Deformable image registration of four head and neck cancer patients was conducted using biomechanical based model.
Patient specific 3D finite element models have been developed using CT and cone beam CT image data of the planning
and a radiation treatment session. The model consists of seven vertebrae (C1 to C7), mandible, larynx, left and right
parotid glands, tumor and body. Different combinations of boundary conditions are applied in the model in order to find
the configuration with a minimum registration error. Each vertebra in the planning session is individually aligned with
its correspondence in the treatment session. Rigid alignment is used for each individual vertebra and to the mandible
since deformation is not expected in the bones. In addition, the effect of morphological differences in external body
between the two image sessions is investigated. The accuracy of the registration is evaluated using the tumor, and left
and right parotid glands by comparing the calculated Dice similarity index of these structures following deformation in
relation to their true surface defined in the image of the second session. The registration improves when the vertebrae
and mandible are aligned in the two sessions with the highest Dice index of 0.86±0.08, 0.84±0.11, and 0.89±0.04 for the
tumor, left and right parotid glands, respectively. The accuracy of the center of mass location of tumor and parotid
glands is also improved by deformable image registration where the error in the tumor and parotid glands decreases from
4.0±1.1, 3.4±1.5, and 3.8±0.9 mm using rigid registration to 2.3±1.0, 2.5±0.8 and 2.0±0.9 mm in the deformable image
registration when alignment of vertebrae and mandible is conducted in addition to the surface projection of the body.