Definition – Hypoxia
A state of reduced oxygen availability in tissues (usually <10 mmHg O₂ in tumors).

Causes in tumors

  • Abnormal vasculature: tortuous, leaky vessels → uneven oxygen delivery.

  • Diffusion-limited hypoxia: cells far (>100–150 μm) from capillaries become hypoxic.

  • Perfusion-limited hypoxia: transient vessel collapse reduces local oxygen.

Consequences

  • Radiation resistance: oxygen “fixes” DNA free radical damage; without it, cells survive.

  • Selection pressure: hypoxic cells can become more aggressive, metastatic, resistant to therapy.

  • Increased mutagenesis: ROS fluctuations in low O₂ lead to genetic instability.

Oxygen Enhancement Ratio (OER)

  • Dose in hypoxia / Dose in normoxia for the same effect.

  • For X-rays: OER ≈ 2.5–3.

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