A processor-implemented method for use in adaptive radiotherapy, the method comprising: accessing a first medical image and a second medical image, the first and second medical images representing a region of interest of a patient at different times, wherein each medical image includes a plurality of voxels and is segmented into a target region and at least one non-target region, accessing a deformation vector field, the deformation vector field including a plurality of vectors, wherein each vector defines a geometric transformation to map a respective voxel in the first medical image to a corresponding voxel in the second medical image; and wherein the method is characterised by generating a modified deformation vector field by: identifying a first vector in the deformation vector field that maps a voxel in the first medical image to a voxel that is in a non-target region in the second medical image, determining whether the first vector causes a distance between the mapped voxel and the target region to increase, when it is determined that the first vector causes the distance between the mapped voxel and the target region to increase, generating a modified first vector by reducing the magnitude of the first vector, and storing the modified first vector in the modified deformation vector field; the method further comprising adapting a treatment plan based upon the modified deformation vector field. accessing a first medical image and a second medical image, the first and second medical images representing a region of interest of a patient at different times, wherein each medical image includes a plurality of voxels and is segmented into a target region and at least one non-target region, accessing a deformation vector field, the deformation vector field including a plurality of vectors, wherein each vector defines a geometric transformation to map a respective voxel in the first medical image to a corresponding voxel in the second medical image; and wherein the method is characterised by generating a modified deformation vector field by: identifying a first vector in the deformation vector field that maps a voxel in the first medical image to a voxel that is in a non-target region in the second medical image, determining whether the first vector causes a distance between the mapped voxel and the target region to increase, when it is determined that the first vector causes the distance between the mapped voxel and the target region to increase, generating a modified first vector by reducing the magnitude of the first vector, and storing the modified first vector in the modified deformation vector field; identifying a first vector in the deformation vector field that maps a voxel in the first medical image to a voxel that is in a non-target region in the second medical image, determining whether the first vector causes a distance between the mapped voxel and the target region to increase, when it is determined that the first vector causes the distance between the mapped voxel and the target region to increase, generating a modified first vector by reducing the magnitude of the first vector, and storing the modified first vector in the modified deformation vector field; the method further comprising adapting a treatment plan based upon the modified deformation vector field.