Relative biological effectiveness and tolerance dose of fission neutrons in canine skin for a potential combination of neutron capture therapy and fast-neutron therapy

To investigate the potential efficacy of fission neutrons from a fast-neutron reactor for the treatment of radioresistant tumors, the relative biological effectiveness (RBE) and tolerance dose of fission neutrons in canine skin were determined. The forelimbs of 34 healthy mongrel dogs received a single dose of fission neutrons (5.6, 6.8, 8.2, 9.6 or 11 Gy) or 137Cs gamma rays (10, 15, 20, 25 or 30 Gy). Based on observations of radiodermatitis for each radiation, the single-fraction RBE of fission neutrons in the sixth month was calculated as approximately 3. The tolerance doses of fission neutrons and gamma rays, defined as the highest doses giving no moist desquamation on the irradiated skin in the recovery phase, were estimated as 7.6 Gy and 20 Gy, respectively. The tolerance dose of 7.6 Gy of fission neutrons included 5.0 Gy of fast neutrons possessing high anti-tumor effects and 1.4 x 10(12) n/cm2 of thermal neutrons, which could be applicable to neutron capture therapy (NCT). The combination of fast-neutron therapy and NCT using a fast-neutron reactor might be useful for the treatment of radioresistant tumors.

Treatment of salivary gland neoplasms with fast neutron radiotherapy

OBJECTIVE

To evaluate the efficacy of fast neutron radiotherapy for the treatment of salivary gland neoplasms.

DESIGN

Retrospective analysis.

SETTING

University of Washington Cancer Center, Neutron Facility, Seattle.

PATIENTS

The medical records of 279 patients treated with curative intent using fast neutron radiotherapy at the University of Washington Cancer Center were reviewed. Of the 279 patients, 263 had evidence of gross residual disease at the time of treatment (16 had no evidence of gross residual disease), 141 had tumors of a major salivary gland, and 138 had tumors of minor salivary glands. The median follow-up period was 36 months (range, 1-142 months).

MAIN OUTCOME MEASURES

Local-regional control, cause-specific survival, and freedom from metastasis.

RESULTS

The 6-year actuarial cause-specific survival rate was 67%. Multivariate analysis revealed that low group stage (I-II) disease, minor salivary sites, lack of skull base invasion, and primary disease were associated with a statistically significant improvement in cause-specific survival. The 6-year actuarial local-regional control rate was 59%. Multivariate analysis revealed size 4 cm or smaller, lack of base of skull invasion, prior surgical resection, and no previous radiotherapy to have a statistically significant improved local-regional control. Sixteen patients without evidence of gross residual disease had a 100% 6-year actuarial local-regional control. The 6-year actuarial freedom from metastasis rate was 64%. Factors associated with decreased development of systemic metastases included negative lymph nodes at the time of treatment and lack of base of skull involvement. The 6-year actuarial rate of development of grade 3 or 4 long-term toxicity (using the Radiation Therapy Oncology Group and European Organization for Research on the Treatment of Cancer criteria) was 10%. No patient experienced grade 5 toxic effects.

CONCLUSIONS

Neuron radiotherapy is an effective treatment for patients with salivary gland neoplasms who have gross residual disease and achieves excellent local-regional control in patients without evidence of gross disease.

Wedge factor dependence with depth and field size for fast neutron beams

The dependence of the wedge factors (WFs) on field size (FS) and depth for a fast neutron beam has been investigated. In a previous study (Popescu et al 1999 Med. Phys. 26 541), a method was presented that allows a simple and accurate way of calculating the wedge-factor dependence on FS and depth in the case of a photon beam. The validity of a similar approach is tested in the present study for neutron beam dosimetry. The clinical neutron therapy system at the University of Washington (UW) has a flattening filter assembly consisting of two filters: a small field filter and a large field filter. Despite this complication, the approach presented in Popescu et al (1999 Med. Phys. 26 541) can be used to describe the WF dependence on FS and depth (d).

In vivo gadolinium neutron capture therapy using a potentially effective compound (Gd-BOPTA)

BACKGROUND

In a previous study, we found that Gadobenate dimeglumine (Gd-BOPTA) resulted in a significantly greater Gd uptake by brain tumor tissue than Gadopentate dimeglumine (Gd-DTPA). Therefore, we investigated whether Gd-BOPTA is an efficient agent for neutron capture therapy (NCT).

