boron compound is critical to determine the radiation effects for both tumors and normal tissue. In the current dose calculation of BNCT, normal brain tissue is considered to have a homogeneous boron concentration. The purpose of this study was to examine the structure-specific boron concentration in normal rat neural tissue. At 10, 30 and 60 min after intravenous injection of 300 mg/kg boronophenylalanine-fructose to 10-week-old CD Fisher rats, neural tissue and blood were collected. Various neural structures were anatomically and histologically identified and specific boron concentrations were analyzed using high-resolution quantitative autoradiography. At 60 min after the injection, only the pituitary gland showed a higher boron concentration than that in blood, with the former being threefold higher. All other neural structures showed lower boron concentrations than that in blood. The present study thus demonstrated an extremely high boron concentration in the pituitary gland following intravenous injection of boronophenylalanine-fructose. In clinical trials of BNCT using an epithermal neutron beam, the radiation dose to the pituitary gland should be carefully evaluated.

A simple model for quantification of the radiobiological effectiveness of the 10B(n,α)7Li capture reaction in BNCT

A simple model has been developed for predicting radiobiological effectiveness of the neutron capture reaction in boron neutron capture therapy. This model was derived from the relationship between the cell survival from the boron capture reaction, the intracellular boron concentration, and the thermal neutron fluence. We found that the cell-killing effect of the boron capture reaction was well described using a power function of the intracellular boron concentration. Hence the relationship between cell survival from the boron capture reaction, intracellular boron concentration, and the thermal neutron fluence could be determined using a simple mathematical equation. We consider that our current approach is more appropriate and realistic than the conventional theoretical mathematical model used to estimate the radiobiological effectiveness of the neutron capture reaction in boron neutron capture therapy.

The Microdosimetry of the10B Reaction in Boron Neutron Capture Therapy: A New Generalized Theory

The microdosimetry of (10)B thermal neutron capture reactions should be considered as an essential step to be followed before studying the radiobiological aspects of boron neutron capture therapy. The boron dose itself is insufficient as the only quantity used to describe the biological effectiveness of the (10)B reaction for two important reasons: the specific microdistribution that the (10)B carrier compound exhibits at the cellular level and the primarily stochastic nature of the energy deposition process, which influences the biological response to the particulate radiation. In this work, these two aspects are analyzed in detail and an innovative rigorous analytical framework is developed in the microdosimetry domain. This formalism provides the necessary microdosimetric tools for more precisely describing the (10)B dose distribution deposited in sensitive microscopic structures and offers improved approaches for analyzing the biological dose--effect relationship of (10)B reactions.

Common challenges and problems in clinical trials of boron neutron capture therapy of brain tumors

Clinical trials for binary therapies, like boron neutron capture therapy (BNCT), pose a number of unique problems and challenges in design, performance, and interpretation of results. In neutron beam development, different groups use different optimization parameters, resulting in beams being considerably different from each other. The design, development, testing, execution of patient pharmacokinetics and the evaluation of results from these studies differ widely. Finally, the clinical trials involving patient treatments vary in many aspects such as their dose escalation strategies, treatment planning methodologies, and the reporting of data. The implications of these differences in the data accrued from these trials are discussed. The BNCT community needs to standardize each aspect of the design, implementation, and reporting of clinical trials so that the data can be used meaningfully.

Improved treatment planning for boron neutron capture therapy for glioblastoma multiforme using fluorine-18 labeled boronophenylalanine and positron emission tomography

Boron neutron capture therapy (BNCT) is a cancer brachytherapy based upon the thermal neutron reaction: 10B(n,alpha)7Li. The efficacy of the treatment depends primarily upon two conditions being met: (a) the preferential concentration of a boronated compound in the neoplasm and (b) an adequate fluence of thermal neutrons delivered to the neoplasm. The boronated amino acid, para-boronophenylalanine (BPA), is the agent widely used in clinical trials to deliver 10B to the malignancy. Positron emission tomography (PET) can be used to generate in vivo boron distribution maps by labeling BPA with the positron emitting nuclide fluorine-18. The incorporation of the PET-derived boron distribution maps into current treatment planning protocols is shown to provide improved treatment plans. Using previously established protocols, six patients with glioblastoma had 18BPA PET scans. The PET distribution maps obtained were used in the conventional BNCT treatment codes. The isodose curves derived from the PET data are shown to differ both qualitatively and quantitatively from the conventional isodose curves that were derived from calculations based upon the assumption of uniform uptake of the pharmaceutical in tumor and normal brain regions. The clinical course of each of the patients who eventually received BNCT (five of the six patients) was compared using both sets of isodose calculations. The isodose contours based upon PET derived distribution data appear to be more consistent with the patients' clinical course.

Microdosimetry model for boron neutron capture therapy: II. Theoretical estimation of the effectiveness function and surviving fractions

A model has been developed to obtain a better understanding of the effects of boron neutron capture therapy (BNCT) on a cellular scale. This model, the microdosimetry model MICOR, has been developed to include all reactions important for BNCT. To make the model more powerful in the translation from energy deposition to biological effect, it has been designed to be capable of calculating the effectiveness function. Based on this function, the model can calculate surviving fractions, RBE values and boron concentration distributions. MICOR has been used to analyze an extensive set of biological experiments performed at the HB11 beam in Petten. For V79 Chinese hamster cells, the effectiveness function is determined and used to generate surviving fractions. These fractions are compared with measured surviving fractions, which results in a good agreement between the measured and calculated surviving fractions (within the uncertainties of the measurements).

