System and method for determining a radiation treatment plan and a radiation therapy machine

A system for generating an output radiation treatment plan for a treatment volume associated with at least one target, each of the at least one target being associated with a respective prescribed radiation dose, the system comprising:

• a data processor configured to:
• calculate an updated treatment plan which when implemented by a radiation therapy machine is configured to cause the respective prescribed radiation dose to be delivered to the at least one target, the updated treatment plan requiring an updated delivery time to complete,
• wherein the data processor is configured to:
• [i] receive a numerical value designating an upper bound on modulation efficiency of the updated treatment plan,
• [ii] receive a current treatment plan,
• [iii] calculate a current delivery time for the current treatment plan,
• [iv] calculate an un-modulated delivery time for the current treatment plan,
• [v] calculate the updated treatment plan by executing an optimization process while satisfying the upper bound on the modulation efficiency, and
• [vi] generate the output radiation treatment plan based on the updated treatment plan.