MATERIALS AND METHODS

Four groups of Fisher344 rats (control, neutron (n), n+ Gd-DTPA, n+ Gd-BOPTA) were subcutaneously injected 9L gliosarcoma cells in both hind legs. Gd-BOPTA and Gd-DTPA (0.05 mmol/g tumor weight) were injected directly into the tumor. At the peak of Gd uptake, thermal neutron irradiation was applied.

RESULTS

Two Gd+ groups showed pronounced tumor growth delay as compared with the control and neutron groups (p = 0.0053, 0.0064, respectively). Furthermore, the BOPTA group showed significantly prolonged delay of tumor growth as compared to the DTPA group (p = 0.033).

CONCLUSION

This is the first report of Gd-NCT to demonstrate that Gd-BOPTA serve as an effective compound for NCT. Better cytocidal effects of Gd-BOPTA warrant further investigation of subcellular Gd distribution.

Fission reactor neutron sources for neutron capture therapy--a critical review

The status of fission reactor-based neutron beams for neutron capture therapy (NCT) is reviewed critically. Epithermal neutron beams, which are favored for treatment of deep-seated tumors, have been constructed or are under construction at a number of reactors worldwide. Some of the most recently constructed epithermal neutron beams approach the theoretical optimum for beam purity. Of these higher quality beams, at least one is suitable for use in high through-put routine therapy. It is concluded that reactor-based epithermal neutron beams with near optimum characteristics are currently available and more can be constructed at existing reactors. Suitable reactors include relatively low power reactors using the core directly as a source of neutrons or a fission converter if core neutrons are difficult to access. Thermal neutron beams for NCT studies with small animals or for shallow tumor treatments, with near optimum properties have been available at reactors for many years. Additional high quality thermal beams can also be constructed at existing reactors or at new, small reactors. Furthermore, it should be possible to design and construct new low power reactors specifically for NCT, which meet all requirements for routine therapy and which are based on proven and highly safe reactor technology.

Simulations of silicon microdosimetry measurements in fast neutron therapy

Experimental silicon microdosimetry measurements were performed at a Fast Neutron Therapy facility. Monte Carlo based calculations of these measurements were made using the GEANT4 toolkit. Reasonable agreement between theoretical and experimental results was obtained and the contribution of elastic and inelastic reaction products to the final microdosimetric spectrum was determined.

Engineering tumor-targeted gadolinium hexanedione nanoparticles for potential application in neutron capture therapy

Microemulsions (oil-in-water) have been employed as templates to engineer nanoparticles containing high concentrations of gadolinium for potential application in neutron capture therapy of tumors. Gadolinium hexanedione (GdH), synthesized by complexation of Gd(3+) with 2,4-hexanedione, was used as the nanoparticle matrix alone or in combination with either emulsifying wax or PEG-400 monostearate. Solid nanoparticles (<125 nm size) were obtained by simple cooling of the microemulsions prepared at 60 degrees C to room temperature in one vessel. The feasibility of tumor targeting via folate receptors was studied. A folate ligand was synthesized by chemically linking folic acid to distearoylphosphatidylethanolamine (DSPE) via a poly(ethylene glycol) (PEG; MW 3350) spacer. To obtain folate-coated nanoparticles, the folate ligand (0.75% w/w to 15% w/w) was added to either the microemulsion templates at 60 degrees C or nanoparticle suspensions at 25 degrees C. Efficiencies of folate ligand attachment/adsorption to nanoparticle formulations were monitored by gel permeation chromatography. Cell uptake studies were carried out in KB cells (human nasopharyngeal epidermal carcinoma cell line), known to overexpress folate receptors. The uptake of folate-coated nanoparticles was about 10-fold higher than uncoated nanoparticles after 30 min at 37 degrees C. The uptake of folate-coated nanoparticles at 4 degrees C was 20-fold lower than the uptake at 37 degrees C and comparable to the uptake of uncoated nanoparticles at 37 degrees C. Folate-mediated endocytosis was further verified by the inhibition of folate-coated nanoparticles uptake by free folic acid. It was observed that folate-coated nanoparticles uptake decreased to approximately 2% of its initial value with the coincubation of 0.001 mM of free folic acid. The results suggested that these tumor-targeted nanoparticles containing high concentrations of Gd may have potential for neutron capture therapy.

