What is the effect of radiation therapy on cancer cells?

• A. It increases DNA replication
• B. It damages cellular DNA to stop replication
• C. It spares DNA
• D. It only affects blood vessels

Answer: (B)

Radiation therapy kills cancer cells primarily by damaging their DNA, which prevents replication and triggers cell death. Ionizing radiation interacts with cellular water to form free radicals that attack DNA, causing breaks in the strands, especially double-strand breaks that are hard to repair. When DNA damage is too extensive, cells halt the cycle and undergo programmed death or mitotic catastrophe, reducing the cancer cell population. Cancer cells are often more vulnerable because they commonly have defective DNA repair mechanisms and are already under replication stress, so they can’t fix the damage as effectively as normal cells. While radiation can also affect tumor blood vessels and contribute to overall tumor control, the central effect driving tumor kill is the DNA damage that stops replication.