Which therapy uses sealed radioactive sources placed inside or near the tumor to deliver radiation from within?

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

Answer: (B)

The main idea here is brachytherapy. This approach uses sealed radioactive sources placed directly inside or very near the tumor, so radiation is delivered from within the target area. Because the source is so close to the tumor, the dose to the tumor can be high while the dose to surrounding normal tissue falls off quickly with distance (thanks to the inverse-square law). This allows effective tumor control with typically shorter overall treatment times and the option to tailor the dose and duration. Sources can be temporary and removed after treatment or permanent and remain in place as seeds, depending on the cancer type and plan. Prostate cancer, certain gynecologic cancers, and some head/neck cancers are common sites treated this way.

In contrast, external beam approaches deliver radiation from outside the body, using devices like linear accelerators to generate high-energy photons or electrons targeted at the tumor, or specialized external systems like the Gamma Knife, which focuses multiple external beams on the target. These do not involve implanted sealed sources.