Dynamic surface matching for patient positioning in radiotherapy

Curative radiotherapy of localised cancer delivers discrete doses of radiation to diseased tissue over many days. Computer driven multileaf collimators now closely conform radiation dose fields to the shape of any tumour. This technology invites the reduction of beam margins in order to maximally spare healthy tissue, reducing side-effects, but placing more emphasis on the precision of dose delivery. However, tumour motion and body surface variation demand more accurate patient set-ups. Software has been developed to assist in the task of aligning a patient on the treatment couch with his/her previously planned CT scan in true three-dimensions. A body surface height-map is constructed from a B-spline skinned model of the planned CT scan outline set and compared with a height-map from an optical sensor. This type of co-registration does not depend entirely on skin surface markers which unavoidably move due to the dynamics of internal/external anatomical forces.