Which radioisotope acts as an external source of ionizing radiation in the treatment cancer?

The radioisotope that acts as an external source of ionizing radiation in the treatment of cancer is Cobalt-60 (Co-60). It emits highly penetrating gamma rays, making it suitable for external beam radiation therapy. Co-60 is commonly used in radiotherapy machines known as teletherapy units or gamma knife systems, where the radiation beam is precisely directed to the tumor site while minimizing damage to surrounding healthy tissues.