Which of the following radioisotopes is commonly used as a source for external beam radiotherapy in the treatment of cancer patients –: (A) Strontium–89, (B) Radium–226, (C) Cobalt–59, (D) Cobalt–60
Answer is D. Cobalt - 60 and Cesium - 137 are most commonly used radioisotopes for teletherapy (External beam radiation).