Noble gas used in the treatment of cancer is ..............

Explanation

**Step-by-Step Solution:**

1. **Identify Noble Gases**: Noble gases are elements found in Group 18 of the periodic table. They include helium (He), neon (Ne), argon (Ar), krypton (Kr), xenon (Xe), and radon (Rn).

2. **Understand the Properties of Noble Gases**: Noble gases are known for having a complete outer electron shell, making them very stable and non-reactive. This stability is why they are referred to as "noble."

3. **Focus on the Application in Cancer Treatment**: The question specifically asks about the noble gas used in cancer treatment. In medical applications, certain noble gases can be utilized for their properties.

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