Dose fractionation would at first examination seem to be a foolish idea - although it allows the recovery of normal tissues it allows the neoplastic material to repeat the starting, favourable part of the survival curve, thus reducing the effect of the radiation.
However, the reality is not as easy as this for the following reason. Tumours have an initial rapid death, which then slows, i.e. a biphasic decay. This is because tumours don't have normal vasculature - there is a central area of necrosis or relative hypoxia which is thus less resistant to the effects of ionising radiation. One dose of radiation kills the sensitive cells in the periphery, allowing aeration of the hypoxic centre. These can be killed by the next dose, i.e. there is a chance for the tumour to oxygenate between each dose thus facilitating the efficacy of the next dose of radiation.
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