Boron neutron capture therapy

A schematic of therapy facility in Otaniemi, Finland.

Boron neutron capture therapy (BNCT) is an experimental form of radiotherapy that uses a neutron beam that interacts with boron injected to a patient. BNCT depends on the interaction of slow neutrons with boron-10 to produce alpha particles and lithium nuclei, without producing other types of ionizing radiation.

Patients are first given an intravenous injection of a boron-10 tagged chemical that preferentially binds to tumor cells. In clinical trials performed so far the neutrons are created in a nuclear reactor, but particle accelerators may also be used to collide protons into targets made of lithium or beryllium.

The neutrons pass through a moderator, which shapes the neutron energy spectrum suitable for BNCT treatment. Before entering the patient the neutron beam is shaped by a beam collimator. While passing through the tissue of the patient, the neutrons are slowed by collisions and become low energy thermal neutrons. The thermal neutrons undergo reaction with the boron-10 nuclei, forming a compound nucleus (excited boron-11) which then promptly disintegrates to lithium-7 and an alpha particle. Both the alpha particle and the lithium ion produce closely spaced ionizations in the immediate vicinity of the reaction, with a range of approximately 5-9 micrometres, or roughly the thickness of one cell diameter. This technique is advantageous since the radiation damage occurs over a short range and thus normal tissues can be spared.

BNCT has been experimentally tested primarily as an alternative treatment for malignant brain tumors called glioblastoma multiforme as well as recurrent, locally advanced head and neck cancer. Although there are reports of some successful outcomes, this approach has not yet been shown to be superior to other current therapies. Hence, BNCT has not entered routine clinical use.

Early history of neutron capture therapy

After the initial discovery of the neutron in 1932 by Sir James Chadwick, a study by H. J. Taylor in 1935 showed the ability of the boron-10 nuclei to capture thermal neutrons. The neutron capture initiated the fission of the boron-10 nuclei into helium-4 alpha particles as well as lithium-7 particles. In 1936, Locher realised the potential of this discovery in the field of medicine and subsequently suggested that neutron capture could be used to treat tumours. A binary system uses two separate components for the therapy of cancer. Each component in itself is relatively harmless to the cells, but when combined together for treatment they produce a highly cytocidal effect which is lethal. As development on neutron capture therapy continued, other radioactive isotopes such as uranium-235 were researched. However, studies in the late 1950s by Lussenhop et al. showed that the amounts of uranium needed for successful neutron capture therapy was too toxic for human use.

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