Basis of practice

Mathematical models based on clinical and laboratory studies have been developed and the linear quadratic has now replaced Ellis as the preferred formula for relating dose and fractionation.

At clinically relevant doses, tumours and early reacting tissues respond to ionizing radiation with a linear relationship between dose and effect—the linear or a component. In the late reacting tissues, in the clinically relevant dose range, a large part of the effect is related to the square of the individual dose given—the /} or quadratic element.

The important implication of the linear quadratic model is that by giving radiotherapy in many small doses, changes in the late reacting tissues should be spared, with little or no alteration in the response of the early reacting normal tissues and of the tumour.

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