An output radiation treatment plan for at least one target in a treatment volume is determined. Each target is associated with a prescribed radiation dose. An updated treatment plan causes the prescribed radiation dose to be delivered to the target when implemented by a radiation therapy machine. The updated treatment plan requires an updated delivery time to complete and is calculated by:[i] receiving a numerical value designating an upper bound on modulation efficiency of the updated treatment plan,[ii] receiving a current treatment plan,[iii] calculating a current delivery time for the current treatment plan,[iv] calculating an un-modulated delivery time for the current treatment plan, and[v] calculating the updated treatment plan by executing an optimization process while satisfying the upper bound on the modulation efficiency.Steps [ii] to [v] are traversed a predetermined number of times. Thereafter, the output radiation treatment plan is generated.
