Neutron capture therapy

Neutron capture therapy (NCT) is a noninvasive therapeutic modality for treating locally invasive malignant tumors such as primary brain tumors and recurrent head and neck cancer. Briefly summarized, it is a two-step procedure: first, the patient is injected with a tumor-localizing drug containing the non-radioactive isotope boron-10 (¹⁰B) that has a high propensity or cross section (σ) to capture slow neutrons. The cross section of the ¹⁰B is many times greater than that of the other elements present in tissues such as hydrogen, oxygen, and nitrogen. In the second step, the patient is radiated with epithermal neutrons, the source of which is either a nuclear reactor or, more recently, an accelerator. After losing energy as they penetrate tissue, the neutrons are absorbed by the capture agent, which subsequently emits high-energy charged particles that can selectively kill tumor cells that have taken up sufficient quantities of ¹⁰B (Fig.1).

All of the clinical experience to date with NCT is with the non-radioactive isotope boron-10, and this is known as boron neutron capture therapy (BNCT). At this time, the use of other non-radioactive isotopes, such as gadolinium, has been limited, and to date, it has not been used clinically. BNCT has been evaluated clinically as an alternative to conventional radiation therapy for the treatment of malignant brain tumors (gliomas), and recurrent, locally advanced head and neck cancer.

After the initial discovery of the neutron in 1932 by Sir James Chadwick, H. J. Taylor in 1935 showed that boron-10 nuclei had a propensity to capture thermal neutrons. This resulted in nuclear fission of the boron-11 nuclei into helium-4 (alpha particles) and lithium-7 ions. In 1936, G.L. Locher, a scientist at the Franklin Institute in Pennsylvania, recognized the therapeutic potential of this discovery and suggested that neutron capture could be used to treat cancer. W. H. Sweet, from Massachusetts General Hospital, first suggested the technique for treating malignant brain tumors and a trial of BNCT against the most malignant of all brain tumors, glioblastoma multiforme, using borax as the delivery agent in 1951. A clinical trial was initiated in a collaboration with Brookhaven National Laboratory in Long Island, U.S.A. and the Massachusetts General Hospital in Boston in 1954.

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