What Is Neutron Capture Therapy?

Neutron capture therapy is a binary radiotherapy technique that delivers highly localized radiation doses to tumor cells by combining the selective uptake of a neutron-capturing agent in malignant tissue with external irradiation by a neutron beam. The therapy exploits neutron capture reactions that produce short-range, high-linear-energy-transfer (LET) particles capable of destroying individual cells within a radius comparable to a single cell diameter, while sparing adjacent healthy tissue. The most clinically developed form is boron neutron capture therapy (BNCT), in which the capturing agent contains boron-10, but gadolinium-157 and other neutron-absorbing isotopes have been investigated as alternative agents.

The field draws on nuclear medicine, radiation oncology, radiochemistry, and nuclear engineering. Its physics basis connects directly to the biological effects of densely ionizing radiation and to the dosimetry challenges of characterizing mixed neutron and gamma fields within tissue.

Boron Neutron Capture Therapy

BNCT rests on the reaction between thermal neutrons and boron-10 nuclei, which has an exceptionally high thermal neutron cross-section of approximately 3,840 barns. When a boron-10 nucleus captures a thermal neutron, it momentarily forms boron-11, which immediately fissions into a helium-4 (alpha) particle and a lithium-7 recoil nucleus. The combined path length of these two products is approximately 10 micrometers, close to the diameter of a single mammalian cell. If sufficient boron has accumulated within a tumor cell, the fission products deposit their energy within that cell and its immediate neighbors, inducing lethal DNA damage without requiring a high neutron dose to surrounding tissue.

The selectivity of BNCT depends entirely on the differential uptake of boron compounds between tumor and normal cells. Two compounds approved for human use are boronophenylalanine (BPA) and sodium borocaptate (BSH). BPA exploits the elevated amino acid metabolism of many tumor types to concentrate boron selectively in malignant cells; BSH accumulates preferentially in brain tumors due to disruption of the blood-brain barrier. A review of clinical applications of BNCT published in PMC summarizes the historical development of these agents and early clinical outcomes at reactor-based facilities.

Neutron Sources and Beam Delivery

Early BNCT programs depended on nuclear research reactors, which supply epithermal neutron beams in the 1 eV to 10 keV range suited to penetrating tissue and thermalizing at depth. Reactors in Brookhaven, Petten (Netherlands), Studsvik (Sweden), and several Japanese institutions served as the primary clinical platforms through the 1990s and 2000s. The principal limitation of reactor-based BNCT was geographic: facilities were few, and treatment required patient transport to research reactors.

Accelerator-based neutron sources have overcome this constraint by generating epithermal neutrons through proton or deuteron bombardment of beryllium or lithium targets at clinically manageable current levels. Japan approved the first accelerator-based BNCT system as a routine clinical treatment for unresectable locally advanced head and neck cancer in 2020, opening the pathway to hospital-based deployment. The IAEA publication on advances in boron neutron capture therapy documents this transition and the international status of clinical programs.

Dosimetry and Radiobiology

Dosimetry for BNCT is more complex than for conventional photon radiotherapy because the treatment field contains multiple radiation components: the boron capture reaction products, fast neutron recoil protons, gamma rays from neutron capture in tissue hydrogen, and the nitrogen capture reaction. Each component has a different biological effectiveness per unit of absorbed dose, quantified by the relative biological effectiveness (RBE) and the compound biological effectiveness (CBE) factor for the boron component. Accurate dose calculation requires knowledge of the boron concentration distribution, which varies across tissue types and tumor regions. Monte Carlo transport codes, including MCNP, are standard tools for treatment planning, as reviewed in the Frontiers in Oncology clinical review of BNCT applications.

Applications

Neutron capture therapy has applications in a wide range of oncological and research contexts, including:

• Treatment of glioblastoma multiforme and recurrent brain tumors
• Head and neck cancers, particularly locally recurrent disease after prior radiotherapy
• Cutaneous melanoma and malignant skin tumors
• Hepatocellular carcinoma and liver metastases in intraoperative settings
• Fundamental radiobiological research on high-LET radiation effects