Margin of Error for a Frameless Image-Guided Radiosurgery System: Direct Confirmation Based On Post-Treatment MRI Scans

Publication date: Available online 20 August 2016
Source:Practical Radiation Oncology
Author(s): Guozhen Luo, Joseph S. Neimat, Anthony Cmelak, Austin N. Kirschner, Albert Attia, Manuel Morales-Paliza, George X. Ding
PurposeTo report on radiosurgery delivery positioning accuracy in the treatment of tremor patients with frameless image-guided radiosurgery using the linear accelerator (LINAC) based ExacTrac system and to describe quality assurance (QA) procedures used.MethodsBetween 2010 and 2015, twenty patients underwent radiosurgical thalamotomy targeting the ventral intermediate nucleus for the treatment of severe tremor. The median prescription dose was 140Gy (range 120–145Gy) in a single fraction. The median maximum dose was 156Gy (range 136–162Gy). All treatment planning was performed with the iPlan system using a 4-mm circular cone with multiple arcs. Before each treatment QA procedures were performed including the imaging system. As a result of the extremely high dose delivered in a single fraction, a well-defined circular mark developed on the post-treatment MRI. Eight out of these twenty patients were selected to evaluate treatment localization errors because their circular marks were available in post treatment MRI. In this study the localization error is defined as the distance between the center of the intended target and the center of the post-treatment mark.ResultsThe mean error of distance was found to be 1.1mm (range 0.4mm to 1.5mm). The mean errors for the left–right, anterior–posterior, and superior–inferior directions are 0.5mm, 0.6mm, and 0.7mm, respectively.ConclusionsThe result reported in this study includes all tremor patients treated at our institution when their post treatment MRI data were available for study. It represents a direct confirmation of target positioning accuracy in radiosurgery with a LINAC-based frameless system and its limitations. This level of accuracy is only achievable with an appropriate QA program in place for a LINAC-based frameless radiosurgery system.



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