**Core Concept**
Dactinomycin (Actinomycin-D) is a chemotherapeutic agent that enhances radiation effects due to its ability to intercalate DNA, thereby increasing the radiosensitivity of cells. This property makes it a critical consideration during radiotherapy to prevent radiation toxicity.

**Why the Correct Answer is Right**
Dactinomycin accumulates in the tissues and increases the radiation-induced damage to the DNA, thus enhancing the radiation effects. This synergistic effect increases the risk of radiation toxicity, such as mucositis, dermatitis, and myelosuppression. As a result, dose adjustment of Dactinomycin is necessary during radiotherapy to minimize the risk of radiation toxicity.

**Why Each Wrong Option is Incorrect**
**Option A:** Vincristine is a vinca alkaloid that works by inhibiting microtubule formation in the mitotic spindle, thereby inducing cell cycle arrest. It does not enhance radiation effects and does not require dose adjustment during radiotherapy.

**Option C:** Cyclophosphamide is an alkylating agent that works by inducing cross-linking of DNA strands, leading to cell death. It does not have a synergistic effect with radiation and does not require dose adjustment during radiotherapy.

**Option D:** 6-Mercaptopurine is an antimetabolite that works by inhibiting purine synthesis, thereby inducing cell cycle arrest. It does not enhance radiation effects and does not require dose adjustment during radiotherapy.

**Clinical Pearl / High-Yield Fact**
When administering Dactinomycin during radiotherapy, it is essential to monitor patients closely for signs of radiation toxicity, and dose adjustments should be made accordingly to minimize the risk of adverse effects.

**✓ Correct Answer: B. Dactinomycin**