<prompt> Isotope s used in high dose rate brachytherapy a lr192b Co60c Csl33d Ra 226 e Pd103acaabbc  <response>: Although cobalt - 60 and cesium - 137 have been used in the past, iridium - 192 is the most commonly used radioisotope in HDR brachytherapy. The physics of radiation therapyNote - Cs-137 is used in HDR brachytherapy, while option c i.e. Cs - 133 not Cs - 137. 