Controllability of depth dose distribution for neutron capture therapy at the Heavy Water Neutron Irradiation Facility of Kyoto University Research Reactor

The updating construction of the Heavy Water Neutron Irradiation Facility of the Kyoto University Research Reactor has been performed from November 1995 to March 1996 mainly for the improvement in neutron capture therapy. On the performance, the neutron irradiation modes with the variable energy spectra from almost pure thermal to epi-thermal neutrons became available by the control of the heavy-water thickness in the spectrum shifter and by the open-and-close of the cadmium and boral thermal neutron filters. The depth distributions of thermal, epi-thermal and fast neutron fluxes were measured by activation method using gold and indium, and the depth distributions of gamma-ray absorbed dose rate were measured using thermo-luminescent dosimeter of beryllium oxide for the several irradiation modes. From these measured data, the controllability of the depth dose distribution using the spectrum shifter and the thermal neutron filters was confirmed.

Folate receptor-mediated liposomal delivery of a lipophilic boron agent to tumor cells in vitro for neutron capture therapy

PURPOSE

This study was aimed at the in vitro evaluations of folate receptor (FR)-targeted liposomes as carriers for a lipophilic boron agent, K[nido-7-CH3(CH2)15-7,8-C2B9H11, in FR-overexpressing tumor cells for neutron capture therapy.

METHODS

Large unilamellar vesicles (-200 nm in diameter) were prepared with the composition of egg PC/chol/K[nido-7-CH3(CH2)15-7,8-C2B9H11] (2:2:1, mol/mol), with an additional 0.5 mol % of folate-PEG-DSPE or PEG-DSPE added for the FR-targeted or nontargeted liposomal formulations, respectively.

RESULTS

Boron-containing, FR-targeted liposomes readily bound to KB cells, an FR-overexpressing cell line, and were internalized via FR-mediated endocytosis. The boron uptake in cells treated with these liposomes was approximately 10 times greater compared with those treated with control liposomes. In contrast, FR-targeted and nontargeted liposomes showed no difference in boron delivery efficiency in F98 cells, which do not express the FR. The subcellular distribution of the boron compound in KB cells treated with the FR-targeted liposomes was investigated by cellular fractionation experiments, which showed that most of the boron compound was found in either the cytosol/endosomal or cell membrane fractions, indicating efficient internalization of the liposomal boron.

CONCLUSION

FR-targeted liposomes incorporating the lipophilic boron agent, K[nido-7-CH3(CH2)15-7,8-C2B9H11], into its bilayer were capable of specific receptor binding and receptor-mediated endocytosis in cultured KB cells. Such liposomes warrant further investigations for use in neutron capture therapy.

The medical-irradiation characteristics for neutron capture therapy at the Heavy Water Neutron Irradiation Facility of Kyoto University Research Reactor

At the Heavy Water Neutron Irradiation Facility of the Kyoto University Research Reactor, the mix irradiation of thermal and epi-thermal neutrons, and the solo irradiation of epi-thermal neutrons are available additionally to the thermal neutron irradiation, and then the neutron capture therapy (NCT) at this facility became more flexible, after the update in 1996. The estimation of the depth dose distributions in NCT clinical irradiation, were performed for the standard irradiation modes of thermal, mixed and epi-thermal neutrons, from the both sides of experiment and calculation. On the assumption that the 10B concentration in tumor part was 40 ppm and the ratio of tumor to normal tissue was 3.5, the advantage depth were estimated to 5.4, 6.0, and 8.0, for the respective standard irradiation modes. It was confirmed that the various irradiation conditions can be selected according to the target-volume conditions, such as size, depth, etc. Besides, in the viewpoint of the radiation shielding for patient, it was confirmed that the whole-body exposure is effectively reduced by the new clinical collimators, compared with the old one.

Treatment planning and dosimetry for the Harvard-MIT Phase I clinical trial of cranial neutron capture therapy

PURPOSE

A Phase I trial of cranial neutron capture therapy (NCT) was conducted at Harvard-MIT. The trial was designed to determine maximum tolerated NCT radiation dose to normal brain.

