Biological Factors Affecting Radiosensitivity

Cell Age & Radiosensitivity

• Older cells/tissues ⇒ more radio-resistant; younger (including stem cells) ⇒ more sensitive.
• Key reasons:
  • Higher metabolic rate $\rightarrow$ more frequent chemical reactions ⇒ greater chance of radiation damage.
  • Higher proliferation rate $\rightarrow$ DNA replicated more often, so damage is copied/amplified.

Oxygen Effect

• Aerobic (well-oxygenated) tissue is more radiosensitive than hypoxic/anoxic tissue.
• Oxygen Enhancement Ratio (OER): $OER = \frac{\text{Dose}<em>{\text{anoxic}}}{\text{Dose}</em>{\text{aerobic}}}$
  • Always >1 (oxygen lowers the dose needed for same effect).
  • Larger for low $LET$ radiation; approaches $1$ for high $LET$ radiation.
• Oxygen fixation hypothesis: O$_2$ "fixes" (locks in) free-radical DNA damage, making it harder to repair.

Patient Age vs. Sensitivity (Whole Organism)

• In utero & early childhood: highest sensitivity (rapid proliferation & metabolism).
• Adolescence $\rightarrow$ adulthood: sensitivity drops as growth stabilises.
• Elderly: slight rise in sensitivity due to reduced repair/healing efficiency (not due to increased metabolism or proliferation).

Clinical Implications

• Radiotherapy: increasing tumour oxygenation can enhance cell kill (smaller dose needed).
• Diagnostic imaging: reducing tissue oxygenation (where practical) can lessen unintended damage to healthy tissue.