Repair

  • Sublethal damage can be repaired if time between fractions is sufficient.

  • Normal tissues generally repair better than tumors → basis of fractionation.

Reoxygenation

  • Tumor hypoxia reduces radiosensitivity.

  • After radiation, some hypoxic cells reoxygenate, making them more radiosensitive for later fractions.

Redistribution

  • Cells in G2/M are most radiosensitive, late S phase most resistant.

  • Radiation pushes surviving cells into sensitive phases → fractionation enhances effect.

Repopulation

  • Tumor cells may proliferate between fractions.

  • Can limit control in prolonged treatments → explains need for accelerated schedules in fast-growing tumors (e.g., head and neck cancer).

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