The radioisotope ⁶⁰Co emits gamma rays that are used to destroy cancer cells. Hundreds of thousands of Americans who suffer from cancer have been treated in this way. Every year millions of cubic meters of medical products and equipment are sterilized by irradiation worldwide. ¹³⁷Cs has found substantial applications as a gamma-ray source in medical therapy, similar in its use to that of ⁶⁰Co.

Cancer treatment with beams of massive ions directly from an accelerator has gained increasing utilization in the last decade. Unlike gamma rays, which distribute their energy equally in healthy as well as cancerous cells, massive particles such as protons or alpha particles will deposit the bulk of their energy just before they stop. If the energy is well-chosen, most of the energy will be dumped into the tumor and not into the surrounding healthy tissue. Using three dimensional water degrader columns, the shape of the tumor can be mapped out and selectively irradiated. Dedicated accelerators are now being built to continue this work at medical centers. In the United States, the Loma Linda Medical Center in California is now operating a proton synchrotron for therapy, and a second facility is being built at Massachusetts General in Boston.

Boron Neutron Capture Therapy (BNCT) is under development for the treatment of glioblastoma multiforma, a brain cancer which afflicts some 12,500 people a year in the US alone and is almost impossible to treat by currently available means. In BNCT, boron is synthesized into compounds that are selectively taken up by cancerous cells in the brain and not by healthy ones. In subsequent irradiation by low-energy neutrons (from a reactor or accelerator), the following neutron-capture reaction occurs:

¹⁰B + n Æ ¹¹B Æ ⁷Li + ⁴He.

The recoiling lithium and helium nuclei have short ranges and large energy losses. These particles destroy the cancerous cells and spare the surrounding healthy tissue.