Purpose: Target delineation within volumetric imaging is a critical step in the planning process of intensity modulated radiation therapy. In endoluminal cancers, endoscopy often reveals superficial areas of visible disease beyond what is seen on volumetric imaging. Quantitatively relating these findings to the volumetric imaging is prone to human error during the recall and contouring of the target. We have developed a method to improve target delineation in the radiation therapy planning process by quantitatively registering endoscopic findings contours traced on endoscopic images to volumetric imaging.Methods: Using electromagnetic sensors embedded in an endoscope, 2D endoscopic images were registered to computed tomography(CT) volumetric images by tracking the position and orientation of the endoscope relative to a CTimage set. Regions-of-interest (ROI) in the 2D endoscopic view were delineated. A mesh created within the boundary of the ROI was projected onto the 3D image data, registering the ROI with the volumetric image. This 3D ROI was exported to clinical radiation treatment planningsoftware. The precision and accuracy of the procedure was tested on two solid phantoms with superficial markings visible on both endoscopy and CTimages. The first phantom was T-shaped tube with X-marks etched on the interior. The second phantom was an anatomically correct skull phantom with a phantom superficial lesion placed on the pharyngeal surface. Markings were contoured on the endoscope images and compared with contours delineated in the treatment planning system based on the CTimages. Clinical feasibility was tested on three patients with early stage glottic cancer.Image-based rendering using manually identified landmarks was used to improve the registration.Results: Using the T-shaped phantom with X-markings, the 2D to 3D registration accuracy was 1.5–3.5 mm, depending on the endoscope position relative to the markings. Intraobserver standard variation was 0.5 mm. Rotational accuracy was within 2°. Using the skull phantom, registration accuracy was assessed by calculating the average surface minimum distance between the endoscopy and treatment planning contours. The average surface distance was 0.92 mm with 93% of all points in the 2D-endoscopy ROI within 1.5 mm of any point within the ROI contoured in the treatment planningsoftware. This accuracy is limited by the CTimaging resolution and the electromagnetic (EM) sensor accuracy. The clinical testing demonstrated that endoscopic contouring is feasible. With registration based on em tracking only, accuracy was 5.6–8.4 mm. Image-based registration reduced this error to less than 3.5 mm and enabled endoscopic contouring in all cases.Conclusions: Registration of contours generated on 2D endoscopic images to 3D planning space is feasible, with accuracy smaller than typical set-up margins. Used in addition to standard 3D contouring methods in radiation planning, the technology may improve gross tumour volume (GTV) delineation for superficial tumors in luminal sites that are only visible in endoscopy.
Funding for this research was provided by the Ontario Institute of Cancer Research for support through the “1 MM Challenge” Program and The Kevin and Sandra Sullivan Chair in Surgical Oncology and the RACH Fund of the Princess Margaret Hospital Foundation.
II.A. Registration and contouring workflow overview
II.C. Endoscope tracking and registration
II.E. Contouring process
II.F. Testing of registration accuracy
II.G. Validation of contouring procedure
II.H. Clinical feasibility
III.A. T-shaped phantom
III.B. Skull phantom
III.C. Clinical feasibility
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