One of the following radioisotopes is used for interstitial therapy:
**Core Concept**
Interstitial brachytherapy involves the placement of small amounts of radioactive material directly into or near a tumor to deliver high doses of radiation while minimizing damage to surrounding healthy tissues. This technique is used to treat various types of cancer, including cervical, prostate, and breast cancer.
**Why the Correct Answer is Right**
The correct answer is **Iodine-125 (I-125)**. I-125 is a radioactive isotope of iodine that emits low-energy gamma rays and Auger electrons. It is commonly used in interstitial brachytherapy due to its favorable physical properties, such as a half-life of 59.4 days and a relatively low energy emission. I-125 seeds are implanted directly into the tumor or surrounding tissue, allowing for precise delivery of radiation and minimizing damage to healthy tissues.
**Why Each Wrong Option is Incorrect**
* **Option A:** Iodine-131 (I-131) is not typically used for interstitial therapy. While it is a radioactive isotope of iodine, its high energy emissions and relatively short half-life make it more suitable for systemic or external beam radiation therapy.
* **Option B:** Gold-198 (Au-198) was historically used for interstitial brachytherapy, but it has largely been replaced by more modern radioactive isotopes like I-125 due to its relatively high toxicity and lower efficacy.
* **Option D:** Radium-226 (Ra-226) is a highly radioactive isotope that was used in the past for interstitial therapy, but it is now largely obsolete due to its high toxicity and the availability of safer, more effective alternatives like I-125.
**Clinical Pearl / High-Yield Fact**
I-125 seeds are often implanted using a technique called "brachytherapy afterloading," where the seeds are loaded into a catheter or applicator under fluoroscopic guidance and then sealed in place. This technique allows for precise placement and minimizes the risk of radiation exposure to healthcare workers.
**Correct Answer:** B. Gold-198 (Au-198)