Effect Of Fractionation

This topic is the other main focus of time, dose and fractionation effects. Again Hall is a good reference.

Observed effects of fraction size

Studies have shown that early responding tissues tend to show a more linear dose-response relationship than late responding tissues, when the surviving fraction of cells is graphed on a logarithmic scale. This means that at low doses, less late effects are seen relative to early effects, and that at high doses late effects are more severe than the early effects.
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The shape of the curve is described by the alpha/beta values for a particular tissue. Early responding tissues tend to have a high alpha/beta value, giving them a linear response. Late responding tissues have a low alpha/beta value (more weight on the beta/quadratic value) and have a more curved shape.

Evidence for effects of fraction size

Hypofractionation

Hypofractionated treatments which had their dose adjusted to give equal acute effects to conventionally fractionated treatments demonstrate increased rates of late effects.

Hyperfractionation

Hyperfractionated treatments where dose is adjusted to give equal acute effects to conventially fractionated treatments show lower rates of late effects.

Isoeffect curves

Isoeffect curves are steeper for late responding tissues in animal studies.
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