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Purpose: Due to long treatment session with frameless image-guided radiosurgery for Trigeminal Neuralgia on a Linac (IGSRS-TN), frequent intra-fraction motion assessment is necessary. In this study, we retrospectively evaluated 19 IGSRS-TN treatments for setup accuracy throughout treatment.
Methods: The study includes 19 consecutive patients treated from 03/2017 onward at our institution, all were prescribed with 85 or 90Gy to isocenter. The plans consist of 9~12 arcs using a 4 mm conical cone. All patients were immobilized with stereotactic mask and positioned using the ExacTrac system. Prior to each arc, a pair of ExacTrac X-ray films was acquired and registered to planning CT at the planned couch angle; Any translational deviation per axis >0.3mm was corrected and followed by verification with a 2nd pair of ExacTrac films. Total elapsed time from the first to last arc, and 3D-deviation from initial pairs of x-ray films, and films capturing treated position, were collected. In comparison, intra-fraction 3D-deviation from 121 randomly sampled cranial SRS treatments with a dose =18Gy, same immobilization, but a tolerance of 1.0mm per axis evaluated once only per treatment, was also provided.
Results: Elapsed time of IGSRS-TN treatment is 37.6±9.8min (range 24.7 to 58.8min). A total of 197 arcs was delivered. At initial versus treated position, ExacTrac films indicated a 3D-deviation of 0.39±0.20mm (range 0.05 to 1.06mm) and 0.22±0.09mm (range 0.05 to 0.45mm), respectively. Setup correction was applied in 48% of the arcs, resulting in a decrease in 3D-deviation by 0.30mm in 27%, and 0.50mm in 8% of arcs. In comparison, the regular cranial SRS treatments indicated an intra-fraction deviation of 0.60±0.27mm (range 0.27 to 1.34mm), up from 0.33±0.18mm (range 0.18 to 1.23mm) pre-treatment.
Conclusion: Setup accuracy degrades from intra-fraction motion at frameless cranial SRS treatment. Frequent intra-fraction correction is effective in maintaining setup accuracy for IGSRS-TN.
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