In boron neutron capture therapy, patients are given boron-10, which selectively accumulates in the tumor. Then the tumor is irradiated with neutrons. When a neutron hits a boron nucleus, it triggers decay into helium and lithium — the released energy destroys the cancer cell. The main challenge: precisely measuring how much boron has actually gathered in a specific person's tumor.
The solution is to listen for the neutron 'echo.' Since the body is mostly water, it lets neutrons through but reflects some back. Boron-10 sharply dampens this echo. By comparing the reflected signal from the tumor and healthy tissue, a detector calculates the concentration. It's similar to spectroscopy: the substance's response reveals its composition.
The method works at a depth of 6 cm, detecting less than 10 parts per million of boron, like a grain of salt in a swimming pool. Amazingly, boron-10 captures neutrons thousands of times more effectively than the carbon and nitrogen in our bodies. The method adds almost no extra radiation and, as predicted by Fermi, paves the way for personalized therapy.
Boron-10 is one of nature's best neutron absorbers, thousands of times more effective than the carbon or nitrogen that mostly make up our bodies.