A computer-implemented method of determining a treatment field trajectory for delivering a desired dose of radiation to a treatment target within a patient using an external-beam radiation treatment system, the method comprising: receiving a radiation treatment plan having a first treatment field of an intensity modulated radiation therapy (IMRT) modality and a second treatment field of a dynamic treatment path modality, the first treatment field having a first spatial point associated with a first set of values for treatment axes of the radiation treatment system, and the second treatment field defining a treatment path having a start spatial point associated with a start set of values for the treatment axes and an end spatial point associated with an end set of values for the treatment axes; identifying, using a computer system, a plurality of intermediate spatial points along the treatment path of the second treatment field, each intermediate spatial point dividing the treatment path into a first section and a second section, and each intermediate spatial point associated with a respective intermediate set of values for the treatment axes; for each of the plurality of intermediate spatial points: determining, using the computer system, a transition time for changing the treatment axes from the respective intermediate set of values to the first set of values of the first spatial point and changing from the first set of values back to the respective intermediate set of values; selecting, using the computer system, an optimal intermediate spatial point among the plurality of intermediate spatial points to achieve an optimal total treatment time, the optimal total treatment time including the transition time and beam-on time for administering the first treatment field and the second treatment field; determining the treatment field trajectory by interleaving the first treatment field between the first section and the second section of the second treatment field; and updating the radiation treatment plan using the determined treatment field trajectory, wherein the radiation treatment plan is to be used by the external-beam radiation treatment system for delivering the desired dose of radiation. receiving a radiation treatment plan having a first treatment field of an intensity modulated radiation therapy (IMRT) modality and a second treatment field of a dynamic treatment path modality, the first treatment field having a first spatial point associated with a first set of values for treatment axes of the radiation treatment system, and the second treatment field defining a treatment path having a start spatial point associated with a start set of values for the treatment axes and an end spatial point associated with an end set of values for the treatment axes; identifying, using a computer system, a plurality of intermediate spatial points along the treatment path of the second treatment field, each intermediate spatial point dividing the treatment path into a first section and a second section, and each intermediate spatial point associated with a respective intermediate set of values for the treatment axes; for each of the plurality of intermediate spatial points: determining, using the computer system, a transition time for changing the treatment axes from the respective intermediate set of values to the first set of values of the first spatial point and changing from the first set of values back to the respective intermediate set of values; selecting, using the computer system, an optimal intermediate spatial point among the plurality of intermediate spatial points to achieve an optimal total treatment time, the optimal total treatment time including the transition time and beam-on time for administering the first treatment field and the second treatment field; determining the treatment field trajectory by interleaving the first treatment field between the first section and the second section of the second treatment field; and updating the radiation treatment plan using the determined treatment field trajectory, wherein the radiation treatment plan is to be used by the external-beam radiation treatment system for delivering the desired dose of radiation.