Cs131

Cesium-131 (Cs-131) is a radioactive isotope of the element cesium (atomic number 55) with a mass number of 131. It is produced for medical and research applications and is often supplied in sealed, biocompatible sources for clinical use. Cs-131 decays by electron capture to xenon-131, with a half-life of about 9.7 days. The decay emits low-energy photons, including principal gamma emissions near 30 keV, which allows localized dose delivery with rapid attenuation in surrounding tissue.

Medical use is the primary context in which Cs-131 is employed. In brachytherapy, Cs-131 is encapsulated in small seeds that are implanted directly into or near tumors, such as prostate cancer. The short half-life and relatively low-energy emissions enable a high initial dose rate and rapid dose fall-off, delivering substantial radiation to the target while limiting long-term exposure to nearby healthy tissue. This can reduce treatment duration compared with some other brachytherapy isotopes and offers dosimetric advantages for certain tumor geometries.

Production and handling involve irradiating the appropriate precursor material in a nuclear reactor or accelerator, followed by chemical processing and sealing of the isotope in protective capsules. Because Cs-131 is used in sealed sources, safety considerations focus on source integrity, shielding requirements, regulatory compliance, and proper disposal after the source reaches end-of-life.

Compared with other brachytherapy isotopes such as iodine-125 and palladium-103, Cs-131 provides a different balance of half-life, energy, and dose-rate characteristics, which can influence treatment planning and outcomes.

---