A computer implemented method of determining a radiation treatment plan for delivering a desired dose of radiation to a treatment volume within a patient using an external-beam radiation treatment system, the method comprising: receiving an initial radiation treatment plan having one or more treatment fields for providing the desired dose of radiation; receiving a maximum value of total treatment time for providing the desired dose of radiation to the patient; determining, using a computer system, a maximum value of total monitor unit count based on the maximum value of total treatment time, a selected dose rate, geometries of the one or more treatment fields, and maximum speed limits of treatment axes of the external-beam radiation treatment system; defining a cost function including a first term relating to a difference between a value of MU count of a radiation treatment plan and the maximum value of total MU count; identifying, using the computer system, a plurality of candidate radiation treatment plans, each of the plurality of candidate radiation treatment plans having a respective multileaf collimator leaf sequence; for each of the plurality of candidate radiation treatment plans: determining a corresponding value of MU count based on the respective MLC leaf sequence; determining a value of the first term of the cost function based on the corresponding value of MU count; and determining a value of the cost function based at least in part on the value of the first term; and selecting an optimal radiation treatment plan among the plurality of candidate radiation treatment plans that minimizes the cost function, wherein the optimal radiation treatment plan is to be used by the external-beam radiation treatment system for delivering the desired dose of radiation. receiving an initial radiation treatment plan having one or more treatment fields for providing the desired dose of radiation; receiving a maximum value of total treatment time for providing the desired dose of radiation to the patient; determining, using a computer system, a maximum value of total monitor unit count based on the maximum value of total treatment time, a selected dose rate, geometries of the one or more treatment fields, and maximum speed limits of treatment axes of the external-beam radiation treatment system; defining a cost function including a first term relating to a difference between a value of MU count of a radiation treatment plan and the maximum value of total MU count; identifying, using the computer system, a plurality of candidate radiation treatment plans, each of the plurality of candidate radiation treatment plans having a respective multileaf collimator leaf sequence; for each of the plurality of candidate radiation treatment plans: determining a corresponding value of MU count based on the respective MLC leaf sequence; determining a value of the first term of the cost function based on the corresponding value of MU count; and determining a value of the cost function based at least in part on the value of the first term; and determining a corresponding value of MU count based on the respective MLC leaf sequence; determining a value of the first term of the cost function based on the corresponding value of MU count; and determining a value of the cost function based at least in part on the value of the first term; and selecting an optimal radiation treatment plan among the plurality of candidate radiation treatment plans that minimizes the cost function, wherein the optimal radiation treatment plan is to be used by the external-beam radiation treatment system for delivering the desired dose of radiation.