Optimal schemes of five weekly radiation doses for a fractionated radiotherapy treatment have been studied in the original article as functions of tumour and healthy tissue parameters. The extremals and the optimal solutions derived there, however, did not account for “singular” situations in which some of the expressions (3.10), providing the elements of the extremal structures, are not defined. For a given set of model parameters, singular situations occur when suitable Kuhn– Tucker multipliers can be found so that the quantities δ, σ and τ defined in (3.1) satisfy the relations in one of the following cases: