M > G2)# The cell is vulnerable to radiation in the stage of mitosis (M), less so during synthesis (S) and relatively insensitive during resting periods.# 'S' phase is the most radioresistant phase of cell cycle.# But, 'S' phase is the most chemosensitive phase of cell cycle.# Radiosensitivity of a cell also depends on its histological type and oxygenation of the tissues.RADIOBIOLOGY# Four important processes that occur after radiation exposure can be summarized as the ' four R's" of radiobiology- The first is repair. Repair is temperature dependent and is thought to represent the enzymatic mechanisms for healing intracellular injury.- The second R is reoxygenation, a process whereby oxygen (and other nutrients) are actually better distributed to viable cells following radiation injury and cell killing.- The third R is repopulation, the ability of the cell population to continue to divide and to replace dying and dead cells.- The fourth R is redistribution, which reflects the variability of a cell's radiosensitivity over the cell cycle.">