**Core Concept**
High dose rate (HDR) brachytherapy involves the temporary placement of a radioactive source close to or within a tumor to deliver a high dose of radiation over a short period. The isotope used in HDR brachytherapy should have a suitable half-life, allowing for repeated treatments with minimal decay, and be compatible with the applicator design.

**Why the Correct Answer is Right**
The correct answer involves two isotopes commonly used in HDR brachytherapy: **Iridium-192 (Ir-192)** and **Cobalt-60 (Co-60)**. **Iridium-192** is a popular choice due to its suitable half-life of 74 days and high gamma radiation output, which allows for efficient delivery of radiation. **Cobalt-60**, with a half-life of 5.27 years, is another commonly used isotope in HDR brachytherapy, offering a high dose rate and suitable for various treatment applications.

**Why Each Wrong Option is Incorrect**
**Option A:** Iridium-192 (Ir-192) is correct, but Cs-133 is not typically used in HDR brachytherapy. **Option B:** Cobalt-60 (Co-60) is correct, but Co-60 is not the primary isotope associated with Cs-133. **Option C:** Incorrect, Ra-226 is not used in HDR brachytherapy due to its low gamma radiation output and long half-life. **Option D:** Incorrect, Ra-226 is not suitable for HDR brachytherapy applications.

**Clinical Pearl / High-Yield Fact**
Radionuclides used in HDR brachytherapy should have a suitable half-life, allowing for repeated treatments with minimal decay. This is crucial for maintaining a high dose rate and ensuring effective tumor treatment.

**Correct Answer:**
✓ Correct Answer: C. ab. Iridium-192 (Ir-192) and Cobalt-60 (Co-60) are commonly used isotopes in HDR brachytherapy.