Page 95 - ARNM-2-3

Advances in Radiotherapy
& Nuclear Medicine Radiotherapy with neutron/gamma tubes

moderating the 2.5 MeV d-d neutrons or the 10 and arrive at the tumor site. The distribution of epithermal and
13 MeV d- Li neutrons. Alternatively, they can be directly thermal neutron flux inside the body has been studied in
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produced by employing the Li(p,n) Be reaction near detailed by Nakagawa in one chapter of the book. Using a
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14
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the threshold interaction energy (1.881 MeV). 12,13 This 1.9 MV and 10 mA H ion beam with a diameter of 2 cm,
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approach to forming epithermal neutrons offers several the total neutron yield is 1.5 × 10 n/s with mean neutron
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advantages. First, as no moderation is needed, the energy of 38 keV and a mean neutron angle of 23 . For
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epithermal neutron flux is higher. Second, the epithermal a H ion beam with a diameter of 5 cm and an ion beam
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neutrons formed at near-threshold energy are directional current of 70 mA, the average neutron yield for a 1% DF
with a mean neutron angle of approximately 20 . Therefore, operation is approximately 1 × 10 n/s, or an average
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0
shielding is not needed on the side and at the back of the epithermal neutron flux of 5 × 10 n/cm /s, which is the
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2
neutron generator. Third, the d-d and the d- Li neutrons recommended value for BNCT treatment. 15
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are emitted isotropically, therefore more than 50% of these A single p- Li neutron tube with a 5-cm diameter beam
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neutrons produced are wasted but not the near-threshold target can provide the required epithermal neutron flux
produced p- Li neutrons. In the past, this approach of for BNCT. Alternatively, one can employ a cluster of mini
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generating epithermal neutrons was only carried out neutron tubes each operated with a 2-cm diameter target.
with a large 2 MV accelerator system such as a linac or Figure 7 shows a schematic diagram of a multi-neutron
a tandem accelerator. Using the new mini p- Li neutron tube arrangement. There are six tubes surrounding the
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tube (Figure 1) and employing an AC high-voltage power center one. These six neutron tubes are focused on the
supply, a compact high-yield epithermal neutron source tumor providing the highest flux of thermal neutrons at
can be developed for boron neutron capture therapy the tumor site. With a total H beam current of 70 mA
−
(BNCT) applications. 14 (i.e., 10 mA from each neutron tube), the combined
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BNCT is based on the high probability of a stable epithermal neutron yield is 1 × 10 n/s for a 1% DF
isotope B capturing a thermal neutron, thereby releasing operation. Since the orientation of these neutron tubes can
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two high-energy ions ( He and Li ). Because of the high be adjusted independently, the epithermal neutron beam
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+
2+
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LET and RBE of these ions, only cells in close proximity from each tube can be focused onto the tumor. As a result,
to the reaction B(n, α) Li are damaged, leaving adjacent the thermal neutron flux reaching the tumor site will be
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cells unaffected. The enhanced uptake of the boron-labeled higher than a single large tube. With recent advancements
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agent in tumor cells versus normal cells results in selective in boron drug development, the mini p- Li neutron tubes
killing of tumor cells. Using the mini p- Li neutron tube, a should be the most compact and versatile tool for the
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high flux of epithermal neutrons can be directly produced treatment of cancer through BNCT.
by operating the generator near the threshold interaction 5. Mini gamma-ray tube for the generation
energy. On entering the patient’s body, these epithermal of 6 MeV photons
neutrons are moderated to thermal neutrons when they
High-energy photons have found important applications in
explosive and special nuclear materials detection, medical
imaging, cancer therapy, radioisotope production, and
structural analysis. Bremsstrahlung photons (or X-rays)
with a continuum energy distribution are generated by
high-energy electron accelerators. The end-point energy

Figure 6. A prototype mini neutron tube with an applicator enclosing the
conical titanium target. The spherical applicator head is changeable. The Figure 7. Treatment of brain tumor with multiple epithermal neutron
mini neutron tube and applicator system are approximately 25-cm long. tubes. Diagram created by the authors.

Volume 2 Issue 3 (2024) 5 doi: 10.36922/arnm.3920