Image registration: A pre-step in patient´s position verification in radiation therapy

Radiation therapy (RT) is one of the modalities employed for treatment of cancer patients by irradiating the region to be treated using ionizing radiation. In radiation therapy, accurate patient positioning is an important requirement for precise dosage delivery. This paper presents a methodology for image registration employed in verification of the patient´s position during radiation therapy for the treatment of cancer patients. Before treatment, portal image of the patient is acquired using Electronic Portal Imaging Device (EPID) and compared with the Digitally Reconstructed Radiograph (DRR) to evaluate patient set-up error. Our approach involves the use of image registration techniques based on anatomical landmarks prior to the estimation of positional errors between the reference i.e. DRR and portal image. In proposed approach, three pairs of visually recognizable corresponding landmarks were located on DRR and the portal image to extract the transformational parameters required for accurate image registration. The proposed approach was applied on model images to verify the algorithm and results suggested that the proposed approach is promising for image registration before evaluating positional error for radiation therapy.