**Core Concept:**
Teletherapy and Brachytherapy are two types of radiation therapy used in the treatment of cancer. Teletherapy involves the use of external radiation sources to deliver high-energy X-rays or gamma rays to the tumor site, while Brachytherapy uses internally placed radioactive sources to deliver high doses of radiation to the tumor. Both methods aim to destroy cancer cells while minimizing damage to healthy tissues.

**Why the Correct Answer is Right:**
The correct answer is a radioactive isotope, specifically Iodine-131 (I-131), which is used in both teletherapy and Brachytherapy. Iodine-131 is a radioactive isotope of iodine, and it emits beta particles. In teletherapy, I-131 is used outside the patient's body, where it is delivered via external sources. In Brachytherapy, I-131 is placed directly into or near the tumor, delivering a high dose of radiation to the targeted area while minimizing damage to surrounding healthy tissue.

**Why Each Wrong Option is Incorrect:**

A. Iodine-125 (I-125) is a radioactive isotope used in Brachytherapy, particularly for prostate cancer treatment, but not commonly used for teletherapy.

B. Cobalt-60 (Co-60) is a radioactive isotope used in teletherapy, particularly for cancer treatment, but not commonly used in Brachytherapy.

C. Cobalt-56 (Co-56) is not a radioactive isotope; it is a stable isotope of cobalt.

D. Curium-244 (Cm-244) is a radioactive isotope primarily used in nuclear research and not in medical applications like teletherapy or Brachytherapy.

**Clinical Pearls:**
Iodine-131 is a valuable tool in nuclear medicine for cancer treatment as it can be used in both teletherapy and Brachytherapy, making it an essential isotope in the arsenal of nuclear medicine practitioners.

**Correct Answer:**
Correct Answer: Iodine-131 (I-131)

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