A new isoeffect curve for change in dose per fraction.

A method is proposed for using survival curve parameters for calculating the change in total dose necessary to achieve an equal response in a tissue when the dose per fraction is varied. The method uses the ratio alpha/beta of the coefficients of the linear quadratic survival formula and accounts only for the effect of repair of cellular injury. Absolute values for alpha and beta are not required. The isoeffect curves vary for different tissues. A dose adjustment to account for differences in the regeneration of surviving cells that might result from changing a treatment regimen must be made separately and will also vary from tissue to tissue. Examples of the use of the curves are given. At present, the curves, particularly those for late effects, are uncertain and caution should be observed in using them until they are defined more accurately with additional data.