Which modality uses sealed radioactive isotopes placed in tissues, body cavities, or on body surfaces?

• A. Linear Accelerators
• B. External Beam
• C. Brachytherapy
• D. Cobalt-60 Unit (Gamma Knife)

Answer: (C)

Brachytherapy. It uses sealed radioactive sources placed directly into tissues, body cavities, or on the surface of the body. Placing the sources close to the tumor delivers a high radiation dose to the target while the dose falls off quickly with distance, helping spare surrounding healthy tissue. The sources are sealed to prevent leakage, and placement can be interstitial (into tissue), intracavitary (within a body cavity), or surface-based (as molds or applicators). In contrast, external beam approaches deliver radiation from outside the body with machines like linear accelerators, and the Gamma Knife cobalt-60 setup, though using sealed sources, delivers beams from outside rather than implanting sources inside the patient.