Microdosimetry model for boron neutron capture therapy: I. Determination of microscopic quantities of heavy particles on a cellular scale

Due to the limitations of existing microdosimetry models, a new model called MICOR has been developed to analyze the spatial distribution of microscopic energy deposition for boron neutron capture therapy (BNCT). As in most existing models, the reactions independent of the incident neutron energy such as the boron and the nitrogen capture reactions can be considered. While other models do not include reactions that are dependent on the neutron energy such as the proton recoil reaction, the present model is designed so that the energy deposition resulting from these reactions is included. The model MICOR has been extended to enable the determination of the biological effects of BNCT, which cannot be done with the existing models. The present paper describes the determination of several microscopic quantities such as the number of hits, the energy deposition in the cell nucleus, and the distribution of lineal and specific energy deposition. The companion paper (Radiat. Res. 155, 000-000 2001) deals with the conversion of these microscopic quantities into biological effects. The model is used to analyze the results of a radiobiological experiment performed at the HB11 facility in the HFR in Petten. This analysis shows the value of the model in determining the dose depositions on a cellular scale and the importance of the extension to the energy deposition of the proton recoil.

Boron neutron capture therapy of brain tumors: biodistribution, pharmacokinetics, and radiation dosimetry sodium borocaptate in patients with gliomas

OBJECTIVE

The purpose of this study was to obtain tumor and normal brain tissue biodistribution data and pharmacokinetic profiles for sodium borocaptate (Na2B12H11SH) (BSH), a drug that has been used clinically in Europe and Japan for boron neutron capture therapy of brain tumors. The study was performed with a group of 25 patients who had preoperative diagnoses of either glioblastoma multiforme (GBM) or anaplastic astrocytoma (AA) and were candidates for debulking surgery. Nineteen of these patients were subsequently shown to have histopathologically confirmed diagnoses of GBM or AA, and they constituted the study population.

METHODS

BSH (non-10B-enriched) was infused intravenously, in a 1-hour period, at doses of 15, 25, and 50 mg boron/kg body weight (corresponding to 26.5, 44.1, and 88.2 mg BSH/kg body weight, respectively) to groups of 3, 3, and 13 patients, respectively. Multiple samples of tumor tissue, brain tissue around the tumors, and normal brain tissue were obtained at either 3 to 7 or 13 to 15 hours after infusion. Blood samples for pharmacokinetic studies were obtained at times up to 120 hours after termination of the infusion. Sixteen of the patients underwent surgery at the Beijing Neurosurgical Institute and three at The Ohio State University, where all tissue samples were subsequently analyzed for boron content by direct current plasma-atomic emission spectroscopy.

RESULTS

Blood boron values peaked at the end of the infusion and then decreased triexponentially during the 120-hour sampling period. At 6 hours after termination of the infusion, these values had decreased to 20.8, 29.1, and 62.6 microg/ml for boron doses of 15, 25, and 50 mg/kg body weight, respectively. For a boron dose of 50 mg/kg body weight, the maximum (mean +/- standard deviation) solid tumor boron values at 3 to 7 hours after infusion were 17.1+/-5.8 and 17.3+/-10.1 microg/g for GBMs and AAs, respectively, and the mean tumor value averaged across all samples was 11.9 microg/g for both GBMs and AAs. In contrast, the mean normal brain tissue values, averaged across all samples, were 4.6+/-5.1 and 5.5+/-3.9 microg/g and the tumor/normal brain tissue ratios were3.8 and 3.2 for patients with GBMs and AAs, respectively. The large standard deviations indicated significant heterogeneity in uptake in both tumor and normal brain tissue. Regions histopathologically classified either as a mixture of tumor and normal brain tissue or as infiltrating tumor exhibited slightly lower boron concentrations than those designated as solid tumor. After a dose of 50 mg/kg body weight, boron concentrations in blood decreased from 104 microg/ml at 2 hours to 63 microg/ml at 6 hours and concentrations in skin and muscle were 43.1 and 39.2 microg/g, respectively, during the 3- to 7-hour sampling period.

CONCLUSION

When tumor, blood, and normal tissue boron concentrations were taken into account, the most favorable tumor uptake data were obtained with a boron dose of 25 mg/kg body weight, 3 to 7 hours after termination of the infusion. Although blood boron levels were high, normal brain tissue boron levels were almost always lower than tumor levels. However, tumor boron concentrations were less than those necessary for boron neutron capture therapy, and there was significant intratumoral and interpatient variability in the uptake of BSH, which would make estimation of the radiation dose delivered to the tumor very difficult. It is unlikely that intravenous administration of a single dose of BSH would result in therapeutically useful levels of boron. However, combining BSH with boronophenylalanine, the other compound that has been used clinically, and optimizing their delivery could increase tumor boron uptake and potentially improve the efficacy of boron neutron capture therapy.

Computational dosimetry and treatment planning for boron neutron capture therapy

The technology for computational dosimetry and treatment planning for Boron Neutron Capture Therapy (BNCT) has advanced significantly over the past few years. Because of the more complex nature of the problem, the computational methods that work well for treatment planning in photon radiotherapy are not applicable to BNCT. The necessary methods have, however, been developed and have been successfully employed both for research applications as well as human trials, although further improvements in speed are needed for routine clinical applications. Computational geometry for BNCT applications can be constructed directly from tomographic medical imagery and computed radiation dose distributions can be readily displayed in formats that are familiar to the radiotherapy community.