Submillimeter accuracy in radiosurgery is not possible

摘要

Arguing against the Proposition is Sonja Dieterich, Ph.D. After completing her Ph.D. in Nuclear Physics at Rutgers University in 2002, Dr. Dieterich received training in Medical Physics at Georgetown University Hospital, Washington, DC, from 2002 to 2003. In 2003, she accepted a faculty position at Georgetown. From 2007 to 2012, she worked at Stanford University Hospital as Clinical Associate Professor and Chief of Radiosurgery Physics. Since April 2012 she is an Associate Professor and Physics Residency Co-Director at the University of California Davis. Dr. Dieterich is Chair of the AAPM Task Group 135 “QA for Robotic Radiosurgery” and a member of the ASTRO Physics and Multi-Disciplinary QA Committees. Her current research interests are the development of QA/QM programs for new technologies, image-guided brachytherapy, and Veterinary Radiation Oncology. The advent of cranial radiosurgical therapy has allowed a nonsurgical approach to the treatment of cranial lesions.1 The obvious benefits apply to both malignant as well as benign lesions. Historically a rigid invasive frame, attached to the patient's skull via small screws, acted as both immobilizer and localizer. With such devices, the frame defines a 3D coordinate system within which the skull and targeted lesion are intended to remain fixed from the time of the planning CT scan to the completion of treatment. More modern radiosurgical systems may use 3D “image-guided” positioning for exact alignment. Accuracy of radiosurgery has often been considered as ultimately relying on the rigidity of the immobilizer, as opposed to the treatment system as a whole. This ignores multiple other factors that must also be included. Typical quoted values for frame-based immobilization accuracy are 0.3–0.6 mm.2–4 This measure relates only to flex between the frame and the skull, and the stability of the treatment apparatus. The system accuracy as a whole must also include end-to-end uncertainties that result from the spatial resolution of the original CT, resolution and linearity of the MRI scan, contouring uncertainty, treatment planning grid resolution, algorithm errors, CT to MRI registration uncertainty, etc. The now-dated AAPM report 54 (Ref. 5) suggested that the total uncertainty can reach 2–3 mm. The planning system is designed to calculate an optimized dose distribution around a physician-determined region of interest (ROI). Since the physician-drawn ROI is a function of the window and level differences on CT and MRI, there is variability between physicians, which even for well-identified normal structures (let alone often less distinct targets) can exceed 1–2 mm.6 A recent study by a Canadian group demonstrated a shift in tumor isocenter of 1.4 mm by simply altering the time of CT imaging after injected contrast.7 An analysis of the registration accuracy of CT with MRI for several algorithms determined typical errors along the x, y, and z axes of approximately 0.68, 1.04, and 0.60 mm, respectively. When added in quadrature this amounts to a vector of 1.4 mm.8 Others have published even greater variability.9 A recent Gamma Knife end-to-end study, evaluating the distance to agreement, determined that the uncertainty was better than expected based on quadrature-sum but still over 1 mm.10 The planning system dose grid is unlikely to have a resolution of less than 1 mm and the dose calculation algorithm will itself contain some uncertainty in placement of dose within the grid due to imperfect modeling of radiation transport. Since a significant percent of target definition is done on MRI datasets it is important to note that even for MRI systems compliant with ACR guidelines, the distortion can reach up to 2 mm.11 Taking all of the above uncertainties into account is essential in deriving a realistic overall accuracy for SRS treatment. It is apparent that radiosurgery treatment using current technology and considering human factors, when observed from end-to-end, cannot reach submillim