In boron neutron capture therapy (BNCT), the patient receives an injection of the isotope boron-10, which selectively accumulates in tumor cells. The tumor area is then irradiated with a beam of thermal neutrons. When a neutron is captured by a boron-10 nucleus, a nuclear reaction occurs: the nucleus splits into helium (an alpha particle) and lithium-7, releasing about 2.3 MeV of kinetic energy. This energy, released directly inside the cancer cell, destroys its biomolecules, leading to cell death. The key challenge in BNCT is to accurately, in real time, and noninvasively measure how much boron-10 has actually accumulated in a given patient's tumor, so as to choose the optimal time for irradiation.

The solution is to detect the 'echo' of neutrons. Since the human body is mostly composed of water (hydrogen atoms), it weakly absorbs neutrons but elastically scatters them, sending some back toward the source. Boron-10, with its anomalously high capture cross-section for thermal neutrons, sharply quenches this reflected radiation. By comparing the intensity of the reflected neutron signal from the tumor region with that from known healthy tissue, the detector calculates the boron concentration. The principle resembles spectroscopy: the characteristic response of a substance to probing radiation reveals its elemental composition, except here the analysis is not of absorption but of changes in backscattering.

The method works at depths up to 6 cm from the surface, detecting concentrations below 10 parts per million, like finding a grain of salt dissolved in a swimming pool. This remarkable sensitivity arises because boron-10 captures neutrons thousands of times more effectively than the main elements of soft tissue, such as carbon and nitrogen. The measurement procedure itself adds almost no radiation dose, as it uses very weak neutron fluxes. This approach, as Enrico Fermi foresaw, his work on neutron moderation laid the foundations for the method, opens the door to truly personalized boron neutron capture therapy.