An output radiation treatment plan (P) for at least one target in a treatment volume is determined. Each of the at least one target is associated with a respective prescribed radiation dose. An updated treatment plan (UTP) is configured to cause the respective prescribed radiation dose to be delivered to the at least one target when implemented by a radiation therapy machine. The updated treatment plan (UTP) requires an updated delivery time (T UTP ) to complete and is calculated by executing the steps: [i] receiving a numerical value (ME) designating an upper bound on modulation efficiency of the updated treatment plan (UTP), [ii] receiving a current treatment plan (CTP), [iii] calculating a current delivery time (T CTP ) for the current treatment plan (CTP), [iv] calculating an un-modulated delivery time (T UM ) for the current treatment plan (CTP), and [v] calculating the updated treatment plan (UTP) 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 (P) generated based on the updated treatment plan (UTP).
