A computer-implemented method of controlling dose distribution outside one or more target volumes within a domain of interest of a patient in a radiation treatment plan for delivering radiation to the one or more target volumes using an external-beam radiation treatment system, the method comprising: receiving, by a computer system, geometrical information about the one or more target volumes, wherein the one or more target volumes includes a first target volume, and wherein the geometrical information includes contour information of the first target volume; receiving, by the computer system, a dose prescription including a lower dose limit for the first target volume; the method being characterised by the following steps: determining, by the computer system, a first dose falloff constraint profile for voxels outside the first target volume based on (i) the contour information of the first target volume and (ii) the lower dose limit for the first target volume, wherein the first dose falloff constraint profile includes a first upper dose constraint for each voxel outside the first target volume defined by a first dose falloff constraint curve; obtaining, by the computer system, a cost function including a first term proportional to an excess of a dose value at each respective voxel outside the first target volume with respect to a corresponding value of the first dose falloff constraint profile at the respective voxel; and determining, by the computer system, an optimal radiation treatment plan using the cost function, wherein the optimal radiation treatment plan produces a dose distribution that is compliant with the first dose falloff constraint profile for voxels outside the first target volume, and wherein the optimal radiation treatment plan includes a control-point sequence and a multileaf collimator leaf sequence to be used by the external-beam radiation treatment system for delivering the radiation. receiving, by a computer system, geometrical information about the one or more target volumes, wherein the one or more target volumes includes a first target volume, and wherein the geometrical information includes contour information of the first target volume; receiving, by the computer system, a dose prescription including a lower dose limit for the first target volume; the method being characterised by the following steps: determining, by the computer system, a first dose falloff constraint profile for voxels outside the first target volume based on (i) the contour information of the first target volume and (ii) the lower dose limit for the first target volume, wherein the first dose falloff constraint profile includes a first upper dose constraint for each voxel outside the first target volume defined by a first dose falloff constraint curve; obtaining, by the computer system, a cost function including a first term proportional to an excess of a dose value at each respective voxel outside the first target volume with respect to a corresponding value of the first dose falloff constraint profile at the respective voxel; and determining, by the computer system, an optimal radiation treatment plan using the cost function, wherein the optimal radiation treatment plan produces a dose distribution that is compliant with the first dose falloff constraint profile for voxels outside the first target volume, and wherein the optimal radiation treatment plan includes a control-point sequence and a multileaf collimator leaf sequence to be used by the external-beam radiation treatment system for delivering the radiation. determining, by the computer system, a first dose falloff constraint profile for voxels outside the first target volume based on (i) the contour information of the first target volume and (ii) the lower dose limit for the first target volume, wherein the first dose falloff constraint profile includes a first upper dose constraint for each voxel outside the first target volume defined by a first dose falloff constraint curve; obtaining, by the computer system, a cost function including a first term proportional to an excess of a dose value at each respective voxel outside the first target volume with respect to a corresponding value of the first dose falloff constraint profile at the respective voxel; and determining, by the computer system, an optimal radiation treatment plan using the cost function, wherein the optimal radiation treatment plan produces a dose distribution that is compliant with the first dose falloff constraint profile for voxels outside the first target volume, and wherein the optimal radiation treatment plan includes a control-point sequence and a multileaf collimator leaf sequence to be used by the external-beam radiation treatment system for delivering the radiation.