A guidance system for optical patient alignment during breast radiotherapy
Schmitt, Jonathan D.
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Breast radiotherapy, particularly IMRT, involves large dose gradients and difficult patient positioning problems. A critical requirement for successful treatment is accurate reproduction of the patient's position assumed during CT simulation and planning. Solving this problem using a simple optical system requires careful imaging geometry calibration. We have developed an optical image-guided technique, which assists in accurately and reproducibly positioning the patient, by displaying her real-time optical image superimposed on a perspective projection image of her 3D CT data. The Single Projection Technique (SPT) accurately determines the 3-D position and orientation of a camera from a single image acquired of a known model. A calibration jig, composed of ten identifiable reflecting spheres, was constructed and CT imaged to provide this model. The coordinates of each point were determined with respect to a fiducial marker. To implement our method, a digital photograph of the jig is acquired, and a centroid-finding technique is applied to this image. The two-dimensional coordinates of each sphere, along with its 3D coordinates serves as input to the SPT program, which calculates the coordinates and orientation of the camera. Using this information, 3D CT patient data is projected onto the camera's imaging plane, and is displayed on a monitor, superimposed on the real-time patient image. This enables the therapist to view both the patient's current and desired positions, and guide proper patient positioning. The SPT can determine the position and orientation of the camera to an accuracy of 0.2 cm and 0.3°, respectively. Investigations are ongoing to determine the accuracy and reproducibility of our method, based on film measurements performed on a breast phantom. We have developed a method to calibrate an optical camera system and superimpose a perspective projection of a CT image on a patient's real-time optical image. Displaying this visual information will assist in accurate setup during breast radiotherapy. Future work will enable us to quantify the setup and dose delivery accuracy of this technique.