Boron Neutron Capture Therapy (BNCT) is a new radiation therapy. In BNCT, there exists some very critical problems that should be solved. One of the severest problems is that the treatment effect cannot be known during BNCT in real time. We are now developing a SPECT (single photon emission computed tomography) system (BNCT-SPECT), with a cadmium telluride (CdTe) semiconductor detector. BNCT-SPECT can obtain the BNCT treatment effect by measuring 478 keV gamma-rays emitted from the excited state of 7Li nucleus created by 10B(n,α) 7Li reaction. In the previous studies, we investigated the feasibility of the BNCT-SPECT system. As a result, the S/N ratio did not meet the criterion of S/N >1 because deterioration of the S/N ratio occurred caused by the influence of Compton scattering especially due to capture gamma-rays of hydrogen.
We thus produced an arrayed detector with two CdTe crystals to test cross talk phenomenon and to examine an anti-coincidence detection possibility. For more precise analysis for the anti-coincidence detection, we designed and made a collimator having a similar performance to the real BNCT-SPECT.
We carried out experiments with the collimator to examine the effect of cross talk of scattering gamma-rays between CdTe elements more practically. As a result of measurement the coincidence events were successfully extracted.
We are now planning to carry out evaluation of coincidence rate from the measurement and comparison of it with the numerical calculations.
Boron Neutron Capture Therapy (BNCT) is a new radiation therapy which attracts the whole world’s attention. This therapy can kill tumor cells by alpha particles(α) and lithium nuclei (7Li) emitted by the reaction of thermal neutron or epithermal neutron with boron (10B). In BNCT, there exists some very critical problems that should be solved. One is that the treatment effect cannot be known during BNCT in real time. Many researchers [
As indicated in
Precisely,10B(n,α) 7Li reaction utilized in BNCT is practically expressed by the next two nuclear reactions:
94% of 7Li is in a first excited state, i.e., 7Li*. 7Li* decays in its half-life of 10–14 seconds to emit a 478 keV gamma-ray via transition from the first excited state to the ground state.
If the intensity distribution of 478 keV gamma-rays could be measured three-dimensionally, we could obtain the three-dimensional distribution of 10B(n,α) 7Li reaction rate in the tumor. The result of the measurement can then be regarded as the treatment effect of BNCT. Emitted 478 keV gamma-rays are collimated by the collimator and measured by a lot of measuring devices in the array detector in order to make an image of gamma-ray intensity. The BNCT treatment effect (local tumor dose) can be estimated from the obtained three dimensional gamma-ray image.
BNCT-SPECT should be so designed that 478 keV gamma-rays have to be measured in a very high background field. The measurement thus has to be carried out carefully from a viewpoint of actual medical situation and site [
Considering the above condition, we selected a CdTe detector so as to detect gamma-rays of 478 keV. The reason is that it has a good energy resolution and it can be used without casing and cooling. We have already made clear an optimum size of the CdTe element, the performance test of which was carried out so far [
However, the signal to noise (S/N) ratio did not meet the criterion of S/N>1. We then tried to improve the S/N ratio, because we confirmed that deterioration of the S/N ratio was caused by the influence of Compton scattering especially due to capture gamma-rays of hydrogen [
We thus produced an arrayed detector with two CdTe crystals to experimentally make clear cross talk phenomenon by Compton scattering gamma-rays and finally to examine an anti-coincidence detection possibility to improve the S/N ratio. We carried out anti-coincidence measurement with a standard gamma-ray source, 137Cs, and confirmed possibility of reduction of noises formed by Compton scattering of incident gamma-rays [
In this study, for more precise analysis for the anti-coincidence detection, we carried out experiments with a collimator having a similar performance to the real BNCT-SPECT.
In this experiment, we used a set of two-element CdTe detector, not an arrayed detector detailed in Ref. [
We designed and produced a collimator which has a similar performance of the one originally designed for the array type CdTe detector [
We examined basic property of coincidence events between two CdTe elements by measuring coincidence signals of the two to confirm improvement of the S/N ratio.
As a result, as shown in
We carried out coincident spectrum measurement for BNCT-SPECT development with a set of two-element CdTe detector with a collimator and standard gamma-ray source of 137Cs.
As a result of measurement the coincidence events were successfully extracted. This result is very valuable for designing of BNCT-SPECT. We are now planning to carry out evaluation of coincidence rate from the measurement and comparison of it with the numerical calculations.
The principle of BNCT-SPECT.
Photos of a set of two-element CdTe detector; (A) Two-element CdTe detector and (B) Set of two-element CdTe detector.
Photos of designed collimator; (A) Front view of the collimator and (B) Oblique view of the collimator.
Electronic circuits of coincidence measurement.
Photos of experimental arrangement; (A) Electronic circuits of coincidence measurement, (B) The two-element CdTe detector and the collimator (from direction A in (A)), and (C) Enlarged view of the collimator (from direction B in (B)).
Measured pulse height spectra (PHS) of 137Cs and coincidence events; (A) The CdTe1 PHS with the collimator, (B) The CdTe 2PHS without the collimator, and (C) The sum PHS of the CdTe1 and the CdTe2.