Abstract
PurposeThe aim of this study was to investigate the intra‐fractional patient motion using the ExacTrac system in LINAC‐based stereotactic radiosurgery (SRS).MethodA retrospective analysis of 104 SRS patients with kilovoltage image‐guided setup (Brainlab ExacTrac) data was performed. Each patient was imaged pre‐treatment, and at two time points during treatment (1st and 2nd mid‐treatment), and bony anatomy of the skull was used to establish setup error at each time point. The datasets included the translational and rotational setup error, as well as the time period between image acquisitions. After each image acquisition, the patient was repositioned using the calculated shift to correct the setup error. Only translational errors were corrected due to the absence of a 6D treatment table. Setup time and directional shift values were analyzed to determine correlation between shift magnitudes as well as time between acquisitions.ResultsThe average magnitude translation was 0.64 ± 0.59 mm, 0.79 ± 0.45 mm, and 0.65 ± 0.35 mm for the pre‐treatment, 1st mid‐treatment, and 2nd mid‐treatment imaging time points. The average time from pre‐treatment image acquisition to 1st mid‐treatment image acquisition was 7.98 ± 0.45 min, from 1st to 2nd mid‐treatment image was 4.87 ± 1.96 min. The greatest translation was 3.64 mm, occurring in the pre‐treatment image. No patient had a 1st or 2nd mid‐treatment image with greater than 2 mm magnitude shifts.ConclusionThere was no correlation between patient motion over time, in direction or magnitude, and duration of treatment. The imaging frequency could be reduced to decrease imaging dose and treatment time without significant changes in patient position.
Highlights
Frameless stereotactic radiosurgery (SRS) has taken on a significant role in treatment of cranial lesions, including primary and metastatic brain tumors, nerve disorders, and arteriovenous malformations
Previous works have shown that intra-fractional positioning accuracy of mask-based immobilization systems range from 1.59 Æ 0.84 mm to 4.7 Æ 1.7 mm using a thermoplastic mask and image guidance from Cone-beam CT (CBCT), CT simulation, portal images, and biplanar diagnostic x ray.[2,3,4,5]
This study investigates the intra-fractional motion during SRS treatment utilizing a thermoplastic mask and repositioning during treatment using ExacTrac stereoscopic kV x ray system based on our institutional imaging protocol
Summary
Frameless stereotactic radiosurgery (SRS) has taken on a significant role in treatment of cranial lesions, including primary and metastatic brain tumors, nerve disorders, and arteriovenous malformations. Previous works have shown that intra-fractional positioning accuracy of mask-based immobilization systems range from 1.59 Æ 0.84 mm to 4.7 Æ 1.7 mm using a thermoplastic mask and image guidance from Cone-beam CT (CBCT), CT simulation, portal images, and biplanar diagnostic x ray.[2,3,4,5] These positioning errors are still too large for SRS treatments, due to irradiating critical organs during the treatment. Multiple systems have been developed for image guidance, including electronic portal imaging devices (EPIDs), stereoscopic kV imaging, CBCT, and MVCT.[7] A study by Ramakrishna et al investigating intra-fractional motion found that there was less than 1.0 mm discrepancy between frame-based and imageguided at initial setup using a stereoscopic kilovoltage x ray system combined with an infrared position tracking system, and a positioning error of 0.7 mm for image-guided setup.[8] These imaging methods are highly reliant on bony anatomy for alignment due to the inability to distinguish brain metastases. Previous works have determined that the skull is a reliable surrogate for tumor position.[2,9,10]
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