1 .5.1 .01

The epithermal neutron irradiation station for boron neutron capture therapy(BNCT) at the FiR I in Otaniemi

Iiro Auterinen and Pekka HiismiikiVTT (Technical Research Centne of Finland) Chemical Technology

P.O. Box 1404, FIN-02044VT1, Finland

Abstracfi The optimal neutron field for BNCT wouldhave only neutrons of the epithermal energy range andwould be free of y-radiation. Such a field penetratesseveral centimeters into the tissue but does not causedamage so much as higher energy neutrons would. Inthe depth thermalized neutrons are then captured bythe r0B that has accumulated into the tumor. Anepithermal neutron irradiation station has beendesigned and is under construction at the FiR I nuclearresearch reactor in Espoo, Otaniemi. The epithermalfield is formed by replacing the graphite in the existingthermal neutron irradiation column with a specially forthis purpose developed new composite material, amixture of aluminum and aluminum fluoride [11. Theperformance of the field, as well as the layout of theirradiation station and auxiliary spaces are reported.

INTRODUCTION

Boron neutron capture therapy (BNCT) is perhaps oneof the most complex cancer therapeutic modalities, and itsultimate success is dependent on the question howadequate concentrations of boron and neuffons can bedelivered to the tumor [2]. In 1990 the idea of a possibleclinical realization on BNCT in Finland was brought up bythe medical radio isotope group at V-ff [3]. As the primaryrequirement the suitability of the FiR I nuclear reactor ,

located at VTT close to Helsinki University CentralHospital, was evaluated and it was found out to have therequired potential. The plans to modify the thermal columnof the reactor stårted and at the same time clinical researchon boron carriers and later research on dosimetry wereinitiated [3]. The basic design work was done by VTI andnow the construction of the BNCT station has started atVTT under contract with the Radtek Inc., Espoo. Radtek isa company formed to combine private capital and statetechnology development funding (TEKES, Sitra) for thispurpose.

OPNMAL NEUTRON SOURCEFOR BNCT

For an efficient boron neutron capture treatment anhigh enough thermal neutron field has to be produced atthe tumor location. The requirement depends on the l0Bconcentration and with the present day boron carriersoperating at below 100 ppm level a total fluence of 1012nlcm2 is needed giving a flux requirement of the order of109 n/cm2ls to keep the treatrnent times tolerable. Until1994 thermal neutron irradiation was used for patienttreatment both in USA and Japan. But thermal neutronirradiation produces a field that peaks very strongly at thesurface and thus does not cover tumor parts deeper in thetissue.

The optimal neutrqr field fa BNCT nould have anly

neutrons of the epithermal energy range. Such a fieldpenetrates several centimeters into the tissue but does notcause damage so much as higher energy neutrons would.In the depth thermalized neutrons are then captured by thel0B that has accumulated into the tumor. The gamma doselevel in the irradiation field has to be well below the levelof the thermal neutron induced gamma from the irradiationvolume.

FiR 1 REACTOR AS NEUTRON SOURCE

The Finnish Research Reactor I (FiR 1) operated by theTechnical Research Center of Finland is a 250 kW TRIGAII pool reactor with a graphite reflector and a core loading

of 3 kg 235IJ12gv,tVo) in the special TRIGA uranium-zirco-nium hydride fuel (8-12 wVo U, 9IVo Zr, 17o Ft). Theadvantages of the TRIGA design for BNCT include a widecore face area and a wide spatial angle covered by thethermal-epithermal column system, large flux-per-Wattfeature and inherent safety of the TRIGA fuel. The reactorwhich was originally build in 1962 is in good shape, forexample the instrumentation was completely renewed in 1981

