A method of segmenting a three-dimensional model to create a patient model, comprising: determining a three-dimensional model from measurements of a patient, wherein the 3D model comprises a plurality of voxels; receiving an indication of a selection of an active label, wherein the selection of the active label enables a user indicator to cause voxels of the plurality of voxels that are interacted with to be associated with the active label; characterised by: receiving an indication of a selection of a paint-over label, wherein the selection of the paint-over label causes voxels of the plurality of voxels not associated with the paint-over label to be write-protected from being associated with the active label; receiving an indication of a selection of a limit-to label that specifies a structure within the 3D model, wherein the selection of the limit-to label causes voxels of the plurality of voxels that are not associated with the structure within the 3D model to be write-protected from being associated with the active label; and causing one or more voxels of the plurality of voxels associated with the paint-over label and the limit-to label to be associated with the active label based on an interaction with the one or more voxels via the user indicator. determining a three-dimensional model from measurements of a patient, wherein the 3D model comprises a plurality of voxels; receiving an indication of a selection of an active label, wherein the selection of the active label enables a user indicator to cause voxels of the plurality of voxels that are interacted with to be associated with the active label; characterised by: receiving an indication of a selection of a paint-over label, wherein the selection of the paint-over label causes voxels of the plurality of voxels not associated with the paint-over label to be write-protected from being associated with the active label; receiving an indication of a selection of a limit-to label that specifies a structure within the 3D model, wherein the selection of the limit-to label causes voxels of the plurality of voxels that are not associated with the structure within the 3D model to be write-protected from being associated with the active label; and causing one or more voxels of the plurality of voxels associated with the paint-over label and the limit-to label to be associated with the active label based on an interaction with the one or more voxels via the user indicator. receiving an indication of a selection of a paint-over label, wherein the selection of the paint-over label causes voxels of the plurality of voxels not associated with the paint-over label to be write-protected from being associated with the active label; receiving an indication of a selection of a limit-to label that specifies a structure within the 3D model, wherein the selection of the limit-to label causes voxels of the plurality of voxels that are not associated with the structure within the 3D model to be write-protected from being associated with the active label; and causing one or more voxels of the plurality of voxels associated with the paint-over label and the limit-to label to be associated with the active label based on an interaction with the one or more voxels via the user indicator.