A device that is a naturally-occurring, accelerator or reactor produced-radioactive isotope used as a source of radiation in high or low dose rate remote afterloading brachytherapy systems designed to deliver curative or palliative radiotherapy treatments. Sources used in remote afterloading radiotherapy systems may come in a variety of strengths and physical forms, e.g., single encapsulated source (sealed source), ribbons, plated, foil or embedded sources or contained radioactive liquids or gels. It is typically housed in a shielded safe within the remote afterloading system and delivered through guide tubes or catheters to the treatment site by various remote-controlled mechanisms.