Radiobiology > Biologic Interactions > Radiosensitivity and the Cell Cycle

Radiosensitivity and the Cell Cycle

Observations of Bergonié and Tribondeau at the beginning of the 20th century: increased sensitivity to radiation seen in populations of cells that are

  • Highly proliferative with rapid cell divisions
  • Less differentiated (i.e. stem cells are more sensitive than mature somatic cells)

Cellular age and its capacity to repair sub-lethal damage are also factors. In addition, the cellular milieu can modify the probability of damage from radiation energies.

The radiosensitivity of proliferating cells varies with the phase of the cell cycle.


  • Most radiosensitive phases: G2-phase and mitosis (M-phase)
  • Least radiosensitive phase: latter part of S-phase (synthesis of DNA)

Note that, for a population of dividing cells, the most time-consuming phase of the cell cycle will therefore be the most represented phase in the population.

S-phase is the lengthiest portion of the cell cycle – about half of the entire cycle time. Thus, roughly 50% of any given population of cells will be in S-phase. Recall, cells in S-phase are relatively more resistant to radiation damage than when in other phases. Also note, important checkpoints occur in G2; these arrest cells with DNA damage from entering mitosis.

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