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Fractionation is widely employed in radiation therapy. The number of fractions N can vary from 1 to more than 25. Associated with this multifraction treatment is the inability to reposition patients exactly from fraction to fraction or from patient to patient. The effect of this positioning error is analysed within the context of computerised treatment planning (dose calculation). A simple method is described in which this analysis can be easily incorporated into most computerised two-dimensional treatment planning systems. The result is a 'modified' isodose plan in which fluctuation in patient positioning from fraction to fraction is taken into account. The effect of N on dose calculation is further analysed. A procedure to express dose predictions on paper plan due to the finite value of N is described in terms of the uncertainty in dose predictions and the most probable dose predictions.
J. Leong (Sun,) studied this question.