METHODS AND MATERIALS

Twenty-two patients with brain tumors were treated by infusion of boronophenylalanine-fructose (BPA-f) followed by exposure to epithermal neutrons. The study began with a prescribed biologically weighted dose of 8.8 RBE (relative biologic effectiveness) Gy, escalated in compounding 10% increments, and ended at 14.2 RBE Gy. BPA-f was infused at a dose 250-350 mg/kg body weight. Treatments were planned using MacNCTPlan and MCNP 4B. Irradiations were delivered as one, two, or three fields in one or two fractions.

RESULTS

Peak biologically weighted normal tissue dose ranged from 8.7 to 16.4 RBE Gy. The average dose to brain ranged from 2.7 to 7.4 RBE Gy. Average tumor dose was estimated to range from 14.5 to 43.9 RBE Gy, with a mean of 25.7 RBE Gy.

CONCLUSIONS

We have demonstrated that BPA-f-mediated NCT can be precisely planned and delivered in a carefully controlled manner. Subsequent clinical trials of boron neutron capture therapy at Harvard and MIT will be initiated with a new high-intensity, high-quality epithermal neutron beam.

Reference dosimetry calculations for neutron capture therapy with comparison of analytical and voxel models

As clinical trials of Neutron Capture Therapy (NCT) are initiated in the U.S. and other countries, new treatment planning codes are being developed to calculate detailed dose distributions in patient-specific models. The thorough evaluation and comparison of treatment planning codes is a critical step toward the eventual standardization of dosimetry, which, in turn, is an essential element for the rational comparison of clinical results from different institutions. In this paper we report development of a reference suite of computational test problems for NCT dosimetry and discuss common issues encountered in these calculations to facilitate quantitative evaluations and comparisons of NCT treatment planning codes. Specifically, detailed depth-kerma rate curves were calculated using the Monte Carlo radiation transport code MCNP4B for four different representations of the modified Snyder head phantom, an analytic, multishell, ellipsoidal model, and voxel representations of this model with cubic voxel sizes of 16, 8, and 4 mm. Monoenergetic and monodirectional beams of 0.0253 eV, 1, 2, 10, 100, and 1000 keV neutrons, and 0.2, 0.5, 1, 2, 5, and 10 MeV photons were individually simulated to calculate kerma rates to a statistical uncertainty of <1% (1 std. dev.) in the center of the head model. In addition, a "generic" epithermal neutron beam with a broad neutron spectrum, similar to epithermal beams currently used or proposed for NCT clinical trials, was computed for all models. The thermal neutron, fast neutron, and photon kerma rates calculated with the 4 and 8 mm voxel models were within 2% and 4%, respectively, of those calculated for the analytical model. The 16 mm voxel model produced unacceptably large discrepancies for all dose components. The effects from different kerma data sets and tissue compositions were evaluated. Updating the kerma data from ICRU 46 to ICRU 63 data produced less than 2% difference in kerma rate profiles. The depth-dose profile data, Monte Carlo code input, kerma factors, and model construction files are available electronically to aid in verifying new and existing NCT treatment planning codes.

Radiation therapy through activation of stable nuclides

In an investigation by the Swedish Cancer Society, an expert group described the present status, critical issues and future aspects and potentials for each of nine major areas of radiation therapy research. This report deals with radiation therapy using activation of stable nuclides.

In vitro cellular accumulation of gadolinium incorporated into chitosan nanoparticles designed for neutron-capture therapy of cancer

