Which modality uses energies of 6–20 MeV to treat deep-seated tumors?

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

Answer: (D)

Key idea: high-energy megavolt beams provide deep penetration and skin-sparing dose delivery, which is essential for treating tumors nestled deep inside the body. Energies in the 6–20 MeV range are produced by linear accelerators, which accelerate electrons and smash them into a high‑Z target to generate megavoltage photons (and sometimes electrons). This lets the beam reach deep-seated tumors with a relatively uniform dose and less dose to the skin.

Brachytherapy uses internal, short-range sources that emit lower-energy photons (often well below 6 MeV), so it’s not the right fit for deep lesions. Cobalt-60 emits gamma rays around 1.17 and 1.33 MeV, far below the 6–20 MeV range, and Gamma Knife uses those same low-energy gamma cascades from cobalt-60. External beam therapies can be delivered with linear accelerators, but the specific 6–20 MeV energy band points to megavoltage beams from linear accelerators as the method used for deep-seated tumors.