Method, computer program product and computer system for particle-based radiotherapy treatment planning

A method of radiotherapy treatment planning for creating a plan for delivery of at least one particle-based arc to a patient using an apparatus arranged to deliver radiation in the form of charged particles from a number of different directions comprising the steps of

• a. determining (S11;
• S21) an initial set of candidate energy layers, each layer involving a particular energy level at a particular beam angle,
• b. performing a calculation on the initial set,
• c. determining (S13;
• S23), based on the outcome of said calculation, at least one additional energy layer to produce an expanded candidate layer set, wherein the at least one additional energy layer involves an energy level not previously used in any of the energy layers,
• d. adding (S14;
• S24) the at least one additional energy layer to the initial set, to produce an expanded candidate energy layer set
• e. optimizing (S16;
• S26) a radiotherapy treatment plan based on the expanded candidate layer set.