The accumulation of gadolinium loaded as gadopentetic acid (Gd-DTPA) in chitosan nanoparticles (Gd-nanoCPs), which were designed for gadolinium neutron-capture therapy (Gd-NCT) for cancer, was evaluated in vitro in cultured cells. Using L929 fibroblast cells, the Gd accumulation for 12 h at 37 degrees C was investigated at Gd concentrations lower than 40 ppm. The accumulation leveled above 20 ppm and reached 18.0+/-2.7 (mean+/-S.D.) microg Gd/10(6) cells at 40 ppm. Furthermore, the corresponding accumulations in B16F10 melanoma cells and SCC-VII squamous cell carcinoma, which were used in the previous Gd-NCT trials in vivo, were 27.1+/-2.9 and 59.8+/-9.8 microg Gd/10(6) cells, respectively, hence explaining the superior growth-suppression in the in vivo trials using SCC-VII cells. The accumulation of Gd-nanoCPs in these cells was 100-200 times higher in comparison to dimeglumine gadopentetate aqueous solution (Magnevist), a magnetic resonance imaging contrast agent. The endocytic uptake of Gd-nanoCPs, strongly holding Gd-DTPA, was suggested from transmission electron microscopy and comparative studies at 4 degrees C and with the solution system. These findings indicated that Gd-nanoCPs had a high affinity to the cells, probably contributing to the long retention of Gd in tumor tissue and leading to the significant suppression of tumor growth in the in vivo studies that were previously reported.

Dose distributions in a human head phantom for neutron capture therapy using moderated neutrons from the 2.5 meV proton-7Li reaction or from fission of 235U

The feasibility of neutron capture therapy (NCT) using an accelerator-based neutron source of the 7Li(p,n) reaction produced by 2.5 MeV protons was investigated by comparing the neutron beam tailored by both the Hiroshima University radiological research accelerator (HIRRAC) and the heavy water neutron irradiation facility in the Kyoto University reactor (KUR-HWNIF) from the viewpoint of the contamination dose ratios of the fast neutrons and the gamma rays. These contamination ratios to the boron dose were estimated in a water phantom of 20 cm diameter and 20 cm length to simulate a human head, with experiments by the same techniques for NCT in KUR-HWNIF and/or the simulation calculations by the Monte Carlo N-particle transport code system version 4B (MCNP-4B). It was found that the 7Li(p,n) neutrons produced by 2.5 MeV protons combined with 20, 25 or 30 cm thick D20 moderators of 20 cm diameter could make irradiation fields for NCT with depth-dose characteristics similar to those from the epithermal neutron beam at the KUR-HWNIF.

Experimental verification of improved depth-dose distribution using hyper-thermal neutron incidence in neutron capture therapy

We have proposed the utilization of 'hyper-thermal neutrons' for neutron capture therapy (NCT) from the viewpoint of the improvement in the dose distribution in a human body. In order to verify the improved depth-dose distribution due to hyper-thermal neutron incidence, two experiments were carried out using a test-type hyper-thermal neutron generator at a thermal neutron irradiation field in Kyoto University Reactor (KUR), which is actually utilized for NCT clinical irradiation. From the free-in-air experiment for the spectrum-shift characteristics, it was confirmed that the hyper-thermal neutrons of approximately 860 K at maximum could be obtained by the generator. From the phantom experiment, the improvement effect and the controllability for the depth-dose distribution were confirmed. For example, it was found that the relative neutron depth-dose distribution was about 1 cm improved with the 860 K hyper-thermal neutron incidence, compared to the normal thermal neutron incidence.

Enhancement of a 252Cf-based neutron beam via subcritical multiplication for neutron capture therapy

Previous studies indicated that an epithermal-neutron beam based on bare 252Cf is not feasible for neutron capture therapy (NCT). It was reported that a clinically useful epithermal-neutron beam requires a minimum of 1.0 g of 252Cf, which is more than twice the US current annual supply. However, it was reasoned that the required quantity of 252Cf could be dramatically reduced when used with a subcritical multiplying assembly (SMA). This reasoning is based on the assumption that the epithermal-neutron beam intensity for NCT is directly proportional to the fission neutron population, and that the neutron multiplying factor of the SMA can be estimated by 1/(1 - k(eff)). We have performed detailed Monte Carlo calculations to investigate the validity of the above reasoning. Our results show that 1/(1 - k(eff)) grossly overestimates the beam enhancement factor for NCT. For example, Monte Carlo calculations predict a beam enhancement factor of 6.0 for an optimized SMA geometry with k(eff) = 0.968. This factor is much less than 31 predicted by 1/(1 - k(eff)). The overestimation is due to the fact that most of the neutrons produced in the SMA are self-shielded, whereas self-shielding is negligible in a bare 252Cf source. Since the beam intensity of a 0.1 g 252Cf with the optimized SMA enhancement is still more than an order of magnitude too low compared to the existing reactor beams, we conclude that the enhancement via an SMA for a 252Cf-based epithermal-neutron beam is inadequate for NCT.

