Beam therapy is one of the basic methods for curing malignant tumors. Recently, the ever increasing attention in therapy was drawn to the use of neutron beams. At present, the mostly promising is the version of neutron capture therapy (NCT).

The concept of neutron capture therapy was introduced in 1936, four years after the discovery of neutrons. The physical principle of NCT is simple and elegant. The nuclear reaction occurs when the stable isotope ¹⁰B is irradiated with thermal neutrons. The energetic a-particle and recoiling ⁷Li ion produced are break down fast on ~10 microns and deposite ~2,3 MeV precisely inside the cell including boron nucleus. That results in the cell destruction.

Therefore, boron neutron capture therapy (BNCT) will make it possible to destroy selectively tumor cells at higher ¹⁰B concentration than in normal ones.

Elastic scattering of neutrons and ¹⁴N(n,p)¹⁴C and ¹H(n,g)²H nuclear reactions resulting in recoil nuclei and g -rays are possible besides nuclear reactions related to neutron capture by boron nuclei at neutron rediation. Although the neutron capture cross-sections for hydrogen and nitrogen are several orders of magnitude lower then those for ¹⁰B, hydrogen and nitrogen are present in such high concentrations that their neutron capture contributes significantly to the total absorbed dose. In order to reduce this “background” dose it is essential that the tumor attain high ¹⁰B concentration.

In 1951 it was first demonstrated that certain boron compounds would allow higher boron concentration in human brain tumor cells in comparision with normal brain tissue. During 1950-60 at the Brookhaven National Laboratory and Massachusetts Institute of Technology first clinical trials were conducted. Unfortunately, these trials failed to show any evidence of therapeutic efficacy of the method. Later, it became clear that major reason for their lack of success was low ¹⁰B concentration that caused high enough “background” dose of recoil protons and g-rays.

Synthesis of boron containing compounds enriched in the ¹⁰B isotope was the begining of a new stage in development of the NCT conception. This compound introduced into patient blood produces in a tumor cell the ¹⁰B isotope concentration up to 40 m g/g that is three times larger than that in a normal tissue cell. This anables the selective destruction of malignant tumors.

The nuclear reactor is the most powerful stationary source of neutrons. And it is quite natural that the beams of reactor neutrons have been widely used in cancer therapy. The required neutron spectrum is formed by special filters. Fast neutron therapy is successfully used in many nuclear centers of Europe, USA, and Japan, and BNCT techniques are developing vigorously. In Russia, great experience in fast neutron therapy has accumulated in research centers of Obninsk and Tomsk. A fast neutron therapy clinic has been recently put into operation in Snezhinsk. However, dangerous ecological effects of the reactor, as well as cancer therapy clinics having to be in the proximity to these reactors bring about active discussions of issues on development and construction of neutron source based on a compact and reasonably priced accelerator.

Accelerator based neutron source for the neutron-capture and fast neutron therapy at hospital is proposed.

Negative hydrogen ion beam is injected into electrostatic tandem accelerator with vacuum isolation. After charge-exchange of negative hydrogen ion in proton inside charge-exchange tube in the center of tandem, a proton beam is formed at the outlet of the tandem, which is accelerated to double voltage of high voltage electrode. A sectional rectifier is used as a high voltage source. Neutron generation is proposed to be carried out by dropping an intensive proton beam onto lithium target using ⁷Li(p,n)⁷Be threshold reaction.

The most efficient operation mode of facility is at proton energy of 1.883 ¸  1.890 MeV that is near the threshold of the ⁷Li(p,n)⁷Be reaction. In this mode provides neutron beam kinematically collimated with good forward direction and average energy of 30 keV, directly applicable for boron neutron-capture therapy.

In the other “ordinary” mode, at proton energy of 2.5 MeV, the neutron source produces neutron beam with maximum energy board of 790 keV which can be used directly for fast neutron therapy and for neutron-capture therapy after moderation.

Creation of the accelerator with proton beam intensity of 20¸ 40 mA will decrease the exposure time for necessary therapeutic dose of 20 Gy to 10 minutes.

The most attractive and elegant operating mode in the near-threshold area requires high monochromaticity and energy stability of the proton beam. This makes use of widely discussed type of high frequency accelerator RFQ impossible, and the requirement can be met only for the case of electrostatic accelerator. Characteristic feature of the neutron source is use of tandem accelerator with vacuum insulation instead of direct accelerator. The tandem design with vacuum insulation also provides the higher reliability compared to tandem based on accelerating columns with ceramic insulation at high intensity of proton beam.

Investigation and design work was carried out on all the components of the proposed neutron source, and rich experience was stored. Besides, in Obninsk they have an experience of cure of malignant tumors of about 350 patients using FNT technique.

Realization of the Project is one of the main objectives of a Inter-Department Program "Development of progressive techniques of cure for tumors using of neutron and neutron zapture therapy on base of reactors and accelerators". The Program was passed by Presidium of Russian Academy of Medical Sciences on May 27, 1998.

The Project was supported by International Sciences and Technology Center (Project # 1484) on October 28, 1999.

Other investments are necessary and welcome.

The project is under realization now. Necessary scientific researches and design work are carried out. Prototype of the neutron source and technical project for commercial realization are planned to be made by the middle of 2002. The institutions involved are able to produce the accelerating based neutron source proposed for exploitation in clinic during the following 2-3 years.

• Budker Institute of Nuclear Physics, Novosibirsk, Russia
• Institute of Physics and Power Engineering, Obninsk, Russia
• Medical Radiological Research Center of Russian Academy of Medical Sciences, Obninsk, Russia
• Russian Federal Nuclear Center — Institute of Technical Physics, Snezhinsk, Russia
• ……… (Cooperation is welcome)

Please, contact with people listed below on any questions the Project and proposals on investments and cooperation.

Prof. Grigori I. Silvestrov

Dr. Serguei Yu. Taskaev

Budker Institute of Nuclear Physics

11 Lavrentiev ave.

630090 Novosibirsk

Russia

phone: 7(3832)394736 (G. Silvestrov)

7(3832)394121 (S. Taskaev)

fax: 7(3832)342163

e-mail: taskaev@inp.nsk.su

Prof. Victor N. Kononov

Institute of Physics and Power Engineering

1 Bondarenko sq.

249033 Obninsk, Kaluga region, Russia

phone: 7(08439)98364

fax: 7(095)2302326

e-mail: kononov@ippe.obninsk.ru

Dr. Alexei S. Sysoev

Medical Radiological Research Center

4 Koroleva st.

249033 Obninsk, Kaluga region, Russia

phone: 7(08439)72085

fax: 7(095)9561440

e-mail: gulidov@mrrc.obninsk.ru