Several biological factors determine how a cell responds to radiation:

  • Cell cycle phase

    • Most sensitive: G2/M phase.

    • Most resistant: Late S phase (due to efficient HR repair).

  • DNA repair capacity

    • Cells deficient in HR or NHEJ (e.g. BRCA mutations) are hypersensitive.

  • Oxygen effect

    • Oxygen fixes DNA damage by forming peroxides.

    • Hypoxic cells can be 2–3 times more resistant.

  • Intrinsic cell type

    • Rapidly dividing, undifferentiated cells (lymphocytes, germ cells) are highly radiosensitive.

    • Non-dividing, differentiated cells (neurons, muscle) are relatively radioresistant.

Definition – OER (Oxygen Enhancement Ratio):
OER = (dose in hypoxia to achieve effect) ÷ (dose in oxygenated conditions). For X-rays, OER ≈ 2.5–3.

PreviousNext