Measurements and calculations of thermal neutron fluence rate and neutron energy spectra resulting from moderation of 252Cf fast neutrons: applications for neutron capture therapy

252Cf is a neutron emitting radioisotope which has promise for both standard brachytherapy and neutron capture enhanced brachytherapy. In this study, experimental measurements and calculations were used to determine the thermal neutron fluence rate, phi(th) [n cm(-2) s(-1) mg(-1)], in the vicinity of 252Cf applicator tube (AT) type sources. Results of these measurements were confirmed with Monte Carlo calculations performed in a distributed manner on multiple workstations using MCNP. Three studies were executed: (1) relative phi(th) as a function of distance from a 252Cf AT source in an A-150 tissue equivalent plastic phantom using thermoluminescent dosimeters (TLDs) of varying 6Li/Li enrichment, (2) phi(th) measured with gold foils in a 114 liter water phantom 5 cm from two 252Cf AT sources, and (3) calculations of the impact of phantom material composition (e.g., A-150, water, brain, muscle) on phi(th) from moderated 252Cf fast neutrons. TLD results and Monte Carlo calculations in A-150 of relative phi(th) typically agreed within 1% and at most differed by 3% for distances from 1 to 6 cm. Foil measurements followed the ASTM E 262-86e protocol, and the ratio of activated plain and Cd encased gold foils (7.31) agreed well with the calculated ratio (7.26). Measured phi(th) at 5 cm (1.70+/-0.10 x 10(7) n cm(-2) s(-1) mg(-1)) was 10% greater than that determined using MCNP (1.55+/-0.12 x 10(7) n cm(-2) s(-1) mg(-1)), but was within the combined uncertainties. Compared with A-150 at a distance of 1 cm, phi(th) was 20%, 22%, and 32% less for water, brain, and muscle, respectively; these ratios decreased to 16%, 16%, and 24% less, respectively, at a distance of 5 cm from the source in a 15 cm diameter phantom. Comparisons of these results generally agreed with those in the literature for a value of 2 x 10(7) n cm(-2) s(-1) mg(-1) in water at 3 cm.

Comparison of radiation effects of gadolinium and boron neutron capture reactions

PURPOSE

Cell survival assays were performed to evaluate the effects of radiations released during neutron capture reactions by gadolinium-157, boron-10 and by the combination of both.

MATERIALS AND METHODS

Single cell suspensions with or without Gd-157 and/or B-10 were exposed to thermal neutrons produced by the Kyoto University reactor, and standard cell survival curves were obtained.

RESULTS

Under the same molarity, cytocidal effects were 1.5 times greater for Gd-157 than for boron when compared at 10% survival levels. The presence of B-10 enhanced the radiation effect of Gd-157 neutron capture by 1.2-fold, suggesting that cells were not sufficiently irradiated as a result of neutron fluency attenuation by the presence of excess neutron capture agents in the medium.

CONCLUSIONS

When an equal number of atoms were present, Gd-157 was effective as B-10 when exposed to an equal number of thermal neutrons. However, there was no benefit observed in the combination of Gd-157 and B-10 for neutron capture therapy. Further studies are needed to determine optimal Gd-157 and B-10 concentrations as a function of tumor dimension.

Measurements of low-energy (d,n) reactions for BNCT. Boron Neutron Capture Therapy

Neutron yields and energy spectra have been measured for various deuteron-induced reactions at low energy. Neutrons of energy > 100 keV emitted in the 9Be(d,n)10B, 12C(d,n)13N, and 13C(d,n)14N reactions at Ed= 1.5 MeV were detected at five angles by means of liquid scintillator detectors. While low-energy neutrons were observed in all studied reactions, only 13C(d,n)14N is characterized by a relatively large yield with spectral features potentially interesting for an accelerator-based neutron source for BNCT.

Gadolinium neutron capture therapy (GdNCT) of melanoma cells and solid tumors with the magnetic resonance imaging contrast agent Gadobutrol

RATIONALE AND OBJECTIVES

The therapeutic gain of neutron capture therapy with a neutral macrocyclic gadolinium (Gd) complex (Gadobutrol) was evaluated through in vitro and in vivo studies in a beam of low-energy neutrons.

