PII S0360-3016(98)00005-4 ● Physics Contribution FRAME SLIPPAGE VERIFICATION IN STEREOTACTIC RADIOSURGERY KARL OTTO, M.SC. AND B. GINO FALLONE,PH.D., FCCPM Medical Physics Unit, McGill University, Montreal General Hospital, Montreal, PQ, Canada H3G 1A4 Purpose: To develop a method for detecting frame slippage in stereotactic radiosurgery by interactively matching in three dimensions Digitally Reconstructed Radiographs (DRRs) to portal images. Methods and Materials: DRRs are superimposed over orthogonal edge-detected portal image pairs obtained prior to treatment. By interactively manipulating the CT data in three dimensions (rotations and translations) new DRRs are generated and overlaid with the orthogonal portal images. This method of matching is able to account for ambiguities due to rotations and translations outside of the imaging plane. The matching procedure is performed with anatomical structures, and is used in tandem with a fiducial marker array attached to the stereotactic frame. The method is evaluated using portal images simulated from patient CT data and then tested using a radiographic head phantom. Results: For simulation tests a mean radial alignment error of 0.82 mm was obtained with the 3D matching method compared to a mean error of 3.52 mm when using conventional matching techniques. For the head phantom tests the mean alignment displacement error for each of the stereotactic coordinates was found to be x  0.95 mm, y  1.06 mm, z  0.99 mm, with a mean error radial of 1.94 mm (SD  0.61 mm). Conclusion: Results indicate that the accuracy of the system is appropriate for stereotactic radiosurgery, and is therefore an effective tool for verification of frame slippage. © 1998 Elsevier Science Inc. Radiosurgery, Portal, Stereotactic, Verification, Frame slippage. INTRODUCTION Stereotactic radiosurgery is an irradiation technique requir- ing high spatial accuracy in dose delivery (1, 2). Conven- tional methods achieve this accuracy by employing a ste- reotactic frame fastened to the patient’s skull. The frame is attached to the patient’s head prior to localization and remains there until the entire treatment procedure is com- plete. Various frame attachments containing fiducial mark- ers or externally visible coordinate systems are available for the localization procedure and treatment setup verification (3). These methods provide excellent accuracy for align- ment of the frame prior to treatment but do not provide any information on the positioning of the anatomy with respect to the frame. Any motion of the frame with respect to the patients anatomy (e.g., slippage of the frame on the surface of the skull) will not be detected. Between target volume localization and irradiation the patient and frame are de- tached from the diagnostic device, moved over to the treat- ment couch, and reattached to the mounting bracket. These procedures are delicate, and any jostling of the patient make cause some frame repositioning. The potential for slippage is particularly high for fractionated treatments where the frame remains on the patients head for several days between irradiations. A report from a quality assurance program on stereotactic radiosurgery (4) has stated that the assumption that a frame does not move during the various steps of the radiosurgical procedure cannot be taken for granted. For this reason a method of evaluating the positioning of the radio- surgical beam with respect to the anatomy prior to treatment is needed. Methods of verifying frame to patient positioning have been previously reported (4). These methods are based on mechanical devices that provide distance measurements from the frame to the skull or other external anatomical landmarks. Unfortunately, these methods are fairly subjec- tive and, therefore, provide limited accuracy (2 mm). Also, any deviation in the position of the frame attachment assembly will not be identified because the target volume isocenter is not directly verified with respect to the radiation isocenter. A technique for independently verifying the location of the target volume with respect to patient anatomy in three dimensions is required to assess possible frame slippage. Conventional radiotherapy treatment verification is per- formed by obtaining either a single or double exposure portal image just prior to irradiation. Patient positioning is assessed by comparing simulator films or digitally recon- structed radiographs (DRRs) with the portal images. Unfor- tunately, translations and rotations of the patient are only visible in the imaging plane, allowing a potentially large error. Out-of-plane rotations (i.e., rotations about an axis Reprint requests to: B. Gino Fallone, Medical Physics Unit, McGill University, Montreal General Hospital, Montreal, PQ, Canada H3G 1A4. Acknowledgements—This work was partly supported by the Med- ical Research Council (Canada) No. MT-13110. Accepted for publication 3 December 1997. Int. J. Radiation Oncology Biol. Phys., Vol. 41, No. 1, pp. 199 –205, 1998 Copyright © 1998 Elsevier Science Inc. Printed in the USA. All rights reserved 0360-3016/98 $19.00 + .00 199