GEOMETRICAL DESIGN OF TIIE BEAM

Our basic idea in producing an epithermal neutron freldsuitable for BNCT is to fill the thermal column spacestarting from the very bottom with a suitable moderatingmaterial (Fig. l). By placing the wide moderator closer tothe reactor core more neutrons are catched into it. Theseneutrons have a wide energy_ spectrum which is thencompacted to the epithermal range due to collisions andcapture reactions in the moderator. The bismuth shieldpasses through neutrons but attenuates efficiently thegammas originating from the reactor core and neutronactivated structures. The irradiation freld produced by thisgeometry is highly isotropic. The design tårget has beenirradiation of glioma. To restrict the neutron field to thetarget volume we propose the use of a limiter block, anirradiåtion "helmet" of lithiated polyethylene with thetffget (head) inside and an orifice limiting the neutronheld. The polyethylene thermalizes efficiently neutronstryrng to penetrate it and the lithium (3l%o(vol) LiDabsorbs them. The hydrogen capture rate in thepolyethylene is so low that these gammas are surpassed bythe capture gammas from the target volume. Around thebeam port there are detectors for on line monitoring of theneutron and gamma fields.

AN OPTIIvIJZED NEUTRON MODERATORMATERIAL

Based on extensive numerical modeling a new optimalneutron moderator material composition was developed forepithermal neutron field generation[4]. The optimizationwas done for high epithermal flux combined with low fastneutron dose both at the body surface as well as at the peakof thermal neutron doses deeper in the tissues. The

Medical& Biological Engineering & Computing Vol.34, Supplement 1, Part 1, 1996The 1Oth Nordic-Baltic Conference on Biomedical Engineering, June 9-13, 1996, Tampere, Finland 299

patented material is a composition of AlF3 (697om) andaluminum(3U%om) and LiF (l%om) tll. A manufacruringprocess based on the hot isostatic pressing (HIP) techniquehas been developed for this material producing solid blockswith practically IOOVo density. The manufacturing of theneeded moderator material will be completed in February1996.

PERFORMANCE OF TI{E FIELD

Calculational modeling has shown that with theproposed design an epithermal irradiation field is producedsuitable for BNCT of human glioblastoma patients, evenwith deep seated and widely spread tumors. The beamquality, measured by comparing the fast neutron dose atthe surface and at the peak dose point to the boron capturedose, is in between that of the BMRR and that of theproposed design for the GTRR, and five to seven timesbetter than at the Petten HFR. The boron dose rate is morethan double compared to the Petten HFR and would equalthe GTRR at 0.7 MW power and BMRR ar 1.3 MW. Wirh30 ppm BSH the maximum healthy tissue tolerance limited(15 RBE.Gy) treatment rime is about 1.5 h.

CONSTRUCTION PHASES AND EXPERIMENTALVERIFICATION PROGRAM

In the frrst construction phase enough space will beallocated for phantom and animal experiments. Theexisting thermal column space elongated with additionalconcrete walls around the heavy concrete sliding door willbe suffrcient. The aim of the first phase is to eståblish withdosimetric measurements and healthy tissue tolerancestudies the suitability of the beam for human patienttreatments. The first phase will also include boron carriertest with animals. In the second phase an irradiation spacefor human patients will be constructed.

REFERENCES

tll P. Hiismäki, I. Auterinen, Patent FI-92890, (1995).

t2l R. F. Barth, A. H. Soloway, R. G. Fairchild and R. M.Brugger, "Boron neutron capture therapy for cancer,realities and prospects," Cancer vol 70, pp. 2995-3N7,1992.

t3l P. Hiismåiki, I. Auterinen, M. Fffkkilä, "The FinnishBNCT program, an overview," in Proceedings of theCUNCT BNCT Workshop. Helsinki: Helsinki University ofTechnology series TKK-F-A7 18, 1994 , pp. 24 .

t4l I. Auterinen, P. Hiismåiki, "Design of an epithermalneuEon beam for the TRIGA reactor in Otaniemi," inProceedings of the CUNCT BNCT Workshop. Helsinki:University of Technology series TKK-F-A718, 1994, pp.14-24.

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Medical & Biological Engineering & Computing Vol. 34, Supplement 1, Part 1, 1996The 1Oth Nordic-Baltic Conference on Biomedical Engineering, June 9-13, 1996, Tampere, Finland300