METHODS

Neutron irradiation for both the in vitro and in vivo studies was performed in a beam of low-energy neutrons produced by the research reactor of the Hahn-Meitner-Institut, Berlin. Malignant melanoma cells of human origin were irradiated in the presence or absence of Gadobutrol. In vivo irradiation was performed on tumor-bearing nude mice. The tumor site was irradiated subsequent to intratumoral injection of Gadobutrol and compared with irradiation in the absence of the Gd complex.

RESULTS

In vitro studies showed a Gd-dependent delay of cell proliferation as a consequence of neutron irradiation. In animals, intratumoral administration of the Gd complex at a dose of 1.2 mmol Gd/kg before neutron irradiation results in a significant delay in tumor growth with respect to the control groups.

CONCLUSIONS

In vitro and in vivo studies showed a therapeutic benefit with the neutral Gd complex and suggest Gd-containing magnetic resonance contrast media are potential candidates for neutron capture therapy. The Gd dose used in the irradiation experiments was four times the presently accepted high dose in clinical magnetic resonance imaging.

Preparation of lecithin microcapsules by a dilution method using the Wurster process for intraarterial administration in gadolinium neutron capture therapy

Lecithin microcapsules containing gadolinium (Gd) were designed and prepared as a dosage form for intraarterial administration to accumulate Gd in tumors in neutron capture therapy. The microcapsules were composed of 1) a lactose core, 2) a layer of distearylamide of gadopentetic acid (Gd-DTPA-SAm) and polyvinylpyrrolidone (PVP) with or without soybean lecithin (SL) and 3) a membrane containing SL, cholesterol, stearic acid and PVP at three different compositions. A dilution method using the Wurster process was developed for small-scale preparation. In spite of using only 2 g of Gd-DTPA-SAm each, three types of microcapsules were obtained with a content of 24.9% as Gd-DTPA-SAm (3.66% as Gd) even at 150% coating level. The swelling type of microcapsules (MC-D1) did not release Gd at all for the entire 120 min of the experiment in a 0.9% saline solution. On the other hand, the rapid-erosion type (MC-D2) and the vesicle-dispersing type (MC-D3) released Gd with a lag time. The percent released depended on the coating level and the SL content in the Gd-fixing layer. A large number of droplet-like particles spouted out, and/or tubular vesicles formed with MC-D2 and MC-D3 in the saline solution. These phenomena implied that the water-insoluble Gd-DTPA-SAm would be entrapped in these particles/vesicles. When MC-D2 and MC-D3 were administered to normal rats via the hepatic artery, a Gd-accumulation as high as 70 and 71% of the injected dose was detected in the whole liver 2 h after administration. In addition, biochemical and histological evaluation of the liver after administration indicated that embolization of the microcapsules actually occurred in the blood vessels, and that necrosis induced by ischemia was not serious. These results suggested that administration of these microcapsules might be multiply repeated in order to accumulate the required amount of Gd in tumors.

A fundamental study on hyper-thermal neutrons for neutron capture therapy

The utilization of hyper-thermal neutrons, which have an energy spectrum with a Maxwellian distribution at a higher temperature than room temperature (300 K), was studied in order to improve the thermal neutron flux distribution at depth in a living body for neutron capture therapy. Simulation calculations were carried out using a Monte Carlo code 'MCNP-V3' in order to investigate the characteristics of hyper-thermal neutrons, i.e. (i) depth dependence of the neutron energy spectrum, and (ii) depth distribution of the reaction rate in a water phantom for materials with 1/v neutron absorption. It is confirmed that hyper-thermal neutron irradiation can improve the thermal neutron flux distribution in the deeper areas in a living body compared with thermal neutron irradiation. When hyper-thermal neutrons with a 3000 K Maxwellian distribution are incident on a body, the reaction rates of 1/v materials such as 14N, 10B etc are about twice that observed for incident thermal neutrons at 300 K, at a depth of 5 cm. The limit of the treatable depth for tumours having 30 ppm 10B is expected to be about 1.5 cm greater by utilizing hyper-thermal neutrons at 3000 K compared with the incidence of thermal neutrons at 300 K.