ABSTRACT

In Boron Neutron Capture Therapy, neutron and gamma dose determination is of prime importance. The neutron and gamma absorbed doses were measured in a cylindrical head phantom, 18cm in diameter and 20cm in height. For this purpose gold foils (bare and cadmium covered) and TLD-700 were used for neutron flux and gamma dose measurements respectively. The contributions to absorbed dose were due to ¹⁰B(n,a)⁷Li, ¹H(n,g)²D, ¹⁴N(n,p)¹⁴C and recoil proton reactions. The KERMA factors for different energy groups were used to convert the neutron flux to the relevant absorbed dose. There was good agreement between experimental values and the calculated dose.

  Keywords:       Boron Neutron Capture Therapy, Phantom, TLD, Dose components, Spectrum unfolding, Neutron Filter.

INTRODUCTION

Pioneering work of boron neutron capture therapy (BNCT) as a  potential, clinical technique to destroy malignant tumors - especially glioblastoma- is usually attributed to two individuals:  William H. Sweet and Hiroshi Hatanaka. Their work started in the early 1950s. In this technique, a neutron beam from a research reactor core passing through a proper moderator and a filter reduce the core neutron energy to epithermal range and a collimator converts them to an epithermal neutron beam. Prior to neutron bombardment,  the patient is administered a boron-10 compound such as BSH (Na₁₂B₁₂ H₁₁ SH). Then the epithermal neutron beam impinges on the patient head. The epithermal neutron beam is normally used for deep-seated brain tumors since the epithermal neutrons are more or less slowed down and turn to thermal neutrons before reaching the tumor.  It is worth mentioning that, because of its anti-body properties, BSH overwhelms the tumor.  BSH is a stable and non-toxic compound.¹ Therefore it can be applied without hesitation. The neutrons during passage through the brain undergo several reactions including ¹⁰B(n,a)⁷Li, ¹H(n,g)²D, ¹⁴N(n,p)¹⁴C and elastic scattering from protons. Because the cross section for ¹⁰B(n,a)⁷Li reaction is rather high,3838 barns, Boron is very suitable for tumor therapy. In this reaction 2.79 MeV is released in the  form of kinetic energy of two ions, ⁷Li and ⁴He,  in a proportion reciprocal to their atomic masses. It should be pointed out that in 94% of reactions, a gamma ray with 478KeV is emitted. The energy of the alpha particles released in this reaction is enough to travel about 10mm in tissue. Therefore, each alpha particle because of its high LET can destroy a cancer cell. Because of the high Boron-10 concentration in the tumor, 40-50ppm, as compared to 10ppm in other parts of the brain, the absorbed dose due to this reaction in the tumor is several times higher than that in normal tissue. This is enhanced by the  peak thermal neutron flux in the tumor. 

  Success in BNCT lends itself in precise dose determination and it plays a key role in this technique. Since direct dose measurement in a human head is not practical, a head phantom is normally used instead. Ordinary distilled water is a suitable material that has very similar properties to normal brain tissue with regard to neutron interaction and energy absorption.  Different shaped head phantoms namely cylindrical, elliptical or cubic are used. We used a cylindrically-shaped phantom, 18cm in diameter and 20cm high filled with distilled water. The measurements were carried out in the head phantom containing a small tumor-shaped cylinder containing boric acid, 50ppm B-10. The phantom was placed in front of the neutron beam tube of  the Tehran Research Reactor (TRR) in such a way as to receive the emerging epithermal neutrons from the collimator. The beam was directed at the small cylinder.  The gamma ray absorbed dose component due to ¹⁰B(n,a)⁷Li, ¹H(n,g)²D, and ¹⁴N(n,p)¹⁴C reactions and core gamma rays was measured using TLD-700 and the neutron flux was measured using gold foils. To convert neutron flux to absorbed dose the relevant KERMA factors were applied².

  MATERIALS AND METHODS

  In 1993 there was an attempt to investigate the possibility of using the BNCT technique at the Tehran Research Reactor³ .  A collimator and an Al-Fe filter were designed, constructed and installed in a beam tube of TRR, see Fig. 1. The filter was composed of 24 pieces of alternating aluminum and iron blocks, 5cm each. To stop core gamma rays from reaching the target, a 15cm bismuth block was placed behind the filter.  

Fig. 1  Collimator and Al-Fe filter detailed design

To determine the central beam hole axis, a radiographic cassette containing an x-ray film was placed in front of the beam hole, once with a gadolinium converter and second time without a converter. The irradiation period extended 24 hours. The x-ray film was scanned by a laser-scanner to determine the center of the neutron beam.

The leakage neutron spectrum was measured at the beam outlet using an He-3 counter and the unfolding technique^3,4. The spectrum from a bare Cf-252 neutron source, a distribution of well thermalized neutrons and the BNCT beam port neutron spectrum are shown in Fig. 2. 

The dose measurements were carried out in a cylindrical head phantom made of Plexiglas, filled with distilled water.  Bare and cadmium covered gold foils, 1cm diameter and 5mm, were used in three radial rows to measure thermal and epicadmium fluxes.  The irradiated gold foils were counted on a HPGe detector. The absolute net area under the prominent Au-198 photo-peak, 412Kev was determined utilizing a detector efficiency factor.    Using the following relations, thermal and epithermal neutron fluxes were determined.⁵

                                                       (1)

 

The KERMA factor for ith group, Fn (Ei) was calculated using the following relation⁸:

                                  F_n(Ei)  =  1.602*10^-8 sN m^-1E_tr                    (4)

      where

 

F_n (Ei)  =   KERMA factor as a function of neutron energy, in J/kg or Grey

s          =   neutron-isotope cross section reaction for specified neutron energy,b

N         =   Number of atoms in the sample

m         =   mass of sample, g

E_tr        =   total kinetic energy transferred to medium.

The incident epithermal neutrons on the human head phantom, undergo elastic scattering from hydrogen nuclei which may lead to recoil protons. As a result, the neutron spectrum extends to the thermal region as they travel towards the small cylinder tumor inside the phantom.

RESULTS AND DISCUSSIONS

  Table 2 and Fig. 4  give the total and the contribution of each component in building up RBE absorbed dose rate in the tumor along its central line. The result is in good agreement with those of Raaijimakers et.al.⁹. In spite of the fact that the neutron flux level is too low to be feasible in the clinical application. However, this facility can be utilized for in-vitro and in-vivo experiments. The results of the project, especially the neutron beam energy and dose distribution in tumor and surrounding tissues encourages us for future planning. The data obtained from this project has provided enough information to extend the project to practical application.

ACKNOWLEDGEMENT

Thanks are due to Tehran Research Reactor (TRR) operating staff in cooperating during implementation of the project and thanks to Neutron Physics Group, specially H.Ghods and H. Zandi, for assisting radiography and foil activation analysis measurements.